Image forming apparatus, storage medium, and image forming method

Abstract

To suppress the deterioration of image quality and productivity by performing appropriate correction processing to the secular change of density unevenness.SOLUTION: An image forming apparatus includes an image acquisition unit that acquires an output image, an image addition unit that adds an additional image to the output image, an image forming unit that includes a plurality of nozzles for discharging a recording material and forms the output image to which the additional image is added on a recording medium, and a reading unit that reads the formed image as a read image, wherein the additional image includes an acquisition region for acquiring a characteristic of the image forming unit based on the read image, and the acquisition region includes a preliminary discharge region for performing preliminary discharge of the image forming unit.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an image processing technique for reducing density unevenness and streaks that occur when forming an image by ejecting ink. [Background technology]

[0002] The printhead used in inkjet printing devices is equipped with multiple nozzles that eject ink. Due to manufacturing errors and other factors, the amount of ink ejected can vary between nozzles. Such variations in ejection amount can result in uneven density in the printed image. Head shading (HS) technology is a well-known method for reducing such uneven density. HS processing increases or decreases the number or size of ink dots printed ultimately based on information about the ejection amount of each nozzle (nozzle characteristics), thereby reducing uneven density in the printed image. The nozzle characteristics are typically acquired by printing patch images (e.g., uniform images for each tone) on paper and then capturing and analyzing the images using a scanner.

[0003] However, it is known that nozzle characteristics change due to factors such as ink adhesion around the nozzles, aging of the piezoelectric elements and heaters that control ink ejection, and printing environments such as temperature and humidity. A technology is also known that continuously reduces density unevenness by updating the nozzle characteristics at regular intervals to deal with such changes in nozzle characteristics. Patent Document 1 discloses a technology that shortens the time required for density unevenness reduction processing by using a judgment chart to determine whether density correction is necessary for each ink color and dot size, and then reacquiring only the characteristics that require density correction. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2015-160352 Summary of the Invention [Problem to be solved by the invention]

[0005] In the above-mentioned Patent Document 1, two types of charts are required: a judgment chart and a correction chart. Furthermore, for colors determined to require correction, a uniform correction process is performed regardless of the cause of the unevenness or the area where it occurs. However, depending on the unevenness that occurs, it may be possible to reduce the unevenness with a simple process tailored to the cause and area, rather than a uniform process. Furthermore, a judgment chart is required to determine whether density correction is necessary, and correction is performed for all gradations and nozzles during correction, resulting in reduced productivity.

[0006] The present invention provides a technique for suppressing degradation of image quality and productivity by performing appropriate correction processing for changes in density unevenness over time. [Means for solving the problem]

[0007] In order to solve this problem, for example, an image forming apparatus according to the present invention has the following configuration: image acquisition means for acquiring an output image; an image adding means for adding an additional image to the output image; an image forming means that includes a plurality of nozzles for ejecting a recording material, and ejects the recording material onto a recording medium to form the output image to which the additional image has been added; a reading means for reading the formed image as a read image; Equipped with the additional image includes an acquisition area for acquiring characteristics of the image forming means based on the read image, The acquisition area includes a preliminary ejection area for performing preliminary ejection of the image forming means. [Effects of the Invention]

[0008] According to the present invention, deterioration in image quality and productivity can be suppressed by performing appropriate correction processing for changes in density unevenness over time. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a diagram showing a hardware configuration of an image forming system. [Figure 2] FIG. 2 is a diagram showing an example of the configuration of a recording head. [Figure 3] FIG. 2 is a diagram showing the functional configuration of an image processing unit. [Figure 4] FIG. 10 is a diagram showing an outline of density unevenness correction processing. [Figure 5] 10 is a flowchart showing processing by an image processing unit. [Figure 6] 10 is a flowchart showing unevenness correction processing. [Figure 7] 10 is a flowchart showing a static correction process. [Figure 8] FIG. 10 is a diagram showing an example of an output chart. [Figure 9] FIG. 10 is a diagram showing an overview of the HS table creation process. [Figure 10] 10 is a flowchart showing a dynamic correction process. [Figure 11] FIG. 4 is a diagram showing an outline of a nozzle characteristic acquisition process. [Figure 12] FIG. 10 is a diagram showing an outline of ink color specification processing. [Figure 13] FIG. 10 is a diagram showing an example of an output chart. [Figure 14] 10 is a flowchart showing processing by an image processing unit. [Figure 15] 10 is a flowchart showing a correction process. [Figure 16] FIG. 10 is a diagram showing an outline of correction processing. [Figure 17] 10 is a flowchart showing a static correction process. [Figure 18] FIG. 10 is a diagram showing an outline of a correction value calculation process. [Figure 19] FIG. 10 is a diagram showing an example of a correction table. [Figure 20] FIG. 10 is a diagram showing an example of a selection screen. [Figure 21] FIG. 10 is a diagram showing a hardware configuration of a fourth embodiment. [Figure 22] FIG. 10 is a diagram schematically illustrating an image forming unit according to a fourth embodiment. [Figure 23] FIG. 10 is a diagram showing the functional configuration of an image processing unit according to a fourth embodiment. [Figure 24] FIG. 13 is a diagram showing an example of a correction table according to the fourth embodiment. [Figure 25] FIG. 13 is a diagram for explaining processing by a header image adding unit in the fourth embodiment. [Figure 26] 10 is a flowchart showing a correction process according to the fourth embodiment. [Figure 27] 10 is a flowchart showing a correction table creation process according to the fourth embodiment. [Figure 28] FIG. 13 is a diagram for explaining the processing content of a correction table creation unit in the fourth embodiment. [Figure 29] FIG. 10 is a diagram showing an example of an HT pattern according to the fourth embodiment. [Figure 30] 10A to 10C are diagrams showing examples of ejection patterns in a preliminary ejection region according to the fourth embodiment. [Figure 31] FIG. 13 is a diagram for explaining the processing of a header image adding unit in the fourth embodiment. [Figure 32] 13 is a flowchart showing a printing process in the fifth embodiment. [Figure 33] FIG. 13 is a functional configuration diagram of an image processing unit according to the sixth embodiment. [Figure 34] FIG. 20 is a diagram showing an example of a header image according to the sixth embodiment. [Figure 35] FIG. 20 is a diagram showing an example of a line profile according to the sixth embodiment. [Figure 36] 13 is a flowchart showing a correction process according to the sixth embodiment. [Figure 37] FIG. 20 is a diagram showing an example of an HT pattern according to the sixth embodiment. [Figure 38] FIG. 4 is a diagram showing an example of conveyance of recording paper. [Figure 39] FIG. 10 is a diagram showing an example of transport of roll paper. [Figure 40] FIG. 10 is a diagram showing a print example of a header image. [Figure 41] FIG. 10 is a diagram showing an example of a header image. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments will be described in detail with reference to the accompanying drawings. Note that the following embodiments do not limit the scope of the claimed invention. Although multiple features are described in the embodiments, not all of these multiple features are necessarily essential to the invention, and multiple features may be combined arbitrarily. Furthermore, in the accompanying drawings, the same reference numerals are used to designate the same or similar components, and redundant explanations will be omitted.

[0011] [First embodiment] <Hardware configuration of image forming system> FIG. 1 is a diagram showing the hardware configuration of an image forming system 1. The image forming system 1 in this embodiment includes an image processing device 11 that functions as an image processing controller built into the image forming system, and an image forming device 12 that forms an image on a recording medium using a recording material. The image processing device 11 includes a CPU 100, a RAM 101, a ROM 102, an image processing unit 106, an I / F (interface) unit 110, and a bus 112. The image processing device 12 includes an image forming unit 107, an image sensor 108, a maintenance unit 109, an I / F (interface) unit 111, a bus 113, and a RAM 114. The image processing device 11 is connected to an operation unit 103, a display unit 104, and an external storage device 105 via the I / F (interface) unit 111. The I / F unit 111 also includes an interface for communicating with a network.

[0012] The components within the image processing device 11 and the image forming device 12 are connected to a bus 112 and a bus 113, respectively, and data can be exchanged via these buses. The components constituting the image processing device 11 and the image forming device 12 will be described below.

[0013] A CPU (Central Processing Unit) 100 executes input data and computer programs stored in a RAM 101 and a ROM 102 to control the overall operation of the image forming system 1. Note that, although a case where the CPU 100 controls the entire image forming system will be described here as an example, the entire image forming system may also be controlled by multiple pieces of hardware sharing the processing.

[0014] RAM (Random Access Memory) 101 has a storage area for temporarily storing computer programs and data read from an external storage device 105 and data received from the outside via an I / F unit 110. RAM 101 is also used as a storage area (including characteristic information of nozzles of a print head) used when CPU 100 executes various processes and as a storage area used when image processing unit 106 executes image processing. ROM (Read Only Memory) 102 has a storage area for storing setting parameters for setting each unit in the image forming system, boot programs, etc.

[0015] The image processing unit 106 is realized as a processor capable of executing a computer program or a dedicated image processing circuit, and performs various image processing to convert image data input as the print target or image data based on a print job into image data for recording that can be recorded by the image forming apparatus 12, according to the recording characteristics of the nozzles, etc. Note that instead of providing a dedicated processor as the image processing unit 106, it is also possible to configure the CPU 100 to perform various image processing as the image processing unit 106.

[0016] The I / F unit 110 functions as an interface for connecting the image processing device 11, the image forming device 12, and external devices. The I / F unit 110 also functions as an interface for exchanging data with a communication device using infrared communication, a wireless LAN (Local Area Network), or the like, and as an interface for connecting to the Internet.

[0017] The operation unit 103 is an input device such as a keyboard or a mouse, and accepts operations (instructions) from an operator. That is, the operator can input various instructions to the CPU 100. The display unit 104 is a display device such as a CRT (Cathode Ray Tube) or a liquid crystal screen, and can display the results of processing by the CPU 100 as images, characters, etc. If the display unit 104 is a touch panel that can detect touch operations, the display unit 104 may function as a part of the operation unit 103.

[0018] The external storage device 105 is a large-capacity information storage device, typified by a hard disk drive. The external storage device 105 stores computer programs and data for causing the OS (operating system) and CPU 100 to execute various processes. The external storage device 105 also stores temporary data generated by the processing of each unit (for example, input / output image data, a color conversion table used by the image processing unit 106, a threshold matrix, information on the position of non-ejecting nozzles that cannot eject ink, nozzle characteristics used for HS processing, etc.). The computer programs and data stored in the external storage device 105 are read as appropriate under the control of the CPU 100, stored in the RAM 101, and then processed by the CPU 100.

[0019] The I / F unit 111 functions as an interface for connecting the image forming apparatus 12 to the image processing apparatus 11 .

[0020] The RAM 114 is used as a storage area for temporarily storing image data for output acquired from the image processing device 12. The image forming unit 107 forms an image on a recording medium using a recording material based on the image data stored in the RAM 114. The image forming unit 107 in this embodiment is an inkjet type that forms an image by ejecting ink from nozzles onto the recording medium, and includes a recording element array in which a plurality of recording elements capable of ejecting ink are arranged.

[0021] 2 is a diagram showing an example of the configuration of a print head in the image forming unit 107. Note that a print head typically has nozzles for four types of ink: cyan (C), magenta (M), yellow (Y), and black (K), but for simplicity of explanation, only black (K) is shown in the figure.

[0022] The print head in this embodiment is a long line head that covers the entire range of the image formation area in a first direction, which is a direction parallel to the nozzle array (x direction). The image forming unit 107 generates a drive signal for controlling the print head based on halftone image data. Based on the drive signal, the print head forms an image on the print medium by generating dots while relatively moving the print medium, such as recording paper, in a second direction, which is perpendicular to the direction parallel to the nozzle array (y direction).

[0023] The image sensor 108 includes an image sensor (a line sensor or an area sensor) for reading or capturing an image formed on a recording medium by the image forming unit 107, and functions as a means for acquiring non-discharge nozzles and nozzle characteristics from the captured image. In this embodiment, the image sensor 108 is described as an inline scanner mounted on the recording head, but an offline scanner or the like may also be used.

[0024] The maintenance unit 109 functions as a means for performing head recovery operations to remove clogged nozzles in the line heads provided in the image forming unit 107. Examples of head recovery operations include moving the print head to a position where a waste ink absorber (such as a sponge) is located and driving the print head to eject a large amount of ink. Another method is to pressurize the ink from the ink tank side and forcibly push the ink out. Another method is to apply negative pressure from outside the nozzles to forcibly suck out the ink and remove the clogged nozzles.

[0025] <Functional configuration of image processing unit 106> Next, the functional configuration of the image processing unit 106 will be described with reference to Fig. 3. The image processing unit 106 is made up of an input color conversion processing unit 301, an ink color conversion processing unit 302, an HS processing unit 303, a discharge failure complement processing unit 304, a dot size conversion processing unit 305, a quantization processing unit 306, a dot size synthesis processing unit 307, and a color signal modification unit 308. Each component of one pixel of image data handled by the image processing unit 106 is represented by 8 bits (256 gradations), and the resolution is the same as the resolution of the nozzle arrangement of the print head, for example, 1200 dpi.

[0026] The input color conversion processing unit 301 converts input image data from the external recording device 105 into image data that corresponds to the color gamut of the printer. The input image data is, for example, data that indicates color coordinates (R, G, B) in a color space coordinate system such as sRGB, which is the representation color of a monitor. The input color conversion processing unit 301 converts this input image data R, G, B into image data (R', G', B') in the color gamut of the printer. This conversion can be performed using known methods such as matrix calculation processing or processing using a three-dimensional LUT (look-up table). In this embodiment, the conversion processing is performed using a three-dimensional input color conversion LUT 309 stored in the RAM 101, in combination with interpolation calculations.

[0027] The ink color conversion processing unit 302 performs conversion processing on the image data converted by the input color conversion processing unit 301, converting the image data into color signals corresponding to the multiple inks used in the image forming unit 107. For example, if the image forming unit 107 uses black (K), cyan (C), magenta (M), and yellow (Y) inks, the image data of RGB signals is converted into image data consisting of 8-bit color signals for each of K, C, M, and Y. This color conversion is also performed using a three-dimensional ink conversion LUT 310, similar to the input color conversion processing unit described above. Note that other conversion methods, such as matrix calculation processing, can also be used, as described above.

[0028] The HS processing unit 303 performs correction on the color signal image data converted by the ink color conversion processing unit 302 in accordance with the ejection characteristics of each nozzle that makes up the print head. In this embodiment, HS processing is performed using an HS table 311 that is generated in advance based on the characteristics of each nozzle. In this embodiment, color signal data 312 after HS processing is stored in RAM 101. Note that, to avoid interference with the ejection failure complement processing described below, in this embodiment, HS correction data is generated using an image in a state where there are no ejection failures. Details of the HS processing will be described later.

[0029] The discharge failure complement processing unit 304 performs discharge failure complement processing on the color signal data 312 based on previously acquired discharge failure nozzle information (discharge failure information) 313. In this embodiment, the ink color data corresponding to the discharge failure nozzle is distributed to the nozzles in the vicinity of the discharge failure nozzle. Note that the discharge failure complement processing is not limited to the above, and it is also possible to configure the HS table 311 in the vicinity of the discharge failure nozzle to be corrected prior to processing by the HS processing unit 303. Alternatively, it is also possible to configure the dot pattern after quantization processing by the quantization processing unit 306, which will be described later.

[0030] The dot size conversion processing unit 305 performs a conversion process to convert each color signal data into an 8-bit size signal corresponding to the dot size. For example, if the image generation unit 107 can form three dot sizes (large, medium, and small), it converts the image data for each of K, C, M, and Y into multiple image data corresponding to the large, medium, and small dots. In other words, if there are four ink colors (CMYK) and three dot sizes (large, medium, and small), the dot size conversion processing unit 305 generates a total of 12 pieces of image data for all combinations of ink colors and dot sizes. Note that the dot size conversion processing 305 can be performed by using an interpolation operation in combination with the size conversion table 314, which is a one-dimensional lookup table in which the color signal value of each color is associated with the signal value of each dot size.

[0031] The quantization processor 306 performs conversion processing on each 8-bit (256 gradation values) image data processed by the dot size conversion processor 305 to the number of gradations that can be represented by the image forming unit 107 and halftone processing to determine the dot arrangement to be formed by the nozzle group, thereby generating halftone image data. In this embodiment, the quantization processor 306 converts the 8-bit image data per pixel into 1-bit binary halftone image data (output image data) in which each pixel has a value of either 0 or 1. In the halftone image data, a pixel with a pixel value (output value) of 0 represents an off dot, and a pixel with a pixel value (output value) of 1 represents an on dot. Note that well-known methods such as error diffusion processing and dithering can be used for halftone processing. In this embodiment, quantization is performed by dithering using a threshold matrix 315.

[0032] The dot size synthesis processing unit 307 synthesizes the dot data for each dot size generated by the quantization processing unit 306 for each pixel and outputs the synthesized data to the image generation unit 107. The output image data is passed to the image forming device 12 via the I / F units 110 and 111 and stored in the RAM 114 within the image forming device 12. The image forming device 12 performs recording processing using the image forming unit 107 based on the image data stored in the RAM 114.

[0033] The color signal modification unit 308 performs modification processing on the color signal data 312 based on the read result of the image sensor 108. Specific processing in the color signal modification unit 308 will be described later.

[0034] The above is a description of the configuration of the image forming system 1. Below, a description will be given of the process for reducing density unevenness and streaks based on this configuration.

[0035] <Overview of Density Unevenness Correction> FIG. 4(a) is a conceptual diagram showing the processes related to density unevenness correction of the image forming system 1 in the present embodiment along the time axis t. Further, FIG. 4(b) is a conceptual diagram showing the quality Q of the formed image on the same time axis t as in FIG. 4(a). Hereinafter, the density unevenness correction in the present embodiment will be described using FIGS. 4(a) and (b).

[0036] As shown in FIG. 4(a), it is assumed that the power of the image forming system 1 is turned on at time t0. At this time, the image forming system 1 acquires the quality Q(t0) of the image formed by the image forming unit 107.

[0037] Here, the quality Q(t) is a conceptual value representing the overall image quality at time t. More specifically, it is a value that at least includes an evaluation of density unevenness and streaks remaining on the formed image after the correction process by the HS processing unit 303. Although the quality Q(t) is shown on one axis in FIG. 4(b) for the sake of explanation, it may actually be a value represented by multiple axes. In the present embodiment, as the quality Q(t), an overall evaluation value considering granularity, color shift, sharpness, character reproducibility, etc. in addition to density unevenness and streaks is used.

[0038] Next, the image forming system 1 compares Q(t0) with a predetermined threshold quality Q0. Then, when the quality at that time is below the threshold (when Q(t0) < Q0), the image forming system 1 executes any one of a plurality of unevenness correction processes. In the example shown in FIG. 4(a), the image forming system 1 holds three different correction processes, which are shown for convenience as "unevenness correction A", "unevenness correction B", and "unevenness correction C". Further, separately from the unevenness correction process, it also has a function of complementing non-printing nozzles by the non-printing complement processing unit 307, which is shown as "non-printing complement" in FIG. 4(a).

[0039] In the example shown in FIG. 4(a), unevenness correction A is performed from time t1 to time t2. Here, unevenness correction A includes outputting, reading, analyzing, and calculating correction values ​​for multiple measurement charts, as well as performing head recovery operations by the maintenance unit 109. Therefore, compared to other unevenness correction processes, unevenness correction A has higher correction accuracy and is less likely to cause deterioration of graininess or color shift due to correction. On the other hand, unevenness correction A requires a long processing time, and printing of a user image is not possible during the correction process.

[0040] As shown in Figure 4(b), unevenness correction A results in a quality Q(t2) that is higher than the threshold Q0. After t2, the image forming system is ready to print the user image. As mentioned above, the nozzle characteristics change as the user image is printed and over time. Therefore, after t2, the unevenness suppression effect of the unevenness correction process decreases, and as a result, density unevenness and streaks remain in the formed image, causing the quality Q to decrease. In the example shown in Figure 4(b), the quality Q decreases to the threshold Q0 (below the threshold Q0) at time t3.

[0041] In this embodiment, the image forming system 1 monitors the quality Q(t) at regular intervals, and when it detects that Q(t) has dropped to the threshold Q0, it executes one of the multiple unevenness correction processes again. At this time, the correction process to be executed is selected based on, for example, the predicted value of the quality Q after each correction process, the processing time required for unevenness correction, and the downtime during which user images cannot be printed.

[0042] 4, unevenness correction B is executed from time t3 to time t4. Here, unevenness correction B does not include the output of a chart or the execution of a head recovery operation. Therefore, unevenness correction processing is executed while continuing to print the user's image or while maintaining a printable state.

[0043] On the other hand, the quality Q(t4) after processing with unevenness correction B will be lower than the quality Q(t2) after processing with unevenness correction A due to the accuracy of the correction and adverse effects of the correction. Alternatively, the improvement in quality with unevenness correction B depends more on the nozzle characteristics before processing than with unevenness correction A, and in some cases it may be difficult to achieve improvement.

[0044] In addition to changes in nozzle characteristics over time, non-discharge nozzles that do not form dots may occur due to air bubbles entering the nozzles or the adhesion of dust. In such cases, the image forming system executes non-discharge complement processing by the non-discharge complement processing unit 304 to suppress the deterioration of quality Q due to non-discharge nozzles. In FIG. 4, non-discharge nozzles occur at times t5, t7, and t9, and complement processing is completed at times t6, t8, and t10, respectively. In this embodiment, non-discharge complement processing is executed while continuing to print the user's image or while maintaining a printable state.

[0045] 4(b), at time t11, quality Q drops again to threshold Q0 due to density unevenness and streaks that are not caused by non-ejecting nozzles. If the image forming system 1 determines that unevenness correction B, which requires less downtime and less time for the correction to be reflected, does not provide sufficient correction accuracy or that the correction will have significant adverse effects, it selects unevenness correction C, which has higher accuracy.

[0046] In the example shown in FIG. 4(a), the image forming system 1 is performing unevenness correction C from time t11 to time t12. Here, unevenness correction C includes the output of a chart for correction processing, and includes downtime during which user images cannot be printed. However, by limiting the correction range (tone, nozzle), the number of charts to be output, the processing time required for analysis, and calculation of correction values ​​is shorter than that of unevenness correction A, and head recovery operation is not included. Therefore, as shown in FIG. 4(a), the downtime in unevenness correction C and the time required for the correction to be reflected are shorter than those of unevenness correction A.

[0047] 4(a), unevenness correction C is performed again from time t13 to time t14. At this time, even though the same unevenness correction C is performed, the quality Q(t14) after the second unevenness correction C is lower than the quality Q(t12) after the previous unevenness correction C due to adverse effects of the correction and accumulation of correction errors.

[0048] When the image forming system determines that unevenness corrections B and C cannot improve quality due to such adverse effects or accumulated correction errors, it performs unevenness correction A again. In the example shown in FIG. 4(a), unevenness correction A is performed again from time t15 to t16. At this time, the quality Q(t16) after correction is approximately equal to the quality Q(t2) after correction by unevenness correction A the previous time.

[0049] In this way, by storing multiple unevenness correction processes with different accuracy, downtime, processing time, occurrence of adverse effects, etc., and using them appropriately based on the predicted value of quality Q after each correction process, the processing time required for unevenness correction, and the downtime during which user images cannot be printed, it is possible to prevent excessive or insufficient processing from being performed while maintaining a certain level of image quality in response to changes in density unevenness over time.

[0050] Specifically, by performing unevenness corrections B and C at times t3, t11, and t13 in Fig. 4, it is possible to maintain the quality of the output image higher than the threshold Q0, while reducing downtime and improving productivity per unit time compared to performing unevenness correction A at all times. Note that if unevenness corrections B and C are part of unevenness correction A, the processing can be shared, which is preferable as it saves circuit size and program memory.

[0051] <Image processing flow> The image processing flow of the image processing unit 106 in this embodiment is shown in Fig. 5. The image forming system 1 in this embodiment stores a plurality of different non-uniformity correction processes depending on the accuracy, downtime, processing time, occurrence of adverse effects, and the like.

[0052] In the explanation of FIGS. 4(a) and 4(b), it has been explained that there are three types of unevenness correction A, B, and C. Because unevenness correction A involves printing multiple charts, it takes a long time to complete the process. As will be explained in detail later, unevenness correction C involves printing a significantly smaller number of charts (one in this embodiment) than unevenness correction A, obtaining a scanned image, and performing correction. Furthermore, unevenness correction B involves correction during normal job processing without printing a chart. Therefore, to simplify the explanation, an example will be described below in which two different unevenness correction processes A and B are provided and used appropriately to reduce downtime caused by the unevenness correction processes.

[0053] One of the two unevenness correction processes is a dynamic correction process that performs high-speed correction without outputting a correction chart, while continuing to print the user image or maintaining a printable state. This correction process corresponds to unevenness correction B (including unevenness correction C) in the explanation using Figures 4(a) and (b) above. In this embodiment, dynamic correction detects the direction of fluctuation in nozzle characteristics and changes color signal data according to a predetermined correction amount, thereby achieving high-speed correction.

[0054] The other is static correction processing, in which various tables used to create output image data are calibrated, and then the calibrated tables are used to create output data again, including HS processing. This correction processing corresponds to unevenness correction A in the explanation using FIG. 4 above. In static correction processing in this embodiment, multiple charts are output for the calibration of each table. Because there are many types and numbers of charts output for HS correction, and the calculation load required for correction is also large, printing of images requested by the user is interrupted or stopped during static correction processing. Each step of the image processing flow will be explained below using FIG. 5.

[0055] In the above explanation, the threshold value for determining the degradation of the quality of the output image was described as Q0. However, in the following, in order to explain a more specific example, an example will be described in which the degradation is determined based on the size of the color difference ΔE between the target color and the color of the actually printed image.

[0056] First, when the image forming system 1 is powered on, the image processing unit 106 refers to a flag stored in the RAM 101 or the external storage device 105 (S501). In this embodiment, this flag indicates whether static correction processing is required. If the image processing unit 106 determines that the flag is on as a result of the reference, the process proceeds to S502, where static correction processing is performed, and then the process proceeds to S503. Details of the static correction processing will be described later. On the other hand, if the image processing unit 106 determines that the flag is off, the process skips steps S502 to S504 and proceeds to S505.

[0057] In S503, the image processing unit 106 creates reference characteristics from the results of the static correction processing in S502. The reference characteristics created here are the color development characteristics of the image formed in response to the input image signal. In the dynamic correction described below, correction is performed so that the characteristics of each nozzle reproduce these reference characteristics. Specifically, the reference characteristics are stored in the RAM 101 or the external storage device 105 as a color characteristic LUT in a device-independent color space such as XYZ or Lab for the RGB combination of the input image. Specific methods for acquiring the reference characteristics will be described later.

[0058] In S504, the image processing unit 106 turns off the flag stored in the RAM 101 or the external storage device 105. This step makes it possible to skip the processes from S502 to S504 until the flag is turned on, thereby reducing downtime at start-up.

[0059] In S505, the image processing unit 106 enters a standby state until a print job is input. When the user inputs a print job via the operation unit 103, the image processing unit 106 advances the process to S506.

[0060] In S506, the image processing unit 106 acquires a print job input by the user. Specifically, the image processing unit 106 acquires a path to each image data item desired to be printed by the user, as well as printing conditions such as the number of copies to be printed, print quality, and recording medium. The acquired information related to the print job is stored in the RAM 101 and is referenced in subsequent processing. Subsequently, in S507, the image processing unit 106 acquires the user image data specified by the print job and performs various image processing operations to convert the data into image data (output image data) that can be output by the image forming unit 107. Specifically, the image processing unit 106 first loads various tables and matrices stored in advance in the external storage device 105 onto the RAM, and then executes processing by the units denoted by reference numerals 301 to 307 in FIG. 3. Thereafter, the image processing unit 106 transfers the acquired output image data to the image forming device 12. At this time, the image processing unit 106 stores color signal data 312 corresponding to each ink color in the RAM 101. In the discharge failure complement process and dynamic correction process of this embodiment, complement and correction processes are applied to this color signal data 312 .

[0061] Next, in S508, the image processing unit 106 instructs the image forming device 12 to print one image on a recording medium in accordance with the print job stored in RAM 101. At this time, the CPU 100 controls the image sensor 108 to capture an image of the image formed on the recording medium. As a result, the captured image is transferred to the image processing device 11 via the I / Fs 111 and 110, and is stored as a scanned image in the RAM 101 or the external storage device 105. Next, in S509, the image processing unit 106 calculates the current color development characteristics of each nozzle from the scanned image stored in S508. This will be described in detail later.

[0062] Next, in S510, the image processing unit 106 compares the current color characteristics of each nozzle calculated in S509 with the reference characteristics created in S503 and determines whether the difference is outside a predetermined tolerance. Specifically, the color difference ΔE between the color characteristics is used for the determination; for example, if ΔE>3, it is determined to be outside the tolerance. Note that the color difference ΔE used in this step is stored in RAM 101 in association with the time of acquisition, as it may be referenced in the unevenness correction process described below. If the image processing unit 106 determines in S510 that the difference is outside the tolerance, the process proceeds to S511, where one of the unevenness correction processes is executed. Details will be described later. On the other hand, if the image processing unit 106 determines in S510 that the difference is within the tolerance, the process skips S511 and proceeds to S512.

[0063] Next, in S512, the image processing unit 106 performs non-discharge detection and correction using the non-discharge complement processing unit 307. Here, known methods can be used for non-discharge detection. For example, a stepped chart can be embedded in advance at the edge of the image, and non-discharge nozzles can be detected from that area in the scanned image. Alternatively, a non-discharge detection chart can be output at regular intervals, and non-discharge nozzles can be detected based on that chart. Based on the positions of non-discharge nozzles detected in this manner, the non-discharge complement processing unit 304 applies non-discharge correction processing to the color signal data 312 stored in RAM 101. It is also possible to detect non-discharge nozzles based on a comparison between the input image and the scanned image, without using a chart. Alternatively, non-discharge nozzles can be detected using a combination of an infrared emitter and an infrared sensor (not shown), rather than obtaining information from an image. Furthermore, non-discharge nozzles can also be detected by monitoring the ink flow rate within the head.

[0064] Next, in S513, the image processing unit 106 determines whether all print jobs acquired in S506 have been completed. That is, it determines whether the specified number of copies have been printed for all specified user images. If the image processing unit 106 determines that the job is incomplete, the process returns to S508 and printing continues. On the other hand, if the job is completed, the image processing unit 106 returns to S505 and waits until the next print job is input.

[0065] <Unevenness correction process flow> Next, the unevenness correction process in S511 of Fig. 5 will be described in more detail with reference to Fig. 6. As described above, in the unevenness correction process of this embodiment, two correction processes (dynamic correction and static correction) are used.

[0066] First, in S601, the image processing unit 106 refers to a flag stored in the RAM 101 or the external storage device 105. The flag referred to here is the same as the flag referred to in S501 and S504 described above. In S605 and S607 described below, the flag is set to off after static correction and on after dynamic correction. Therefore, if no dynamic correction has been performed after static correction, the flag is off.

[0067] If the flag referenced in S601 is off, the process proceeds to S606, where dynamic correction is performed. The flag is then turned on in subsequent S607. On the other hand, if the flag is already on, the process proceeds to S602, where it is determined whether the previous (most recent) dynamic correction has worsened the streaks. More specifically, the color difference ΔE used in the immediately preceding S510 is compared with ΔE' used in the S510 immediately before that, and if ΔE>ΔE', it is determined that the streaks have worsened.

[0068] If the image processing unit 106 determines in S602 that the density unevenness has not worsened, it determines that further unevenness correction is possible through dynamic correction, and proceeds to S606 to perform dynamic correction again. On the other hand, if the image processing unit 106 determines in S602 that the streaks have worsened due to dynamic correction, it determines that the currently occurring density unevenness cannot be corrected through dynamic correction, and performs static correction in S603. Note that the determination in S602 may also include not only density unevenness and color shift, but also character reproducibility and graininess. For example, the variance value within a substantially uniform area in the scanned image in S503 is stored as a reference variance value. Furthermore, if the variance value of the dynamic area of ​​the scanned image in S508 is larger than the reference variance value by a certain amount or more, static correction processing may be performed.

[0069] In this embodiment, after performing static correction in S603, the image processing unit 106 creates reference characteristics (S604) and turns off the flag (S605), similarly to S503 and S504. As described above, in this embodiment, the flag indicates whether static correction is necessary and whether dynamic correction has been performed at least once after static correction. In other words, by using this flag, static correction is performed if the power is turned off and on after dynamic correction has been performed at least once. This is because dynamic correction prioritizes the time required for correction, the amount of processing, and the number of charts to be printed, and there is a high possibility of correction errors or adverse effects occurring. In other words, in this embodiment, the flag indicates that errors or adverse effects have occurred in the dynamic correction and that performing static correction can further improve the quality of the image forming system.

[0070] <Static correction processing flow> The flow of the static correction processing in S603 described above is shown in Figure 7. As described above, in the static correction processing in this embodiment, multiple charts are output and analyzed in order to calibrate the tables used in the processing units 301 to 307 within the image processing unit 106. Furthermore, in this embodiment, HS correction data is generated in a state where there are no discharge failures, in order to avoid interference with the discharge failure complement processing. The static correction processing in this embodiment will be described in more detail below using Figure 7.

[0071] First, in S701, the image processing unit 106 determines a first primary color for each ink color (CMYK). Here, the first primary color is the coloring characteristic corresponding to the maximum signal value (255 in 8-bit) generated by the ink color conversion processing unit 302. It is extremely rare for the amount of ink ejected from each nozzle to be perfectly consistent due to factors such as manufacturing errors and interactions within the print head. Therefore, if all nozzles are used to output large dots (solid output) to all of the grids at an output resolution (for example, 1200 dpi x 1200 dpi), a solid image with different densities for each column corresponding to each nozzle will be formed on the print medium, despite the uniform input.

[0072] In such cases, the HS processing unit 303 can adjust the density of nozzles that are darker than the other nozzles by thinning out the dots or reducing the dot size. However, it is not possible to eject dots that exceed the output resolution, and it is not possible to output dots larger than large dots. In other words, it is not possible to adjust nozzles that are lighter than the other nozzles to output a darker color. Therefore, it is preferable to set the primary color to match the nozzle with the lightest density. Furthermore, if the density of the lightest nozzle changes over time, it is preferable to change the primary color.

[0073] Specifically, the image processing unit 106 outputs a solid output chart 800 shown in FIG. 8A at each dot size using the image forming device 12. The solid output chart 800 shown in FIG. 8A is composed of solid patches 801-804 and non-ejecting nozzle detection patterns 805-808 corresponding to each ink color (CM, M, Y, K). The solid patch 801 is a solid patch corresponding to K ink, and large-sized dots are formed in the entire area of ​​a grid of output resolution (e.g., 1200 dpi x 1200 dpi) using K ink alone. Reference numeral 805 denotes a non-ejecting nozzle detection pattern for K ink. Similarly, reference numerals 802-804 denote solid patches corresponding to C, M, Y inks, respectively, and reference numerals 806-808 denote non-ejecting nozzle detection patterns corresponding to C, M, Y inks. Furthermore, the image processing unit 106 determines a primary color for each ink color from the solid output chart captured by the image sensor 108. That is, if the ink colors are the four colors CMYK, the four corresponding primary colors are determined.

[0074] Specifically, the image processing unit 106 averages the image areas corresponding to the solid output patches 801-804 in the image data obtained from the image sensor 108 in the conveyance direction to convert them into one-dimensional data, and determines the color closest to the paper color among the obtained one-dimensional data as the primary color. At this time, by first converting the scanned image of the solid output chart 800 using a scan color conversion LUT before converting it into one-dimensional data, the primary color can be determined as a device-independent value. Here, the scan color conversion LUT is an LUT that associates the device values ​​of the image sensor with device-independent color characteristics (Lab values, XYZ values, density values). For example, the scan color conversion LUT can be generated by scanning a calibration chart whose color characteristics are known in advance with the image sensor 108 and associating values ​​on the scanned image.

[0075] Considering that the thinnest nozzle can be corrected by the surrounding nozzles using HS processing, it is also possible to select the color closest to paper white as the primary color after filtering using a filter equivalent to visual function (VTF). It is anticipated that if a non-ejecting nozzle is included in the primary color detection, the primary color will be extremely light. Therefore, in this embodiment, prior to determining the primary color, non-ejection detection processing is performed based on non-ejection detection patterns 805-808 for each ink color (head). If a non-ejecting nozzle is detected as a result, the maintenance unit 109 performs a recovery operation, and then the solid output chart 800 is output again. In this way, by repeatedly performing non-ejection detection and recovery operations from the chart output, a solid output chart that does not include non-ejecting nozzles can be obtained.

[0076] Returning to the explanation of Figure 7, in the next step S702, the image processing unit 106 creates an HS table 311 to be used by the HS processing unit 303. Specifically, the image processing unit 106 first outputs a nozzle-by-nozzle characteristic acquisition chart 810 shown in Figure 8(b). The nozzle-by-nozzle characteristic acquisition chart 810 shown in Figure 8(b) is made up of a gradation patch portion 811 and a non-discharge nozzle detection pattern 812. Note that the chart 810 may also include markers that make it easier to align each nozzle, as will be described separately below.

[0077] The gradation patch section 811 is made up of uniform patches with nine different input values. For example, the input values ​​can be values ​​(0, 32, 64..., 224, 255) obtained by dividing the range (0 to 255) evenly into nine parts. In this embodiment, the nozzle characteristic acquisition chart 820 shown in FIG. 8(b) is output for each ink color (CMYK), and by analyzing each of them, the characteristics of the number of ink colors x the number of nozzles are obtained.

[0078] Specifically, dot size conversion, quantization, and dot size synthesis are performed on the nozzle characteristic acquisition chart 810, and the result is formed as an image on a recording medium by the image forming device 12. Furthermore, each patch area of ​​the gradation patch section 811 is extracted from the scanned image of the formed image, and each is averaged in the transport direction to make it one-dimensional. By associating the one-dimensional data obtained in this way with the position of each nozzle, the characteristics of each nozzle relative to the input value can be obtained.

[0079] Furthermore, the image processing unit 106 generates an HS table 311 based on the acquired nozzle characteristics so that all nozzles have uniform characteristics for inputs of 0 to 255. Details will be described later. In this embodiment, an HS table 311 is created for each nozzle. These tables are also saved in RAM 101 and used in subsequent HS processing. If a non-discharge nozzle occurs when outputting the nozzle characteristic acquisition chart 810, the HS processing and non-discharge interpolation processing will overlap in the area corresponding to the non-discharge nozzle and its surrounding nozzles. As a result, excessive correction will occur in the overlapping area, which may result in black streaks and uneven density.

[0080] Therefore, in this embodiment, prior to creating the HS table 311, non-discharge detection processing is performed based on the non-discharge detection pattern 812. Furthermore, if a non-discharge nozzle is detected as a result, the maintenance unit 109 performs recovery operations, and then the chart 810 is output again. At this time, by repeating non-discharge detection and recovery operations from the chart output, an HS table that does not include non-discharge nozzles can be obtained, and overlapping of the HS processing and the non-discharge interpolation processing can be avoided. Note that a correction value for each nozzle may be calculated after performing filter processing on the one-dimensional data described above. For example, if a filter corresponding to the visual characteristic VTF is used as the filter, it is possible to prioritize correction of streaks in frequency bands that are easily visible.

[0081] Returning to the explanation of Figure 7, in S703, the image processing unit 106 recalculates the ink conversion table 310, taking into account the changes to the primary colors and the HS table 311. Note that the ink color conversion table can be created using a known method. For example, the ink color conversion table can be created by substituting black using a known UCR (Under Color Removal) process from the CMY values ​​calculated using C=255-R, M=255-G, and Y=255-B. The created table is saved in the RAM 101 as an ink conversion LUT, and is used in subsequent ink color conversion processes.

[0082] Next, in step S704, the image processing unit 106 creates an input color conversion table. For example, patches are output by dividing the color solid in the RGB space evenly, and an LUT is created by associating R'G'B' that realizes Lab, which is the target value of the RGB. The created LUT is saved in the RAM 101 as the input color conversion LUT 309 and is used for subsequent input color conversion processing.

[0083] It is preferable to regenerate the ink conversion table 310 and input color conversion table 309 in response to changes, particularly if the primary colors change in S701 due to a change in printing medium or changes over time, because this changes the color gamut that can be reproduced by the image forming system. Also, when calculating each color conversion table based on image quality items such as graininess, sharpness, and gloss, the balance between these often changes due to HS processing. For this reason, from the perspective of image quality, it is preferable to change each color conversion table 309, 310 when changing the HS table 311. Calibration of the various tables is completed through S701 to S704.

[0084] Next, in S705, the image processing unit 106 updates the output image data created in S507 using the various tables created in S701 to S704. That is, using the updated various tables by the static correction process, various image processes are performed by each of the units 301 to 307, new output image data is generated, and it is passed to the image forming apparatus 12. Further, the image forming apparatus 12 overwrites the received output image data to the RAM 114, and performs image formation with the overwritten data for the print instruction in subsequent S508.

[0085] By following the static correction process described above, after updating the various tables considering the change over time, new output image data is generated. As a result, in addition to the unevenness correction by the static correction process, the color development characteristics are also corrected. In addition to the update of the various tables described above, the threshold matrix 315 used in the quantization processing unit 306 may be changed considering the increase or decrease of dots by the HS process. Alternatively, after creating the HS table 311 in S702, the dot size conversion table 314 may be recreated based on the characteristics after HS (granularity, sharpness, streak unevenness, color shift, character reproducibility). In that case, the change in the dot size conversion table may affect the density unevenness. Therefore, it is preferable to execute S702 again using the newly generated dot size conversion table 314 and update the HS table 311 again.

[0086] <Generation of HS Table 311> Hereinafter, referring to FIGS. 9(a) and (b), the creation process of the HS table 311 in S702 will be described more specifically. The horizontal axis in FIG. 9(a) is the signal value of the gradation patch portion 811, and the vertical axis is the distance D from the recording medium color (paper white) on, for example, the Lab space. The distance D from the paper white can be calculated by the following formula (1). In formula (1), Lw, La, and Lb are the Lab values of the recording medium color, respectively.

[0087]

Equation

[0088] Next, calculation of the correction value will be described with reference to FIG. 9(b). First, a target value Dt corresponding to the input value In is calculated from the target characteristic 903. Furthermore, a signal value corresponding to the target value Dt is obtained from the nozzle characteristic 902 as a correction value In'. The obtained correction value In' is then associated with the input value In and stored in RAM 101 as the HS table 311 for the nozzle of interest. At this time, correction values ​​are calculated for all values ​​from 0 to 255 as the input value In and stored as a table for the nozzle of interest. Alternatively, only values ​​corresponding to nine gradations may be calculated and stored as a table. In that case, when using the correction table, values ​​other than the nine gradations may be calculated from these nine values ​​using known interpolation processing (for example, linear interpolation).

[0089] 9(a) and 9(b) plot only one nozzle characteristic 902 for the sake of simplicity, but in reality, a curve is obtained that only represents the characteristics of the number of ink colors multiplied by the number of nozzles. Therefore, by repeating the above process for all nozzles, the HS table 311 corresponding to all nozzles is calculated. The color space used to calculate the HS table 311 is arbitrary, and it is also possible to calculate the correction amount using block density, tristimulus values ​​XYZ, optical density, scanner RGB, etc., without using the distance D from paper white.

[0090] <Getting reference characteristics> Here, the acquisition of reference characteristics in S503 and S604 will be explained in more detail. In this embodiment, the reference characteristics are tristimulus values ​​XYZ values ​​for the signal value of each nozzle. Specifically, in acquiring the reference characteristics, first, the nozzle characteristic acquisition chart 810 shown in FIG. 8(b) is corrected using the HS table 311 generated in S702 and output. Then, a color conversion process is performed on the captured image to convert the device values ​​(RGB values) of the image sensor 108 into XYZ values. This process can be performed using a color conversion LUT calculated in advance. Furthermore, by cutting out each patch area from the captured image after color conversion and averaging them in the transport direction, one-dimensional XYZ data can be obtained for each input value.

[0091] By associating the one-dimensional XYZ data obtained in this way with the position of each nozzle, reference characteristics (tristimulus values ​​XYZ) for the input value of each nozzle can be obtained. At this time, a nozzle characteristic chart can be output for each ink color (CMYK), and the product of each can be used for the secondary and tertiary colors. Alternatively, reference characteristics for secondary and tertiary colors can be stored by outputting and capturing patches including the secondary and tertiary colors. Note that when calculating secondary and tertiary colors from the product of primary colors, it is preferable to use values ​​obtained by normalizing the tristimulus values ​​XYZ with the tristimulus values ​​of the recording medium (paper white) as the reference characteristics.

[0092] FIG. 19(a) shows an example of the reference characteristics. As shown in the figure, in this embodiment, the reference characteristics are stored as an LUT that stores tristimulus values ​​XYZ for input RGB values ​​at regular intervals. Note that the table shown in FIG. 19(a) may be stored for each nozzle, or the characteristics of all nozzles may be averaged and one table may be stored for all nozzles. Alternatively, it is possible to use the target characteristics of static correction indicated by reference numeral 903 in FIG. 9 as the reference characteristics without outputting a characteristic chart for each nozzle. Furthermore, Lab values ​​or block densities may be stored as the reference characteristics instead of XYZ values ​​as color characteristics. Alternatively, color characteristics for each number of dots after HT (Halftone) processing may be stored instead of input signal values.

[0093] <Dynamic correction processing flow> The flow of the dynamic correction process in S606 described above is shown in Fig. 10. As described above, the dynamic correction process in this embodiment achieves faster unevenness correction than static correction by performing correction according to a predetermined correction amount without outputting a chart. The dynamic correction process will be described in more detail below according to the flow shown in Fig. 10.

[0094] First, in S1001, the image processing unit 106 acquires the nozzle characteristics of all nozzles calculated in S509. Specifically, in this embodiment, average XYZ values ​​are acquired for each nozzle from the scanned image. This will be explained in more detail using FIG. 11(a). Reference numeral 1100 in FIG. 11(a) is an example of input image data input in S506. At this time, tristimulus values ​​XYZ are obtained for each pixel by using the above-mentioned scan color conversion LUT for the scanned image of the input image 1100. Furthermore, the obtained tristimulus values ​​XYZ are averaged in the transport direction (y direction) and associated with the nozzle position, thereby calculating the average XYZ values ​​for each nozzle for the input image.

[0095] Curve 1101 in Figure 11(b) shows an example of the average Y value for each nozzle calculated in this way. Note that the horizontal axis of Figure 11(b) represents the nozzle number (or nozzle position), and the vertical axis represents the Y value calculated from the scanned image.

[0096] 10, in S1002, the image processing unit 106 acquires an average color signal value for each nozzle. Specifically, first, in S507 or S705, the image processing unit 106 acquires the color signal data corresponding to each ink color that was saved in the RAM 101 or the external storage device 105. Then, by averaging the acquired color signal data in the transport direction (y direction), the average color signal data for each nozzle for the input image can be calculated.

[0097] Reference numeral 1102 in Fig. 11(c) is an example of averaging the K signal of the color signal image data 312 corresponding to the input image data 1100 shown in Fig. 11(a). Note that the horizontal axis in Fig. 11(c) represents the nozzle number, and the vertical axis represents the average K signal value calculated from the K signal image. Returning to the flowchart in Fig. 10, next, in S1003, the image processing unit 106 calculates target characteristics for each nozzle position for the input image 1100 based on the reference characteristics for each nozzle acquired in S503 or S604. Specifically, the average K signal value indicated by reference numeral 1102 in Fig. 11(c) is converted into XYZ values ​​for each nozzle using the reference characteristics. Similarly, the CMY signal values ​​are converted into XYZ values, and the target XYZ values ​​for each nozzle are obtained by calculating their product.

[0098] Instead of averaging the K signal values ​​to make them linear, it is also possible to convert the K signal values ​​for each pixel of the color signal image into XYZ values ​​and then make the XYZ values ​​linear. Alternatively, by retaining the XYZ values ​​for the on / off states of dots for each pixel in the output image after HT processing, it is also possible to obtain target XYZ values ​​for each nozzle from the output image. Curve 1103 in Figure 11(d) shows an example of the target Y value for each nozzle calculated in this way. Note that the horizontal axis of Figure 11(d) represents the nozzle number, and the vertical axis represents the calculated target Y value.

[0099] Next, in S1004, the image processing unit 106 determines the nozzle position to be subjected to dynamic correction. Specifically, the current nozzle characteristics (reference numeral 1101 in FIG. 11(b)) acquired in S1001 are compared with the target characteristics for each nozzle calculated in S1003. Specifically, both characteristics are converted into Lab values, and the nozzle position where the absolute value of the color difference ΔE between the two is equal to or greater than a predetermined threshold value is set as the target for correction. Furthermore, the ink color to be subjected to correction is determined at the determined nozzle position. These processes will be described in detail later.

[0100] Next, in S1005, the image processing unit 106 (color signal modification unit 308) acquires a correction value for each nozzle identified in S1004. Specifically, the magnitude of the current characteristics and the reference characteristics is acquired for each nozzle to be corrected, and a correction coefficient set for each is acquired. For example, the correction coefficient is set to 0.99 for a nozzle whose acquired value is larger than the reference. On the other hand, the correction coefficient is set to 1.01 for a nozzle whose acquired value is smaller than the reference. Details will be described later.

[0101] Next, in S1006, the image processing unit 106 (color signal modification unit 308) reads out pixel values ​​of the row region formed by the nozzles to be corrected from the color signal image stored in S508 or S707. Furthermore, the signal values ​​of the read image are multiplied by the correction coefficients of the corresponding nozzles to obtain corrected signal values. Next, in S1007, the image processing unit 106 operates the discharge failure complement processing unit 304, the dot size conversion processing unit 305, the quantization processing unit 306, and the dot size synthesis processing unit 307 to generate new output image data from the changed color signal image data 312. Finally, in S1008, the image processing unit 106 passes the new output image data to the image forming apparatus 12, thereby completing the dynamic correction process. The image forming apparatus 12 overwrites the corrected image data in the RAM 114 and performs image formation based on the overwritten image data in response to the print instruction issued in S508.

[0102] In this way, dynamic correction allows for high-speed correction while continuing to print the user image or maintaining a printable state by directly correcting the color signal image without outputting a correction chart. While the explanation in S1002 above describes averaging the entire input image in the y direction to make it one-dimensional, the input image may be divided into small regions and processed for each small region. In this case, the nozzle characteristics (large, small, or approximately the same) are calculated for each region, and if a certain number of nozzles are judged to be large or small, the nozzle is selected as the correction target. Furthermore, dynamic correction may involve updating part of the nozzle characteristic information without outputting a correction chart.

[0103] The correction value may also be changed according to the difference between the current nozzle characteristics and the reference characteristics. Specifically, the correction amount may be increased as the characteristics of a dark nozzle become darker than the reference characteristics. Alternatively, the correction amount may be increased as the characteristics of a dark nozzle become lighter. Alternatively, the correction value may be changed according to the signal value of the small dot image. Specifically, because density unevenness is more visible in midtones, the correction amount in highlight and shadow areas may be reduced compared to the correction amount in midtone areas. Furthermore, only specific ink colors out of multiple ink colors may be subject to dynamic correction. For example, of the CMYK inks, only the K ink, which is expected to contribute significantly to density unevenness, may be subject to dynamic correction processing. In this case, the correction color specification process described below is not necessary.

[0104] <Details of correction nozzle identification process> As described above, in S1004, the color signal to be corrected is identified for the nozzle position that is the target of dynamic correction. In this embodiment, the ink color to be corrected is determined from the Lab values ​​of the area corresponding to the nozzle position xt to be corrected and the Lab values ​​of the reference characteristics. Specifically, the difference vector between both characteristics in Lab space is projected onto a line that passes through each ink color and the paper white. Then, the nozzle number xt of the ink color that has the largest projection vector is selected as the correction target. The identification of the ink color to be corrected in S1004 will be described in more detail with reference to FIGS. 12(a) to 12(d).

[0105] Figure 12(a) is a plot in Lab space of the color of the recording medium (paper white) and the average color development characteristics of all nozzles when each ink (CMYK) is solidly output to the recording medium. That is, each axis in Figure 12(a) represents L*, a*, and b*, respectively.

[0106] Furthermore, points 1200 to 1204 in FIG. 12(a) represent the color development characteristics of paper white (point 1200), C ink (point 1201), M ink (point 1202), Y ink (point 1203), and K ink (point 1204), respectively. In this case, three-dimensional vectors from paper white to the color development characteristics of each ink can be defined in Lab space. Note that, although actual processing is defined and executed in Lab space as shown in FIG. 12(a), for simplicity of explanation, the following explanation will be given on a two-dimensional plane.

[0107] FIG. 12(b) is a view of FIG. 12(a) from the L* axis direction. Note that points 1200 to 1204 indicate the color development characteristics of paper white, C ink, M ink, Y ink, and K ink, respectively. In this case, four straight lines connecting paper white and the points corresponding to each ink color can be defined as dotted lines 1205 to 1208 in FIG. 12(c). For example, dotted line 1205 is a straight line connecting C ink and paper white.

[0108] Each of these lines represents the color development tendency of the corresponding ink. More specifically, when the corresponding ink is increased or decreased, it is expected that the line will move approximately parallel to the corresponding line. Therefore, the ink to be corrected is determined by comparing these color development tendencies with the tendency of deviation from the reference characteristics.

[0109] The point indicated by reference number 1209 in Figure 12(d) is a point on which the target characteristics of the nozzle of interest are plotted. Similarly, reference number 1210 is a point on which the color development characteristics of the current nozzle xt are plotted. Note that the distance D from the paper white mentioned above is the length of the vector directed from point 1200, which indicates the paper white, to points 1209 and 1210 in Figure 12(d).

[0110] Reference numeral 1211 in Figure 12(d) is a vector pointing from point 1209, which indicates the reference characteristics, to point 1210, which indicates the current characteristics, and is a differential vector that indicates the color difference and its tendency taking into account the visual characteristics of both. Then, by calculating the magnitude of the vector obtained by projecting this differential vector onto the straight line that indicates the color development tendency of each ink color, as shown in Figure 12(c), the ink color that is suitable for correcting the difference can be identified. Specifically, the magnitudes of the projected vectors are compared, and the ink that yields the largest projected vector is selected as the ink to be corrected.

[0111] Specifically, if the difference vector indicated by reference numeral 1211 in FIG. 12(d) is projected onto the straight line indicating the color development tendency of cyan ink, indicated by reference numeral 1205 in FIG. 12(c), and a larger projection vector is obtained than the other straight lines, then cyan nozzle number xt is identified as the nozzle to be corrected. Note that these nozzle identification processes are performed not on the ab plane but in the three-dimensional space of Lab, as described above. Furthermore, for ink colors that are not ejecting dots at nozzle number xt, i.e., ink colors for which the color signal value corresponding to nozzle number xt is 0, it is preferable not to obtain the straight line 1207 and its projection vector and not to identify the ink as the ink to be corrected.

[0112] As mentioned above, the difference vector is a vector that represents the difference between the reference characteristic and the current characteristic, and by calculating its magnitude, the amount of deviation from the reference characteristic can be predicted. Therefore, in the above-mentioned S513, the inside or outside of the allowable range may be determined based on the magnitude of this difference vector, rather than the distance D from the paper white. Also, a threshold value for the magnitude of the projection vector may be stored in advance, and the magnitude of each nozzle of each ink may be determined relative to this threshold. In this case, nozzles of ink colors corresponding to vectors larger than the threshold may be collectively targeted for correction. Also, this process may be used to determine whether the vector is inside or outside the allowable range in S512. <Modifications regarding correction timing> Among the correction processes described above, several variations on the timing of correction are shown below. In the explanation using FIG. 5 above, it was explained that the determination of whether unevenness is tolerable (S510), correction (S511), and detection and compensation of discharge failures (S512) are performed each time an image is output. However, there are cases where it is not possible to scan and analyze an image each time an image is output due to processing speed restrictions. In such cases, each of these may be performed at different times, taking into consideration the impact on quality and the processing load if a problem occurs.

[0113] Furthermore, in the explanation using Figure 6, the static correction process (S603) is executed based only on the results of the dynamic correction process (S602), but it may also be executed taking into account, for example, the number of printed sheets, the elapsed time, and the operating time since the previous static correction. For example, it may be determined whether or not to execute non-discharge detection, dynamic correction, and static correction after 10 sheets, 100 sheets, or 10,000 sheets are output, respectively. Alternatively, they may be executed without fail after each of the above outputs. Alternatively, it may be determined and executed based on different indices, such as 10 sheets, 5 minutes, or 1 day.

[0114] Alternatively, dynamic correction may always be performed without determining whether the value is outside the allowable range in S511. In this case, instead of S601 and S602, a determination as to whether the value is outside the allowable range may be performed, and static correction may be performed if the value is outside the allowable range.

[0115] Furthermore, the image processing device 11 and the image forming device 12 can operate independently of each other. For example, the job acquired in S506 may be loaded into the RAM 114 in the image forming device 12, and the image forming unit 107 may output the job as needed according to the job content. In this case, the image processing device performs dynamic correction in response to the transmission of the scanned image by the image sensor 108 via the I / Fs 111 and 110. The image forming device 12 may replace the image as soon as a new output image is received via the I / Fs 110 and 111 without stopping printing. In this case, the dynamic correction may be applied, for example, five or ten sheets after the scanned image.

[0116] 5, if a user issues a shutdown command during any step, the image forming system operates until it enters a print job standby state in S505, stores various tables in the external storage device 105, and then shuts down. At this time, it is preferable to check whether a flag is on, and if it is on, include a static correction process and a process to turn the flag off in the shutdown process. By checking the flag in this way when shutting down the system, it is possible to skip the processes (S501 to S504) that occur after power is turned on, which increases the likelihood of further reducing downtime for the user.

[0117] Furthermore, the user may be able to set the timing for executing dynamic correction and static correction. That is, the user may be able to set static correction to be executed only when the power is turned off. Alternatively, if the standby state in S505 continues for a certain period of time, for example, 30 minutes or more, the flag may be turned off and static correction may be executed. Alternatively, dynamic correction and static correction may be suppressed while printing the same image or the same job. For example, if the quality of the image forming system is gradually deteriorating and a long time has passed since the last correction, executing correction processing may result in larger differences between output images, making unevenness more noticeable. In such cases, it is preferable to suppress execution of correction processing while printing the same job or the same image. For example, correction may be prohibited while printing the same job or the same image in accordance with a user instruction. Alternatively, the threshold for determining whether correction is possible may be changed to make it more difficult to execute correction processing.

[0118] <Modifications regarding the correction target> Among the correction processes performed by the image processing unit 106, several variations of the correction target are described below. In the above explanation, dynamic correction processes are performed on color signal image data 312, achieving high-speed correction. However, if dynamic correction is performed on quantized output image data, the dot size conversion process and quantization process in S1006 become unnecessary, enabling faster processing. For example, the image forming device 12 stores multiple drive signals for small dots, and dynamic correction switches between them to perform correction on quantized output image data. More specifically, the output image for small dots is a 4-bit (0 to 15) image rather than a 1-bit (0 or 1) image for each pixel. Meanwhile, the print head forms dots using ejection signals corresponding to 0 to 15, but the signals are designed in advance so that the larger the value, the greater the ejection volume.

[0119] In static correction, correction processing is performed using the median value of the possible ejection signals (for example, 4). Then, in dynamic correction processing, the output image is read, and the signal value of that column is uniformly changed depending on whether the column is darker or lighter than the reference characteristic. For example, if the column is dark, the pixel value is uniformly changed by -1. Instead of a uniform change, a probabilistic change based on the difference from the reference characteristic may also be performed. However, when performing probabilistic processing on binary data, the resulting pattern often has worse dispersity than the pattern obtained by correcting the data before quantization. Therefore, when considering the adverse effect of worsening graininess due to correction processing, it is preferable to correct the multi-value data before quantization.

[0120] In the configuration diagram shown in Figure 3, dot size conversion processing is performed after HS processing. However, a configuration in which dot decomposition occurs before HS processing is also possible. In other words, an image corresponding to the dot size may be generated using a one-dimensional LUT for each color signal data after ink color (CMYK) conversion. For example, if there are three types of dot sizes, a total of 12 pieces of color signal image data are generated, consisting of four ink colors and three dot sizes.

[0121] In this case, the HS processing unit only needs to perform correction processing on each color signal data (12 types in the above example).

[0122] Alternatively, multiple HT matrices can be prepared for each dot size, and the quantization processing unit 306 can perform dot size conversion. In this case, the HS processing unit 304 performs HS processing on each image for each dot size type (for example, large, medium, and small). In the above explanation, the determination of whether dynamic correction is necessary and the correction processing are performed for each nozzle, but this can also be performed across multiple nozzles. For example, the nozzle tx to be corrected may be corrected to 1.01 times, and its adjacent nozzles may be corrected to 1.005 times.

[0123] <Modification of the third correction process> In this embodiment, the two correction methods, dynamic correction and static correction, are used interchangeably. However, it is also possible to store three or more correction methods. For example, in addition to the dynamic correction and static correction, it is also possible to store a third correction method corresponding to unevenness correction C. For example, in the third correction method, only one correction chart such as that shown in FIG. 13(a) is output, and correction processing is performed based on the scanned image.

[0124] The chart 1300 shown in FIG. 13(a) is composed of uniform patches 1301-1304 of halftones (input value 128) of each ink color (CMYK). For example, uniform patch 1301 is a uniform patch of C ink. In this case, in the third correction process, areas corresponding to each uniform patch 1301-1304 are extracted from the scanned image of the chart 1300 described above, converted into tristimulus values ​​XYZ, and then averaged in the transport direction (y-axis direction) to make it one-dimensional. The obtained one-dimensional data is then compared with reference characteristics, and a correction value for each nozzle is determined based on the magnitude of the comparison. In this way, by using uniform patches, it is possible to obtain nozzle characteristics with higher accuracy than those obtained from a user image.

[0125] In the third correction process, correction may be performed without using the reference characteristic, with the aim of making the patches 1301 to 1304 uniform across the surface. In this case, the one-dimensional data obtained from each uniform patch is further averaged with respect to the nozzle direction and the tristimulus values ​​X, Y, and Z to obtain a single average value as the target value. The obtained average value is then compared with the average value of the tristimulus values ​​corresponding to each nozzle, and a correction value for each nozzle is determined based on the magnitude of the comparison.

[0126] Alternatively, the signal value for each nozzle may be multiplied by a correction value obtained by dividing the average tristimulus value for each nozzle by a target value. Furthermore, if uniform patches fit on one sheet, uniform patches of multiple different gradations for each ink color may be included. Alternatively, these values ​​may be changed each time the image is output. For example, steps S1001 to S1004 are performed in the same manner as the dynamic correction described above until the nozzle to be corrected is determined. A chart that outputs correction values ​​to areas requiring correction may then be generated and output, enabling efficient correction even with a single chart. For example, Figure 13(b) shows a chart 1310 generated when the Nth nozzle of K is the nozzle to be corrected. Note that reference numeral 1311 in Figure 13(b) schematically represents the nozzle row corresponding to the K ink. In this case, the chart 1310 is composed of a group of uniform patches 1312 with different gradations for rows corresponding to K nozzles n-10 to n+10. In this case, the patch group 1312 is formed solely in the K ink color.

[0127] 13(c) shows a chart 1320 that is generated when the nozzles to be corrected are K nozzle number n and C nozzle number m. Note that n and m in chart 1320 are examples of charts that are generated when they are not present within the 10 adjacent nozzles. Note that reference numerals 1321 and 1322 in FIG. 13(c) schematically represent the nozzle rows corresponding to K ink and C ink, respectively.

[0128] In this case, the chart 1320 is composed of a patch group 1323, which is uniform in different gradations for the columns corresponding to K numbers n-10 to n+10, and a patch group 1324, which is uniform in different gradations for the columns corresponding to C numbers m-10 to m+10. In this case, the patch group 1323 is formed with K ink alone. The patch group 1324 is formed with C ink alone. In this case, the patch groups 1323 and 1324 may each be composed of patches with different signal values; for example, it is preferable to determine the signal values ​​based on the color development characteristics of K ink and C ink. Alternatively, signal values ​​with large differences or residual errors due to correction may be output with emphasis.

[0129] Chart 1330 in FIG. 13(d) is an example of a chart generated when K nozzle number n and C nozzle number m' are correction target nozzles, and n and m' are among the 10 adjacent nozzles. Reference numerals 1331 and 1332 in FIG. 13(d) schematically represent nozzle rows corresponding to K ink and C ink, respectively. In this case, chart 1330 is composed of uniform patches 1333 with different gradations for rows corresponding to K nozzle numbers n-10 to n+10, and uniform patches 1312 with different gradations for rows corresponding to C nozzle numbers m'-10 to m'+10. Patch 1323 is formed solely with K ink. Patch 1324 is formed solely with C ink. Patch 1334 in FIG. 13(d) corresponds to a signal value of 0. In the example shown in the figure, the patch set 1334 does not include a patch corresponding to a signal value of 0, since it can be used in common for the nth ink of K ink and the m′th ink of C ink.

[0130] The third correction process described above is performed when it is determined in S602 in FIG. 6 that the condition has worsened. Furthermore, the results of the third correction process are also evaluated, and if the condition has worsened, static correction is performed. Alternatively, static correction may be performed only between jobs or at shutdown. Alternatively, the dynamic correction described in FIG. 10 above may not be performed, and the third correction process may be the dynamic correction shown in S606 in FIG. 6.

[0131] Alternatively, the third correction means may change the HS table 311 instead of the color signal image data 312. Alternatively, a configuration may be adopted in which both the image data 312 and the HS table 311 can be changed, and the color signal image data may be changed if the number of nozzles to be corrected is less than a predetermined number, and the HS table may be changed if the number of nozzles to be corrected is greater than a predetermined number.

[0132] In the third correction process, instead of using the chart 1300, a gradation chart 1340 may be used as shown in FIG. 13(e). The chart 1340 is composed of gradation patches 1341-1344 for the input value range (e.g., 0-255) of each ink color (CMYK). For example, patch 1341 is a gradation patch for C ink. When using the gradation chart 1340, averaging in the transport direction (y direction) is not performed, and the scanned image is directly converted from scanner RGB to Lab, and then associated with the nozzle position. In this case, one-dimensional data in the y direction for each x position indicates the nozzle characteristics for the nozzle position x. In other words, the nozzle characteristics (distance D from the paper white relative to the signal value) shown in 902 in FIG. 9(a) can be obtained from each of the gradation patches 1301-1304. In this case, in the third correction process, the correction amount for each nozzle is calculated based on the nozzle characteristics obtained from the gradation patch, and the color signal image data 312 is corrected.

[0133] In the examples shown in Figures 13(a) to (e), only single colors are shown, but if density unevenness occurs in mixed colors such as blue (C+M) or red (M+Y), patches of secondary colors may also be output.

[0134] <Modification of Stepwise Static Correction> In the flowchart shown in Fig. 6, if the answer is Yes in S602, static correction is performed according to the flowchart shown in Fig. 7. However, it is also possible to perform the static correction process in stages. Instead of performing all of the table creation steps in S701 to S704, it is possible to first perform only S701 and S702, and then determine whether or not to perform S703 and S704 depending on the results.

[0135] [Second embodiment] In the first embodiment described above, an example was shown in which reference characteristics were created immediately after static correction processing. Furthermore, it was explained that a single reference characteristic was used to determine whether correction was necessary, regardless of the output image. However, depending on the characteristics of the image sensor 108 and the image specified by the user, the reference characteristics predicted from the input values ​​may not match well with the nozzle characteristics predicted from the device values ​​of the scanned image. This is because the charts used to obtain the most recent nozzle characteristics and reference characteristics are different. For example, if an image is formed mostly of blue (the secondary color of C and M), emphasis is placed on the quality of printing blue. Alternatively, if an image is formed mostly of lines, such as a text image or a design drawing, emphasis is placed on the quality of the lines.

[0136] Therefore, in the present embodiment described below, reference characteristics are obtained from the output image immediately after static correction processing for each output image, thereby matching the latest nozzle characteristics with the image from which the reference characteristics are obtained. As a result, it is possible to determine the need for correction with higher precision according to the output image. FIG. 14 is a diagram showing an example of the processing flow in the image processing unit 106 in this embodiment. The processing flow in this second embodiment will be described below with reference to FIG. 14.

[0137] First, in S1401, the image processing unit 106 acquires a print job input by the user, similar to S505 described above. Next, in S1402, the image processing unit 106 executes various image processes to obtain output image data, similar to S506 described above. Next, in S1403, the image processing unit 106 instructs the image forming apparatus 12 to print one output image I on a recording medium in accordance with the print job stored in RAM 101, and also instructs the image sensor 108 to read the image. Thereafter, the number of sheets to be output for the print job stored in RAM 101 is decreased by one. Alternatively, if the number of sheets already output is stored in RAM 101 separately from the print job, the number can be increased by one.

[0138] Next, in S1404, the image processing unit 106 references the number of sheets to be printed or the number of sheets already output in the RAM 101 and determines whether the print job acquired in S1401 has been completed. If the image processing unit 106 determines that the job is incomplete, the process proceeds to S1405. In S1405, the image processing unit 106 performs correction processing, including determining whether correction processing is possible. Details of S1405 will be described later. On the other hand, if the image processing unit 106 determines in S1404 that the job has been completed, the process proceeds to S1406.

[0139] In S1406, the image processing unit 106 acquires the quality Q of the image formed by the image forming system 1. Specifically, as the quality Q(t), a comprehensive evaluation value is used that takes into consideration density unevenness and streaks as well as graininess, color shift, sharpness, character reproducibility, etc. In this case, each evaluation value may be acquired using a known method, and the quality Q may be calculated by, for example, weighting and adding up each evaluation value.

[0140] Next, in S1407, the image processing unit 106 compares the quality Q with a predetermined threshold Q0. Specifically, if the calculated current quality Q is equal to or less than the threshold Q0 (Q≦Q0), the image processing unit 106 determines that the current quality Q is outside the allowable range. In this case, the image processing unit 106 proceeds to S1408 and performs static correction processing. Note that the static correction processing may be performed in the same manner as in S502 and S603 described above, and therefore a detailed description thereof will be omitted.

[0141] On the other hand, if the image processing unit 106 determines in S1407 that Q>Q0 (is within the allowable range), or if static correction is performed in S1408, this process ends and the system enters a standby state until the next print job is input. Note that the above flow may be configured to always perform static correction without including S1406 and S1407. Alternatively, the execution of static correction may be determined in S1406 based on the time elapsed since static correction was performed or the number of printed sheets, rather than on the quality Q.

[0142] <Correction process flow> The processing flow of the correction process (S1405) in the second embodiment is shown in Fig. 15. The processing in each step will be described below with reference to Fig. 15.

[0143] First, in S1501, the image processing unit 106 performs discharge failure detection and correction based on the scanned image by the discharge failure complement processing unit 307. The processing in this step can be performed in the same manner as in S512 described above, and therefore details will be omitted here.

[0144] Next, in S1502, the image processing unit 106 compares the remaining number of sheets P planned to be output with a predetermined threshold th1. If P>th1, the image processing unit 106 proceeds to S1503. On the other hand, if P≦th1, the image processing unit 106 skips the remaining steps S1503 to S1511 and terminates this correction process. At this time, the remaining number of sheets P to be output can be obtained by referencing the number of sheets planned to be output and the number of sheets already output for the print job stored in RAM 101. Furthermore, th1 can be determined by using the time required for dynamic unevenness correction or the number of sheets output by the image forming apparatus 12 until the correction is applied after execution. For example, if approximately five sheets are output after scanning the formed image before the dynamic correction is applied, th1=5 can be set. If the correction process is not expected to be applied to the current job based on the remaining number of sheets to be printed, the correction process can be omitted, thereby reducing downtime and calculation load. Note that th1 may be set by the user operating the operation unit 103.

[0145] In S1503, the image processing unit 106 determines whether or not reference characteristics for image I have been created. If they have not been created, the image processing unit 106 proceeds to S1510. Then, in S1510, the image processing unit determines whether or not the image scanned in S1403 includes any uncomplemented discharge failures. Specifically, the image processing unit 106 determines whether or not the detection of discharge failure nozzles in S1501 and the discharge failure complementation process have been executed.

[0146] If there are uncompensated ejection failures, that is, if ejection failure complementation processing has been executed in S1501, the reference characteristics will include the effects of the ejection failures. For this reason, the image processing unit 106 does not create reference characteristics from the current scanned image, and ends the correction processing as is. On the other hand, if there is no uncompensated ejection failure, the image processing unit 106 advances the process to S1511 and creates a reference characteristic.

[0147] In this embodiment, the Lab values ​​for each pixel are used as the reference characteristics. Specifically, in S1511, the image processing unit 106 performs color conversion processing for each pixel on the image obtained by scanning in S1403, converting the device values ​​(RGB values) of the image sensor into Lab values. The two-dimensional Lab values ​​thus obtained are set as reference characteristics L0(x, y), a0(x, y), and b0(x, y) for image I, where x and y respectively indicate positions on the scanned image. Furthermore, in this embodiment, it is assumed that the scan resolution and the nozzle resolution are the same, and x is equal to the nozzle number.

[0148] After the above reference characteristics are created, the determination in S1503 in the flow shown in FIG. 14 indicates that the reference characteristics are complete, and the process proceeds to S1504. In S1504, the image processing unit 106 acquires the nozzle characteristics for the current image I. In this embodiment, the image processing unit 106 also acquires the current nozzle characteristics as two-dimensional Lab values. Specifically, similar to S1511, the most recent scanned image is color converted to obtain the current nozzle characteristics L(x, y), a(x, y), b(x, y).

[0149] Next, in S1505, the image processing unit 106 determines whether the current nozzle characteristics L(x,y), a(x,y), and b(x,y) are outside the allowable range. Specifically, the image processing unit 106 calculates the color difference ΔE(x,y) for each pixel from the reference characteristics L0(x,y), a0(x,y), and b0(x,y) and the current nozzle characteristics L(x,y), a(x,y), and b(x,y). Then, for example, if a pixel where ΔE>3 is included is determined to be outside the allowable range. Alternatively, the number of pixels where ΔE>3 for each nozzle may be counted, and if a nozzle with a number equal to or greater than a predetermined number of pixels exists, it may be determined to be outside the allowable range. Furthermore, it is also possible to use a value ΔEave(x) obtained by averaging the color difference ΔE(x,y) in the y direction and determine that an x ​​where ΔEave(x)>3 exists is outside the allowable range.

[0150] It is also possible to make a judgment after filtering the current, reference, and tristimulus values ​​XYZ. For example, by processing with a filter equivalent to the visual characteristic VTF and then making a comparison, corrections will be made only when streaks or unevenness at frequencies that are easily noticeable to the eye occur, which increases the likelihood of reducing downtime for users. Also, the color space used for judgment can be XYZ values, optical density, block density, or scanner RGB values ​​instead of Lab values.

[0151] If the image processing unit 106 determines that the result of the above determination is within the allowable range, it determines that correction is not necessary and ends the correction process. On the other hand, if the image processing unit 106 determines that the result is outside the allowable range, it proceeds to step S1506.

[0152] In S1506, the image processing unit 106 again refers to the remaining number of sheets P to be output and compares it with a predetermined threshold value th2. If the image processing unit 106 determines that P≦th2, the process proceeds to S1509, where dynamic correction processing is performed. Details will be described later. On the other hand, if the image processing unit 106 determines that P>th2, the process proceeds to S1507. In S1507, the image processing unit 106 determines whether static correction processing is permitted. If the image processing unit 106 determines that static correction processing is permitted, the process proceeds to S1508, where static correction processing is performed. On the other hand, if the image processing unit 106 determines that static correction processing is not permitted, the process proceeds to S1509, where dynamic correction processing is performed.

[0153] Whether static correction is permitted or not may be acquired, for example, by being included in the print job acquired in S1401. Alternatively, in S1507, a user interface 2000 such as that shown in FIG. 20 may be displayed on the display unit 104, and the user may select via the operation unit 103. In the example shown in FIG. 20, the user may select via the operation unit 103 whether static correction is permitted (by pressing the execute button 2001 in FIG. 20) or not permitted (by pressing the cancel button 2002 in FIG. 20). At this time, it is desirable that the selection screen 2000 includes, as information for selection, the number of remaining prints (reference numeral 2004 in FIG. 20) and the estimated time required for correction (reference numeral 2003 in FIG. 20), as shown in FIG. 20, and presents them to the user. Alternatively, the remaining printing time may be displayed instead of the number of remaining prints 2004. It is also desirable that if the user does not press a button within a certain time, the static correction is automatically permitted or not permitted.

[0154] <Dynamic correction processing> The dynamic correction process in S1509 described above can be performed, for example, according to the flow shown in FIG. 10. More specifically, the nozzle position x that includes the pixel that is outside the tolerance in S1509 is set as the nozzle position xt to be corrected, the ink color to be corrected in S1004 described above is selected, and the value corresponding to that nozzle is changed based on the magnitude of the characteristic. Alternatively, the correction value can be calculated from the current characteristic obtained in S1504. That is, for each nozzle, the CMYK values ​​of each pixel on image I are associated with the Lab values ​​of each pixel obtained in S1504 to create a reverse lookup LUT. Then, the reference characteristics L0, b0, and a0 for each pixel are converted to CMYK values ​​using the reverse lookup LUT for the corresponding nozzle, and these are used as ink values ​​after dynamic correction, thereby creating color signal image data after dynamic correction.

[0155] <Modification> In the explanation using FIG. 15 above, the calculation of the reference characteristics (S1511) and the acquisition of the current characteristics (S1504) were described as being performed from a user image, but they can also be performed using a measurement chart. For example, in S1511 and S1504, the gradation chart 1300 shown in FIG. 13(e) is output at regular intervals (time or number of sheets), and the nozzle characteristics of each nozzle are calculated from patches 1301 to 1304 in the chart. In the calculation of the reference characteristics (S1511), the nozzle characteristics immediately after static correction can be stored in RAM 101 as reference characteristics. In S1503, the necessity of correction and the amount of correction are calculated based on the latest nozzle characteristics and the reference characteristics.

[0156] In this case, however, because outputting the measurement chart will result in wasted paper and downtime for the user, it is preferable to perform the determination in S1502 at appropriate intervals. For example, S1502 may be performed only when the remaining number of output sheets P is 100 or greater and is divisible by 100.

[0157] Using a measurement chart in this way makes it impossible to create reference characteristics based on the formed image, but it is more robust to variations in the user image, sensor reading accuracy, and nozzle position misalignment, and may even be able to acquire characteristics with high accuracy depending on the sensor's performance. Furthermore, while the determination in S1506 is based solely on the remaining number of prints (number of unprinted prints), it may also be based on the current nozzle characteristics or the color difference from the reference characteristics. For example, it may be based on the number of pixels outside the tolerance range. Alternatively, a different determination criterion may be used, such as whether pixels with ΔE > 5 are included. Alternatively, the determination may be based on whether the number of pixels outside the tolerance range has increased as a result of dynamic correction.

[0158] [Third embodiment] In the explanation so far, the recording head has been considered as a single head extending in the direction parallel to the nozzle array (x direction) as shown in FIG. 2. However, a method of combining multiple recording heads to cover the entire area of ​​the printing paper is often adopted. For example, in the example shown in FIG. 16(a), a recording medium 1600 is covered by combining three recording heads (1601 to 1603). In this case, the recording medium 1600 is divided into areas 1604 to 1606 formed by the different recording heads 1601 to 1603.

[0159] Depending on the physical configuration of the printhead, uneven density patterns may appear due to differences in characteristics that depend on the position. For example, variations may occur between the center and edges of a chip, between chips, or between heads. When driving a head in block units, unevenness may occur due to the drive blocks. For example, when driving a head divided into 32 blocks, uneven density may occur at a 32-nozzle cycle within the printhead. Even between printheads, differences in installation accuracy and electrical characteristics may result in differences in the average output characteristics for the same signal. In such cases, differences between heads often shift overall while maintaining the characteristics within the head.

[0160] Figure 16(b) shows the characteristics for each nozzle obtained in such a case. The horizontal axis in Figure 16(b) is the nozzle number x, and the vertical axis is the distance D from the paper white of the image formed on the paper surface when the same signal value is applied to each nozzle.

[0161] In this case, characteristic 1607 in Figure 16(b) corresponds to region 1604. In the example shown in Figure 16(a), it is assumed that each head has 5,000 nozzles, and nozzle numbers x = 1 to 5,000 (print head 1601) correspond to region 1604. Similarly, characteristic 1608 corresponds to region 1605 (nozzles 5001 to 10,000, i.e., print head 1602), and characteristic 1609 corresponds to region 1606 (nozzles 10,001 to 15,001, i.e., print head 1602).

[0162] In this case, as shown in Figure 16(b), characteristics 1607 to 1609 can be broken down into deviations in the average value (unevenness between heads) and similar unevenness in each region (unevenness within the head). Note that unevenness within a head in particular is often determined by the nozzle arrangement on the plate that forms the nozzles, and there is often relatively little change over time. In addition, the cycle of the unevenness is relatively high frequency, so residual unevenness due to changes over time is not easily noticeable.

[0163] On the other hand, unevenness between heads is low-frequency unevenness and is easily noticeable. Furthermore, because it is likely to change over time and be affected by environmental changes such as humidity and temperature, it is preferable to correct it more frequently than unevenness within a head. Therefore, in the static correction of this embodiment, the average deviation and similar unevenness for each region are decomposed, and the amount of correction for each is calculated and applied. On the other hand, dynamic correction only corrects deviations from the average, thereby maintaining consistent quality with a lower load.

[0164] <Static correction processing> The static correction process in the third embodiment will be described below with reference to FIG. 17. First, in S1701, the image processing unit 106 causes the image forming device 12 to output a chart that can acquire the intra-head unevenness characteristics and inter-head unevenness characteristics, and acquires a scanned image. In this embodiment, the chart shown in FIG. 8(b) is output for each ink color (CMYK) as a chart that can acquire both characteristics. At this time, the formed image is acquired by the image sensor 108 and stored in RAM 101 as a scanned image. Note that it is preferable to detect non-discharge nozzles using the non-discharge detection pattern 812 and re-output until an output without any non-discharges is obtained.

[0165] Next, in S1702, the image processing unit 106 creates an in-head correction table. FIG. 19B is an example of the in-head correction table obtained in this step. Details of the table and how it is created will be described later. Next, in S1703, the image processing unit 106 acquires the target characteristics. In this embodiment, too, the straight line connecting the primary color Dp and the paper white is set as the target characteristics. An example of the target characteristics is shown by the dashed line 1807 in FIG. 18C.

[0166] Next, in S1704, the image processing unit 106 creates an inter-head correction table. FIG. 19(c) is an example of the inter-head correction table obtained in this step. Details of the table and how it is created will be described later. Next, in S1705, the image processing unit 106 creates output image data using the inter-head correction table and intra-head correction table created above. Specifically, first, input color conversion processing and ink color conversion processing are performed on the input image I to obtain color signal image data. Furthermore, the HS processing unit 303 applies the intra-head correction table shown in FIG. 19(b) and the inter-head correction table shown in FIG. 19(c) to each pixel of the obtained color signal image data 312 after ink color conversion, depending on the nozzle by which the pixel is formed.

[0167] For example, if the signal value of the pixel corresponding to nozzle 0 is 20, a correction value of "+1.70" is obtained using the intra-head correction table and interpolation calculation shown in Figure 19(b). In this case, the input signal value after intra-head correction can be calculated as 20 + 1.70 = 21.70. Furthermore, if the head that forms that pixel is the left head, the post-correction signal value can be calculated as 12.02 using the inter-head correction table and interpolation calculation shown in Figure 19(c). In this way, HS processing can be performed on the color signal image data by applying the two tables corresponding to the nozzle number for each ink color of all pixels.

[0168] Furthermore, by performing non-discharge complement processing, dot size conversion processing, quantization processing, and dot size synthesis processing on the color signal image data after HS, an output image corresponding to the input image I can be obtained. By following the steps S1701 to S1705 described above, the static processing is completed. Note that the color signal image data 312 after HS correction is saved in RAM and is subject to dynamic correction, which will be described later.

[0169] <Creating a head correction table> As described above, in S1705, the image processing unit 106 creates an intra-head correction table. As shown in FIG. 16(b), in this embodiment, the characteristics in each head region (0 to 4999, 5000 to 9999, 10000 to 14999) are approximately similar to each other. Therefore, the intra-head correction table is created as a table common to all three heads in order to correct the similar characteristics within each head. Specifically, first, the image processing unit 106 averages the region corresponding to patch 811 in the transport direction (y direction) of the scanned image of chart 810 obtained in S1701 to convert it into one-dimensional data. Furthermore, the image processing unit 106 divides the obtained one-dimensional data into sections corresponding to regions 1604, 1605, and 1606, and calculates the average value of these.

[0170] That is, for each patch 811, average color characteristics corresponding to the right head 1601, center head 1602, and left head 1603 are obtained. As a specific example, the average color characteristics of the center head 1601 are shown as curve 1802 in FIG. 18(a). Note that the horizontal axis in FIG. 18(a) is the signal value, and the vertical axis is the distance D from the paper white. Also, 1801 in the figure is the upper limit value of the horizontal axis, which is 255 if the input signal value is 8 bits. Furthermore, 1803 in FIG. 18(a) shows the color characteristics of the nozzle with nozzle number (relative number) 0 within the center head. In the head configuration shown in FIG. 16, relative nozzle number 0 of the center head corresponds to nozzle number 5000 (absolute number) across all three nozzles.

[0171] In this case, if the average coloring characteristic 1802 is set as the target characteristic for the center head, the correction amount Δin for the input value in of the center head's relative nozzle number 0 can be calculated as shown in Figure 18(a). That is, the target value Dt corresponding to the input value In is calculated from the average coloring characteristic 1802. Furthermore, the signal value In' corresponding to the target value Dt is obtained from the nozzle characteristic 1803 of the relative number 0. The correction amount Δin is then calculated as Δin = In' - In.

[0172] By performing the above process using the average color characteristics of the right and left heads and the nozzle characteristics of relative nozzle number 0 of each head, a number of correction amounts Δin for relative nozzle number 0 is obtained equal to the number of heads. In this embodiment, three correction amounts Δin are obtained, and their average value is used as the correction amount Δin common to all three heads for relative nozzle number 0. In other words, if the above process is described using absolute numbers instead of relative numbers, the correction value for nozzle number n can be calculated as the average value of Δin obtained using absolute numbers 0 + n, 5000 + n, and 10000 + n. Here, n is an integer greater than or equal to 0 and less than the number of nozzles in the head; in the example above, 0≦n≦4999.

[0173] The above process calculates correction values ​​for input values ​​In=0 to 255, and a table associating the input values ​​with the correction values ​​is created to obtain a correction table within the head. Alternatively, only values ​​corresponding to nine gradations may be calculated and stored as a table. In that case, when using the correction table, values ​​other than the nine gradations can be calculated from these nine values ​​using known interpolation processing. Figure 19(b) is an example of an intra-head correction table obtained in this way. In the example shown in Figure 19(b), a correction value from the average value is stored for each of the nine input signal values ​​at each position within the head.

[0174] <Creating a head-to-head correction table> In the above-mentioned S1703, an inter-head correction table is created by the image processing unit 106. Specifically, first, the one-dimensional data obtained in S1703 is corrected using the intra-head correction table shown in Fig. 19(b). Alternatively, the above-mentioned inter-head correction table is applied to the color signal image data 312 of the gradation chart 810 and output again, and one-dimensional data is again obtained from the scan data of the output image.

[0175] Figure 18(b) shows the one-dimensional data for each nozzle obtained in this way. For simplicity, only the nozzle characteristics obtained from the patches for one gradation are plotted in Figure 18(b). In reality, the characteristics for each head, indicated by reference numerals 1804 to 1806 in Figure 18, are obtained for the number of patches. As shown in Figure 18(b), the nozzle characteristics 1804 to 1806 of each head are corrected to the average value within each head using the intra-head correction table. However, because the average value for each head is different, the density of the images formed by each head is different.

[0176] That is, an image formed by the same gradation patch has different densities for the regions 1604 to 1606.

[0177] Therefore, in this embodiment, the density difference between the heads is corrected by creating and applying an inter-head correction table for correcting the characteristics of each head. Specifically, the image processing unit 106 first averages the area corresponding to the patch 811 in the transport direction (y direction) of the scanned image of the chart 810 obtained in S1701 to make it one-dimensional. The image processing unit 106 further divides the obtained one-dimensional data into sections corresponding to the areas 1604, 1605, and 1606, and calculates the average value of these. In other words, for each patch 811, the color development characteristics corresponding to the right head 1601, center head 1602, and left head 1603 are obtained.

[0178] As a specific example, curves 1808 to 1810 in FIG. 18(c) show the color characteristics of each head. For example, curve 1808 is the color characteristics D corresponding to the center head 1602. Similarly, curve 1809 shows the color characteristics D of the left head 1601, and curve 1810 shows the color characteristics D of the right head 1603. At this time, the corrected input values ​​In' (corresponding to the left head), In'' (corresponding to the center head), and In''' (corresponding to the right head) for each input value In are obtained as shown in FIG. 18(c). That is, first, the target value Dt corresponding to the input value In is calculated from the target characteristic 1807. Furthermore, the signal values ​​In', In'', and In''' corresponding to the target value Dt are obtained as correction values ​​from each nozzle characteristic 1808 to 1810. By calculating correction values ​​for all values ​​of the input value In from 0 to 255, an inter-head correction table in which a different correction value is associated with each head is obtained.

[0179] Alternatively, only the values ​​corresponding to the nine gradations may be calculated and stored as a table. In that case, when using the correction table, values ​​other than the nine gradations can be calculated from these nine values ​​by known interpolation processing. Figure 19(c) is an example of an inter-head correction table obtained in this way. In the example shown in Figure 19(c), the correction values ​​In' for the left head, In'' for the center head, and In'' for the right head are stored in association with each of the nine input signal values ​​In.

[0180] <Dynamic correction processing> In the dynamic correction of this embodiment, only the density difference between the heads is re-corrected. Specifically, as the dynamic correction, steps S1704 and S1705 in the static correction process are executed again. In this way, if the dynamic correction is a part of the static correction, the process can be shared, and as a result, the circuit size and program memory usage can be reduced.

[0181] Note that the target characteristics in creating the inter-head correction table (S1704) can be the target characteristics from the most recent static correction, or the characteristics of the center head can be used as the target characteristics. Also, when creating the inter-head correction table (S1704), if the chart shown in Figure 8(b) is output for each ink color, four charts will be output. To reduce wasted paper and downtime, the number of gradation patches for each ink (CMYK) can be limited and consolidated onto one sheet.

[0182] Specifically, a chart may be output that consists of a total of 13 patches: paper white + each ink color (CMYK) x 3 gradations (input values ​​85, 170, 255). Alternatively, because density unevenness due to yellow is less noticeable than density unevenness due to other inks, a total of 13 patches may be output: paper white + each ink color (CMYK) x 4 gradations (input values ​​64, 128, 192, 255).

[0183] Alternatively, instead of using uniform patches, the head characteristics may be obtained from a gradation chart shown in FIG. 13(e) and the correction values ​​may be calculated.

[0184] Alternatively, dynamic correction may be performed to correct only the K ink. In that case, the chart shown in FIG. 8(b) may be output only for the K ink.

[0185] Alternatively, the misaligned colors may be estimated from the user image, and the gradation chart shown in FIG. 8(b) may be output and corrected for the misaligned colors.

[0186] Alternatively, instead of obtaining the head color characteristics from a chart, they can be obtained from a user image. That is, an area where the input signal is approximately the same for each head is predetermined as the characteristic acquisition area. Then, the difference in that area of ​​the scanned image can be obtained, and corrections can be made for each head based on the magnitude of the difference. For example, if the center head is used as the reference and the D values ​​of the left and right heads are low, the pixel values ​​within the heads can be uniformly multiplied by 1.01.

[0187] In this embodiment, the target of dynamic correction is the image 312 that has already been inter-head corrected. That is, the density difference between the heads is calculated based on the image that has already been inter-head corrected, and the image that has already been inter-head corrected is then further corrected. At this time, correction may be made to the inter-head correction table 1704 rather than the image, and the image may be corrected using the corrected table.

[0188] Furthermore, when using a chart instead of a user image, a chart image before inter-head correction, i.e., a chart image in which only density unevenness within the head has been corrected in S1702, can be output, and the correction table can be replaced before regenerating the color signal image data 312.

[0189] <Modification> In S1704, the image processing unit 106 may output a chart 1610 for calculating an inter-head correction table for each ink color (CMYK) as shown in Fig. 16(c) and calculate correction values ​​from the scanned image. The chart 1610 shown in Fig. 16(c) is a chart for calculating post-correction signal values ​​that minimize density unevenness at the joints between adjacent heads when the characteristics of the central head are used as a reference. Specifically, the chart 1610 is made up of three blocks 1611 to 1613.

[0190] In this case, blocks 1611 to 1613 have different signal values ​​in the areas (1614 to 1615) corresponding to the center head, for example, the respective signal values ​​are 64, 128, and 255. Furthermore, in each block, the areas corresponding to the left and right heads have the same patch 1617. Specifically, patch 1617 shown in Figure 16(c) is a gradation patch with signal values ​​ranging from 0 to 255.

[0191] Correction values ​​for the left and right heads are calculated by analyzing each block from the scanned image of the chart 1610. More specifically, patch 1617 is searched in the direction of change in gradation (y direction) to find the y position where the density difference with the area (1614-1615) corresponding to the center head in the block is smallest. Then, the corresponding signal value is calculated from the y position and used as the corrected signal value. This search is performed independently for each of the left and right heads. By performing the above process for each block (1611-1613), three pairs of input signal value in and corrected signal value in' are obtained for each of the left and right heads. These combinations are used as a dynamic inter-head correction LUT, and by further combining this with interpolation calculation processing, the density difference between the heads is corrected.

[0192] Specifically, a chart 1610 is output to which the intra-head correction LUT and inter-head correction LUT created in the static correction process are applied, and the dynamic inter-head correction LUT is created from the scanned image. Furthermore, the dynamic inter-head correction LUT is further applied to color signal image data 312 to which the intra-head correction LUT and inter-head correction LUT have already been applied in the static correction process, thereby obtaining dynamically corrected color signal image data 312. The above-mentioned search for the y position will be described in more detail using Figure 16(d).

[0193] The horizontal axis in FIG. 16(d) represents the nozzle position. In the example shown in FIG. 18(d), the head printing with nozzle number 5000 switches from the left head to the center head. The vertical axis in FIG. 16(d) represents the distance D from the white paper. At this time, the color characteristics of the y position indicated by the dotted line 1618 in FIG. 16(c) from the scanned image of chart 1610 are shown as curve 1619 in FIG. 16(d). Note that color characteristics 1619 can be calculated using a color conversion LUT that converts scanner RGB to Lab and equation (1).

[0194] In this case, the density difference ΔD between the heads can be calculated from the maximum value Dmax and minimum value Dmin of D in the area corresponding to the left head and the center head (0≦x<10000) by ΔD=Dmax-Dmin. Furthermore, ΔD is calculated for all y positions within the block, and the y position where ΔD is smallest is found. Then, the signal value corresponding to the y position can be set as the corrected signal value In'.

[0195] By detecting the y position and calculating In' as described above for the regions corresponding to the right head and the center head (5000≦x<15000), the signal value In' after correction for both heads can be calculated.

[0196] In addition, by averaging areas of the scanned image with the same signal value in the x direction within each block, it is possible to reduce quantization errors caused by the threshold matrix, errors in the sensor reading position, errors due to noise and sensitivity, and variations in nozzle characteristics. Therefore, it is preferable to average areas that can be considered to have the same signal value before searching for the y position. Furthermore, it is preferable that the width of the x direction to be averaged is a multiple of the matrix size (for example, 256 pixels or 512 pixels).

[0197] In addition, the y position may be searched only from the joints between the heads and their surrounding areas, rather than from the entire adjacent heads. For example, the maximum value Dmax and minimum value Dmin may be calculated in the area 4900≦x<5100.

[0198] Furthermore, the dynamic correction process may be performed only on the head joints and their surrounding areas. In this case, it is preferable to use the weighted average of the signal values ​​before and after applying the dynamic head-to-head correction LUT as the corrected signal value In'.

[0199] Specifically, it is preferable to perform weighted averaging so that the weight before correction increases with increasing distance from the boundary, thereby smoothly varying the portion where correction processing is performed and the portion where correction processing is not performed.

[0200] In the above-described chart 1610, the number of blocks is not limited to 3, but may be, for example, 9 or 17. The more blocks there are, the shorter the interval between interpolations becomes, and therefore the error due to the interpolation calculation can be reduced.

[0201] On the other hand, increasing the number of blocks reduces the height of each block, making it more susceptible to quantization errors and other effects caused by the characteristics of the matrix.

[0202] Therefore, it is preferable to determine the number of blocks taking into consideration the reduction in errors in the interpolation calculations that occurs when the number of blocks is increased and the increase in the errors that occurs when the height of the blocks is narrowed.

[0203] When the above chart 1610 is used, it is also possible to configure the system so that the correction value is calculated from the position visually confirmed by the user, rather than analyzing the image scanned by the image sensor.

[0204] For example, a chart 1610' (not shown) with scales added to the chart 1610 is prepared, and this chart is output periodically. For example, the chart 1610' is output every time 100 sheets of a user image are printed. The user then visually checks the output of the chart and inputs the scale positions that minimize the density difference between the heads via the scanning unit 103. At this time, it is preferable to input the scale positions for each of the left and right heads. The image processing unit 106 then calculates corrected input values ​​for the left and right heads based on the input scale positions, creates an LUT, and applies it to the areas of the color signal image data 312 corresponding to each head. In this way, even an image forming system 1 that does not have a sensor can dynamically correct unevenness between heads.

[0205] In addition to the dynamic correction in this embodiment and its modified examples described above, the dynamic correction processing described in the first and second embodiments may be separately performed on the table in the head.

[0206] <Fourth embodiment> (Hardware configuration of image forming system) 21 is a diagram showing the hardware configuration of an image forming system according to an embodiment of the present invention. The image forming system according to this embodiment includes a CPU 2100, a RAM 2101, a ROM 2102, an operation unit 2103, a display unit 2104, an external storage device 2105, an image processing unit 2106, an image forming unit 2107, an image acquisition unit 2108, an I / F (interface) unit 2109, and a bus 2110.

[0207] A CPU (Central Processing Unit) 2100 controls the operation of the entire image forming system using input data and computer programs stored in RAM and ROM (described later). Note that, although the case where the CPU 2100 controls the entire image forming system will be described here as an example, the entire image forming system may also be controlled by multiple pieces of hardware sharing the processing.

[0208] A RAM (Random Access Memory) 2101 has a storage area for temporarily storing computer programs and data read from an external storage device 2105 and data received from the outside via an I / F unit 2109. The RAM 2101 is also used as a storage area used when the CPU 2100 executes various processes and as a storage area used when the image processing unit 2106 executes image processing. A ROM (Read Only Memory) 2102 has a storage area for storing setting parameters for setting each unit in the image forming system, a boot program, and the like.

[0209] The operation unit 2103 is an input device such as a keyboard or a mouse, and receives operations (instructions) from an operator. That is, the operator can input various instructions to the CPU 2100.

[0210] The display unit 2104 is a display device such as a CRT (Cathode Ray Tube) or a liquid crystal screen, and can display the processing results by the CPU 2100 as images, characters, etc. If the display unit 2104 is a touch panel that can detect touch operations, the display unit 2104 may function as a part of the operation unit 2103.

[0211] The external storage device 2105 is a large-capacity information storage device typified by a hard disk drive. The external storage device 2105 stores computer programs and data for causing the OS (operating system) and CPU 2100 to execute various processes. The external storage device 2105 also stores temporary data generated by the processing of each unit (for example, input / output image data and a threshold matrix used by the image processing unit 2106). The computer programs and data stored in the external storage device 2105 are read as appropriate under the control of the CPU 2100, stored in the RAM 2101, and processed by the CPU 2100.

[0212] The image processing unit 2106 is realized as a processor capable of executing a computer program or a dedicated image processing circuit, and executes various image processing operations to convert image data input as a print target into image data that can be output by an image forming apparatus (described later). Note that instead of providing a dedicated processor as the image processing unit 2106, the CPU 2100 can also be configured to perform various image processing operations as the image processing unit 2106.

[0213] The image forming unit 2107 forms an image on a recording medium using a recording material based on image data received directly from the image processing unit 2106 or via a RAM or an external recording device.

[0214] The image acquisition unit 2108 is an image sensor (a line sensor or an area sensor) for capturing an image formed on a recording medium by the image forming unit 2107.

[0215] The I / F unit 2109 functions as an interface for connecting the image forming system to an external device. The I / F unit 2109 also functions as an interface for exchanging data with a communication device using infrared communication, a wireless LAN (Local Area Network), etc., and as an interface for connecting to the Internet. This allows data, such as input images, to be exchanged with external devices.

[0216] Each of the above-mentioned units is connected to a bus 2110, and data can be exchanged via the bus 2110. However, the image forming system may be configured such that each of the above-mentioned units (e.g., the image forming unit 2107) is connected via an I / F unit 2109.

[0217] [Hardware configuration of the image formation unit and image acquisition unit] 22(a) to 22(d) are diagrams schematically illustrating an image forming unit 2107 according to one embodiment of the present invention. Note that the image forming unit 2107 in this embodiment is an IJ printer that forms an image by ejecting ink from nozzles onto a recording medium.

[0218] 22(a), the image forming unit 2107 includes a plurality of recording heads 2201 to 2204 corresponding to black (K), cyan (C), magenta (M), and yellow (Y), respectively. The recording heads 2201 to 2204 are of the so-called full-line type, in which a plurality of nozzles for ejecting ink are arranged in a predetermined direction within a range corresponding to the width of the recording paper 2206.

[0219] At this time, the recording heads 2201 to 2204 are configured by combining a plurality of head modules, as shown in Fig. 22(b). The head modules 2201a, 2201b, and 2201c that make up the recording head 2201 are arranged alternately in the paper transport direction.

[0220] 22(c), the head module 2201a is made up of a plurality of chip modules 2201a-1 to 2201a-5, each of which is connected to an independent board.

[0221] Figure 22(d) is a diagram of one of the chip modules as viewed from the paper surface, showing that the chip module has multiple nozzles. In the example shown in Figure 22(d), the chip module has 16 nozzles. In this case, the nozzle arrangement resolution of the nozzle arrays for each ink color is, for example, 1200 dpi.

[0222] A recording sheet 2206 serving as a recording medium is conveyed in the direction indicated by arrow 2207 in the figure by a conveying roller 2205 (and other rollers, not shown) rotating due to the driving force of a motor (not shown). As the recording sheet 2206 is conveyed, ink is ejected from the multiple nozzles of each of the recording heads 2201 to 2204 in accordance with recording data, thereby sequentially forming an image for one raster line corresponding to the nozzle array of each recording head. In this way, by repeating the ink ejection operation from each recording head onto the conveyed recording sheet, it is possible to print, for example, an image for one page.

[0223] 22(a), the image acquisition unit 2108 is a line sensor that covers the entire surface of the recording paper and is installed downstream of the recording heads 2201 to 2204. That is, after an image is formed by the recording heads 2201 to 2204, the recording paper 2206 is transported to the image capturing unit 2108. The image capturing unit 2108 sequentially captures and acquires images of the transported recording paper as, for example, RGB information or luminance information, and stores the images in the external storage device 2105 as two-dimensional image data.

[0224] If the resolution of the sensor output value (RGB) acquired by the image acquisition unit 2108 differs from the resolution of the input image (CMYK), it is preferable to perform resolution conversion on the sensor output value to match the two. The resolution conversion can be performed using a known nearest neighbor method, bilinear interpolation, bicubic interpolation, or the like.

[0225] Furthermore, when there is a large amount of paper skew or aberration of the spectroscopic sensor, it is preferable to perform geometric correction on the sensor output value, which can be performed using known affine transformation or projective transformation.

[0226] The resolution conversion process and geometric correction process are performed by, for example, the image processing unit 2106. Alternatively, the image acquisition unit 2108 may perform these processes in units of a predetermined number of lines when acquiring a raster image, and then transmit the sensor output values ​​to the color conversion processing unit. At this time, the image forming unit 2107 may form an image by adding markers that facilitate the above conversion.

[0227] [Functional configuration of the image processing unit] The configuration of the image processing unit 2106 will be described below with reference to FIG.

[0228] As shown in FIG. 23, the image processing unit 2106 includes an input color conversion processing unit 21061, a correction processing unit 21062, an HT (halftone) processing unit 21063, a header image adding unit 21064, a correction table creating unit 21065, and a correction table 21066.

[0229] The input color conversion processing unit 21061 converts input image data from the external recording device 2105 into image data corresponding to the color reproduction gamut of the printer. The input image data has color coordinates (R, G, B) in a color space coordinate system such as sRGB, which is the representation color of the monitor.

[0230] The input color conversion processing unit 21061 performs processing to convert the data into color signals corresponding to the multiple inks used in the image forming unit 2107. For example, if the image forming unit 2107 uses black (K), cyan (C), magenta (M), and yellow (Y) inks, the image data of the RGB signal is converted into image data consisting of 8-bit color signals for each of K, C, M, and Y.

[0231] The CMYK data output by the input color conversion processing unit 21601 represents the usage amount (ejection amount) of each recording material that is ejected onto the paper surface to represent an image by the image forming unit 2107. For this conversion, known methods can be used, such as matrix calculation processing or processing using a three-dimensional LUT (look-up table).

[0232] The input data is not limited to data representing RGB, but may also be data directly representing CMYK. However, even in this case, due to limitations on the total amount of ink and color management, it is preferable that the input color conversion processing unit 21061 performs processing using a four-dimensional LUT that converts the input CYMK data into different data C'M'Y'K'.

[0233] The correction processing unit 21062 performs correction processing to stabilize colors. For example, it refers to a correction table 21066 corresponding to each of the print heads 2201 to 2204 and changes each of the CMYK image data in a direction that cancels out unevenness and streaks that occur on a print head-by-print head basis.

[0234] FIG. 24 is a diagram showing an example of a correction table in this embodiment.

[0235] 24 stores corrected color signal values ​​corresponding to each input color signal (0, 16, 32, ..., 240, 255) for each print head 2201 to 2204. For example, if the input color signal value of a pixel corresponding to print head 2201a in the K color signal image data is 32, correction processing unit 21062 changes the pixel value of that pixel to 28.

[0236] In this way, by correcting the color signal values ​​for each CMYK color signal by referring to the conversion table corresponding to each print head, it is possible to cancel out density variations that occur in units of print heads.

[0237] It is also possible to perform correction processing for each head module, chip module, or nozzle, rather than for each print head.It is also possible to perform correction processing for each nozzle block, which is divided into a set number of nozzles, such as every eight nozzles.

[0238] In this case, for example, to perform correction processing for each nozzle, the correction table 21066 has a number of columns equal to the number of nozzles. Alternatively, a number of correction tables equal to the number of nozzles are stored as the correction table 21066 in advance.

[0239] In the correction table shown in Fig. 24, for input color signal values ​​that are not included in the correction table, color signals are calculated using interpolation processing from nearby signal values ​​stored in the table. Of course, it is also possible to store converted color signal values ​​for all color signal values ​​without using interpolation processing. Alternatively, correction processing can be performed using function conversion or matrix conversion instead of a correction table.

[0240] Returning to Fig. 23, the HT (halftone) processing unit 21063 performs HT processing on the corrected image data from the correction processing unit 21062, generating image data that can be represented by the image forming unit 2107. For example, 8-bit image data per pixel is converted into binary HT image data in which each pixel has a value of either 0 or 1. Dither processing, which is a well-known method, can be used for the HT processing. Alternatively, other methods such as error diffusion can also be applied. The header image adding unit 21064 adds a header image to the image data after the above-mentioned HT processing.

[0241] A more specific description will be given below with reference to Figures 25(a) to (d). Reference numeral 501 in Figure 25(a) is a diagram schematically showing image data after HT processing. Also, arrow 2207 indicates the printing direction of the image data, and corresponds to arrow 2207 in Figure 22. That is, triangular figure 25011 in image data 2501 is printed by image forming unit 2107 prior to circular figure 25012, and is also read by image acquisition unit 2108.

[0242] A header image adding unit 21064 acquires image data 2501 after HT processing, adds an image area 2502 to be printed earlier, and adds a header image 2503 to the area.

[0243] Fig. 25(b) shows a schematic diagram of the image data after the header image 2503 is added, and Fig. 25(d) shows an enlarged view of the header image 2503 added in this embodiment.

[0244] The header image shown in FIG. 25(d) is made up of preliminary ejection regions 25031 to 25034, a line pattern region 25035, and a gradation data acquisition region 25036.

[0245] The preliminary ejection areas 25031 to 25034 each correspond to one of the inks used in the image forming unit 2107. For example, the preliminary ejection area 25031 is a rectangular area formed with cyan (C) ink. At this time, in the binary image data corresponding to the preliminary ejection area 25031, the signal value of C ink for all pixels is 1 (ON), and a so-called solid image of C ink is formed in that area.

[0246] That is, if the resolution of the image forming unit in the nozzle direction (x direction) and transport direction (y direction) is 1200 dpi, 1200 x 1200 = 1,440,000 droplets of C ink per square inch are ejected onto the preliminary ejection area 25031. In this case, if the height of the preliminary ejection area 25031 in the transport direction is 0.1 inches (= 2.54 mm), for example, 120 droplets of C ink are ejected onto this area from all nozzles.

[0247] Similarly, a preliminary ejection area 25032 is a preliminary ejection area for magenta (M) ink, 25033 is a preliminary ejection area for yellow (Y) ink, and 25034 is a preliminary ejection area for black (K) ink, and solid images of each ink are formed, respectively.

[0248] 25(b), by providing preliminary ejection areas 25031-25034 at positions that will be printed prior to the image data 2501, ink that has concentrated in nozzles with low ejection frequency can be discharged before the image data is printed. This makes it possible to prevent defects such as density fluctuations, blurring, and ejection failures that are caused by ink concentration from occurring in the image data 2501.

[0249] In particular, when forming images continuously using full-line type recording heads such as those shown by reference numerals 2201 to 2204 in Figure 22(a), it is difficult to discharge concentrated ink outside the transport route of the recording paper 2206. Therefore, it is preferable to provide a preliminary ejection area to the image data as described above and discharge a certain amount of ink into that area in order to suppress defects caused by ink concentration without reducing the image formation speed.

[0250] The number of preliminary ejection regions included in the header image 2503 depends on the number of inks or print heads held by the image forming unit 2107. For example, if R (red), O (orange), W (white), etc. are held in addition to CMYK, it is preferable to also include preliminary ejection regions for those colors. For example, if a total of six types of ink (CMYKRW) and the corresponding print heads are held, it is preferable that the header image 2503 include a total of six preliminary ejection regions.

[0251] 25(d), the gradation data acquisition region 25036 is a rectangular region formed using only one of the inks used by the image forming unit 2107. In this embodiment, the gradation data acquisition region 25036 is formed using an HT image corresponding to one of the color signal values. The height and width of the rectangle are approximately the same as those of the preliminary ejection region, but the height does not have to be the same as those of the preliminary ejection region.

[0252] Specifically, the results of performing the same dithering process as used by the HT processing unit 21063 on a rectangular area with uniform color signal values ​​are stored in advance as an HT pattern. At this time, one of these multiple patterns is selected and added as the gradation data acquisition area 25036. The method for selecting a pattern will be described in detail later.

[0253] The line pattern area 25035 is an area that includes a line pattern formed with the same ink color as the ink that forms the gradation data acquisition area 25036 .

[0254] Although the line pattern area 25035 is not a required component, it is preferable to form it within the header area 2502, especially when acquiring nozzle characteristics on a nozzle-by-nozzle basis, because referencing the line pattern makes it easier to align the nozzle position with the acquired image.

[0255] Furthermore, as shown in FIG. 25(b), it is preferable that the gradation data acquisition region 25036 is formed at approximately the same timing as the preliminary ejection regions 25031 to 25034.

[0256] For example, suppose that preliminary ejection areas 25031 to 25034 are added to image area 2502 in Figure 25(c). In this case, it is not preferable to add gradation data acquisition area 25036 to image area 2502', which is printed after the image data, rather than within image area 2502. This is because ink concentration occurs depending on the frequency of use when forming image data 2501, and there is a possibility that gradation data cannot be properly acquired in gradation data acquisition area 25036.

[0257] 23, the correction table creation unit 21065 receives the read data corresponding to the gradation data acquisition area 25036 from the image reading unit 2108, and creates a correction table to be used by the correction processing unit 21062. Details of the creation process will be described later.

[0258] [User image printing flow] In light of the above-described configuration of the image processing unit 2106, the flow of printing a user image and correction processing based on the formed image in this embodiment will be described below with reference to the flowchart shown in FIG.

[0259] First, the user inputs a print job to the image forming system via the operation unit 2103. Specifically, the user specifies the name of the input image file in the external storage device 2105 and the number of copies to be printed.

[0260] When a print job is submitted, the image processing unit 2106 acquires input image data stored in the external storage device 2105 based on the specified input image file name (step S2601). Hereinafter, it is assumed that the acquired input image data is image data in RGB 8-bit format.

[0261] In the next step S2602, the input color conversion processing unit 21061 performs input color conversion on the input image data. Through this processing, the input image data is converted into image data composed of color signal values ​​that represent the amount of each recording material (CMYK) to be applied.

[0262] Next, in S2603, the header image adding unit 21064 initializes a number counter (hereinafter simply referred to as a counter) cnt to zero.

[0263] Furthermore, in S2604, the correction processing unit 21062 performs correction processing based on the correction table 21066 on the image data converted in S2602.

[0264] Next, in S2605, the HT processing unit 21603 performs HT processing on the image data after the correction processing.

[0265] Next, in S2606, the header image adding unit 21064 adds a header image based on the counter cnt to the image data after the HT processing.

[0266] Specifically, the ink color and color signal value forming the gradation data acquisition area 25036 are changed based on the counter cnt.

[0267] For example, let us take the case where ink colors are changed among four types, CMYK, and color signal values ​​are changed among five types, 0, 64, 128, 192, and 255. In this case, when counter cnt=0, the HT pattern corresponding to the color signal value of 0 for C ink is acquired and added as the gradation data acquisition area 25036. Also, when counter cnt=1, the HT pattern corresponding to the color signal value of 64 for C ink is added as the gradation data acquisition area 25036.

[0268] Here, the HT pattern corresponding to a color signal value of 64 for C ink in this embodiment is image data obtained by performing HT processing on a rectangular area with a uniform color signal value of 64. In this case, if dithering is used in the HT processing (S2605) of the input image data, it is preferable to use the same dithering in generating the HT pattern for the gradation data acquisition area 25036. In other words, if the gradation data acquisition area 25036 is formed with C ink, it is preferable to generate the HT pattern to be added to the gradation data acquisition area 25036 using a dither matrix used for the image data of C ink in the HT processing (S2605).

[0269] At this time, the size of the rectangular area is equal to the size of the gradation data acquisition area 25036, and the image data obtained is, for example, binary image data.

[0270] Similarly, for counters cnt=2, 3, and 4, HT patterns corresponding to color signal values ​​of 128, 192, and 255 for C ink are acquired and added, respectively.

[0271] Furthermore, for counters cnt=5 to 9, HT patterns corresponding to M ink color signal values ​​of 0, 64, 128, 192, and 255 are acquired and added, respectively. Also, for counters cnt=10 to 14, HT patterns corresponding to Y ink color signal values ​​of 0, 64, 128, 192, and 255 are acquired and added, respectively, for counters cnt=15 to 19.

[0272] In the HT pattern corresponding to a color signal value of 0, all pixels are 0 (OFF), and when this pattern is selected, no ink is ejected into the gradation data acquisition area 25036.

[0273] On the other hand, in the HT pattern corresponding to a color signal value of 255, all pixels are 1 (ON), and when this pattern is selected, a so-called solid image is formed in the gradation data acquisition area 25036.

[0274] By changing the ink color and color signal value based on the counter cnt in this way, even if only gradation data corresponding to a single color signal value of a single ink is acquired from each gradation data acquisition area 25036, gradation data for the color signal value of each ink can be acquired by combining data from multiple gradation data acquisition areas.

[0275] It is not necessary to change to all ink colors that can be ejected by the image forming unit 2107. For example, the ink to be formed may be fixed to K ink, which has a high contribution to density, and only the signal value may be changed. In this case, the correction process described below is also performed only on the K ink.

[0276] Alternatively, the change may be made to only the three inks CMK without changing to yellow (Y) ink, which causes less perceptible fluctuations.

[0277] Furthermore, the gradation values ​​are not limited to the five types mentioned above (0, 64, 128, 192, 255). For example, values ​​obtained by dividing 0 to 255 into 33 equal intervals (0, 8, 16, . . . , 232, 240, 248, 255) may also be used. Furthermore, it is not necessary to include 0 or 255, and the intervals between gradation values ​​may be uneven.

[0278] It should be noted that instead of adding a previously generated HT pattern in S2606, it is also possible to add it as color signal data before the HT processing (S2605), and then perform the HT processing (S2605) together with the input image data.

[0279] 26, next, in S2607, the image forming unit 2107 forms an image on paper based on the HT image to which the header image has been added, thereby creating a formed image 2400.

[0280] In the following S2608, the image acquisition unit 2108 captures the formed image 2400 and acquires two-dimensional captured image data.

[0281] In the next step S2609, the correction table creating unit 21065 refers to the counter cnt and determines whether one set of data required for correction has been acquired.

[0282] For example, if new gradation data acquisition areas have been formed and images have been taken for all combinations of ink colors and color signal values, one set of required data has been acquired.

[0283] Specifically, in the example described above where there are four types of ink colors and five types of color signal values, it is determined that one set has been acquired if counter cnt = 19. Also, if counter cnt < 19, it is determined that one set has not been acquired.

[0284] If it is determined in S2609 that one set has been acquired, the process proceeds to S2610, where a correction table is created and updated based on the acquired gradation data by the correction table creation unit 21065. Details of the process in S2610 will be described later.

[0285] On the other hand, if it is determined in S2609 that one set has not been acquired, the process proceeds to S2611.

[0286] In S2611, the image processing unit 2106 determines whether the submitted print job is complete. Typically, the print job is complete when the number of prints specified by the user in S2601 has been printed. Printing is also complete when the user separately instructs to cancel printing. If it is determined that the print job is complete, the flow ends.

[0287] On the other hand, if it is determined that the print job is not completed, the process proceeds to S2612, where the header image adding unit increments the counter cnt by 1. Thereafter, the process returns to S2606, where printing continues.

[0288] In S2613, similarly to S2611, the image processing unit 2106 determines whether the print job is completed. If it is determined that the print job is completed, the flow ends.

[0289] On the other hand, if it is determined that the print job is not completed, the process returns to S2603, where the counter is again initialized to 0 (S2603). Furthermore, correction processing is performed based on the updated correction table (S2604), and HT processing is performed on the corrected image data (S2605), after which the process returns to S2606 and printing continues.

[0290] By following the steps S2601 to S2613 above, the image specified by the user can be printed for the specified number of copies. In addition, by adding a header image to the printing and performing correction processing to suppress unevenness and streaks based on the reading results, it is possible to maintain the quality of the printed image even with changes over time.

[0291] In the above description of S2609, it was stated that one set has been acquired when all combinations of ink colors and color signal values ​​have been formed and imaged. In this case, instead of acquiring all combinations once, it is also possible to consider one set to be acquired when, for example, five sets of images have been acquired. In this case, correction processing is performed in S2610 based on an average read value obtained by averaging the five read values ​​for each combination. By performing correction based on the average read value in this way, the frequency of correction is reduced while also preventing a decrease in correction accuracy due to errors contained in the read values, such as sensor reading errors or scratches or dirt on the paper surface.

[0292] Alternatively, when formation and imaging for a predetermined color signal value for any ink color has been completed, it may be considered that one set has been acquired. In the above example, when counter cnt=4, formation and imaging for five gradations (0, 64, 128, 192, 255) for C ink are completed. At this time, it may be considered that one set for C ink has been acquired, and correction processing may be performed only for C ink.

[0293] According to the flow shown in FIG. 26, the header image 2503 is added to every page to be printed. However, there may be cases where performing preliminary firing on every page is too frequent. In such cases, the header image may be added intermittently. For example, the header image 2503 may be added at a rate of one page out of every ten pages.

[0294] In addition to the above, depending on the degree of aging of the print head or module, acquiring gradation data for each page may be too frequent. In such cases, it is possible to add a header image with only a preliminary ejection area to every 10 pages, and then add a header image with an additional line chart and gradation data acquisition area to every 10 pages, i.e., every 100 pages.

[0295] [Update correction table] 27 is a diagram showing a flow of creating a correction table in this embodiment. The process of creating a correction table will be described below according to the flow shown in FIG.

[0296] First, in S2701, the correction table creating unit 21065 refers to the image read in S2608 described above, and acquires the read value corresponding to the gradation data acquisition area 25036.

[0297] For example, the image data of the printed matter 2400 captured in three channels (RGB) by the image reading unit 2108 can be converted using a color conversion table prepared in advance to match the color characteristics of the optical sensor, thereby obtaining a reading value for one channel.

[0298] More specifically, a color conversion table that converts Y in the CIEXYZ color space into a linear 16-bit value is stored in advance, and the value obtained by converting the RGB value of each pixel using the color conversion table can be used as the read value of each pixel.

[0299] Alternatively, the L* value of CIELab* or a value linear to the density can be used as the read value. Furthermore, if the measurement image is recorded with color inks such as C, M, and Y, a value corresponding to saturation can be used instead of a value corresponding to brightness. Alternatively, the output values ​​of the R, G, and B channels can be used directly as read values, corresponding to the complementary colors of C, M, and Y, respectively.

[0300] In addition to the above, the correction table creation unit 21065 calculates a read value corresponding to each correction unit of the correction table from the read value for each pixel. For example, if the correction unit is a nozzle unit, the read values ​​for the pixels formed for each nozzle are averaged.

[0301] Specifically, the read values ​​for each pixel corresponding to the gradation data acquisition area 25036 are averaged in the paper transport direction to obtain read values ​​corresponding to each nozzle position as one-dimensional data in the nozzle row direction. More specifically, the resolution in the transport direction (y direction) is assumed to be 1200 dpi, and the height of the preliminary ejection area 25031 in the transport direction is assumed to be 0.1 inch (= 2.54 mm). In this case, the read values ​​for each nozzle can be calculated by averaging the read values ​​for 120 pixels in the y direction for each x position corresponding to each nozzle.

[0302] Alternatively, if the correction unit is a head module unit, the read values ​​for each head module are calculated by averaging the read values ​​in a rectangular area formed by each head module.

[0303] In this case, as shown in FIG. 25(d), if the header image 2503 includes a line pattern, the pattern can be used as a clue to accurately estimate the x-position correspondence on the read image corresponding to each nozzle.

[0304] It should be noted that, for example, there may be cases where the paper meanders significantly, causing a shift of one pixel or more on the scanned image even within the image height (height in the y direction) of the header image 2503. In such cases, it is preferable to place two line patterns on both sides of the gradation data acquisition area 25036, rather than just on one side of the gradation data acquisition area 25036, sandwiching the gradation data acquisition area in the transport direction. By placing multiple line patterns in this way, the amount of meandering can be estimated, and the correspondence between each nozzle and the x position on the image can be estimated more accurately.

[0305] 27, in the next step S2702, the correction table creation unit 21065 calculates a measurement curve corresponding to each correction unit based on the read values ​​of multiple gradation data acquisition areas 25036 formed with the same color. Here, the measurement curve is a curve that represents the relationship between the read values ​​and the color signal values ​​that form the gradation data acquisition area 25036.

[0306] An example of the calculated measurement curve is shown in Figure 28(a). The horizontal axis of Figure 28(a) is the color signal value of the gradation data acquisition area 25036, and the vertical axis is the read value of each correction unit obtained from the gradation data acquisition area.

[0307] Reference symbol 2801 in the figure is the upper limit value of the horizontal axis, which is 255 if the input signal value is 8 bits, for example. Reference symbol 2802 is a measurement curve obtained by interpolation calculation from one or more color signal values ​​corresponding to the same correction unit, for example, the same nozzle, and their corresponding read values. In this case, for example, piecewise linear interpolation can be used as the interpolation method. Alternatively, spline curves or bicubic interpolation methods can be used.

[0308] Note that different measurement curves are obtained according to the characteristics for each number of nozzles or head modules, but to avoid cluttering the diagram, only one of these curves is shown in Figure 28(a).

[0309] Returning to the flowchart in Fig. 27, in step S2703, the correction table creation unit 21065 acquires target characteristics 2803. Here, the target characteristics are curves or straight lines that indicate the relationship between each color signal value and the read value that should be formed after applying the correction table. For example, as shown in Fig. 28(a), a straight line that changes linearly with respect to the gradation can be used as the target characteristic.

[0310] The target characteristic is not limited to the above, and any one of the modules or nozzles may be used as a reference, and the measurement curve of that module or nozzle may be used as the target characteristic 2803. Alternatively, the target characteristic 2803 may be a curve obtained by averaging two or more measurement curves.

[0311] Alternatively, the target characteristics 2803 may be determined based on other values. For example, a curve in which the distance D from the recording medium color (paper white) in the CIE Lab space and the color signal value are linear may be stored in advance as the target characteristics. Here, the distance D from the paper white can be calculated using the following equation (1).

[0312] Returning to the flowchart of Fig. 27, in the following step S2704, the correction table creating unit 21065 acquires the corrected color signal values ​​corresponding to each color signal value, and creates the correction table shown in Fig. 24 as an example.

[0313] With reference to FIG. 28(b), the acquisition of the corrected color signal values ​​will be specifically described.

[0314] First, the nozzle or module number x for calculating the corrected color signal value and the color signal value 2804 are determined. At this time, a target value 2805 corresponding to the color signal value 2804 can be calculated from the target characteristics 2803. Furthermore, a corrected color signal value 2806 corresponding to the target value 2806 can be acquired from the measurement curve 2803 corresponding to the nozzle or module x.

[0315] At this time, the correction table creation unit 21065 associates the acquired corrected color signal value 2806 with the input color signal value 804, and stores it as a correction table 21066 for the nozzle or module number x.

[0316] At this time, it is also possible to calculate correction values ​​for all values ​​from 0 to 255 as input color signal values ​​2804 and store them as a table for the nozzle of interest.

[0317] Alternatively, as shown in Fig. 24, a total of 17 signal values, 0, 16, 32, . . . , 240, and 255, may be determined in advance, and only the values ​​corresponding to these signal values ​​may be calculated and stored as a table. In this case, when using the correction table, color signal values ​​that are not stored may be calculated, for example, by known interpolation processing using adjacent values.

[0318] The above process is repeated for all ink colors (CMYK) and nozzles or modules to create a correction table.

[0319] After creating the correction table, the correction table creating unit 21065 updates the newly created correction table as a correction table 21066 in the memory 2101 or the external storage device 2105 in S2705.

[0320] By following the flow shown in S2701 to 2705 above, it is possible to create or update a new correction table such as that shown in Fig. 24. By updating the correction table in this way, even if the characteristics of each nozzle or head module have changed since the previous creation, the values ​​in the correction table are changed in accordance with the changes, and as a result, it is possible to suppress the occurrence of unevenness and streaks caused by changes over time.

[0321] In the above explanation, the correction processing unit 2402 performs correction processing on the input image data (CMYK). However, the same effect can be obtained by performing correction processing on the threshold matrix for each image data used in the HT processing unit 2403.

[0322] [Detection of non-discharge nozzles] If the image includes a line pattern area, non-discharge nozzles may be detected from that area. When performing detection, it is preferable to change the phase of the line pattern according to the counter cnt so that all nozzles can be detected. In this case, it is preferable to match the number of phase changes, i.e., the line spacing, with the gradation changes in the gradation data acquisition area 25036, so that they can be managed with a single counter cnt.

[0323] For example, suppose four gradations (64, 128, 192, 255) are to be obtained for each ink color, and the line pattern is formed at intervals of four nozzles as shown in Figures 29(a) to (d).

[0324] 29(a) to 29(d) are diagrams conceptually illustrating HT patterns. Specifically, the grids in FIGS. 29(a) to 29(d) schematically show positions where the image forming unit 108 can place droplets. For example, if the print resolution in the nozzle arrangement direction x and the recording paper transport direction y is 1200 dpi, the height and width of each grid are 25.4 / 1200 [mm]. Furthermore, a black circle within a grid indicates that a dot will be placed at that position (ON).

[0325] 29(a) to (d), the retained nozzles are assigned nozzle numbers from left to right as 1, 2, 3, 4, etc., and a line is formed using the nozzles whose remainder when the nozzle number is divided by 4 is equal to the counter cnt. For example, if the counter cnt = 1, as shown in FIG. 29(a), a line is formed using the nozzles corresponding to C ink nozzle numbers 1, 5, 9, 13, etc., and non-discharge of the nozzles corresponding to these nozzle numbers is detected from the read image.

[0326] Similarly, when the counter cnt=2, 3, 4, line patterns in which the phases of the ejecting nozzles are different from each other may be formed, as shown in FIGS. 29(b), (c), and (d).

[0327] 29(a) to (d) are formed with M ink to detect non-ejection if counter cnt=5 to 9. Also, if counter cnt=10 to 14, line patterns as shown in Fig. 29(a) to (d) are formed with Y ink, and if counter cnt=10 to 14, line patterns as shown in Fig. 29(a) to (d) are formed with K ink to detect non-ejection.

[0328] In this way, by changing the nozzle position and ink color that form the non-discharge line in accordance with the counter cnt, non-discharge detection for all nozzles becomes possible.

[0329] [HT processing] As explained above with respect to Figures 25(a) to (d), in this embodiment, two HT images are printed: the input image (reference number 2501 in Figures 25(a) to (d)) and the gradation data acquisition area (reference number 25036 in Figure 25(d)).

[0330] In the description of the header image addition process (S2606) above, it was explained that it is preferable to use the same dithering process also in generating the HT pattern of the gradation data acquisition area 25036.

[0331] However, it is also possible to generate an HT pattern to be added to the gradation data acquisition area 25036 using an HT process different from the HT process applied to the input image data. For example, if the HT process applied to the input image data is a dither process, it is also possible to generate an HT pattern using an error diffusion method. Alternatively, it is also possible to use an HT pattern to add a regular pattern such as a checkered pattern.

[0332] Incidentally, the characteristics of a print head, module, or nozzle may depend on the ejection status of surrounding nozzles. For example, a power source or ink may be shared with surrounding nozzles or modules. In such cases, the amount, speed, and angle of the ejected ink droplets may differ between when the ink droplets are ejected simultaneously with the surrounding shared nozzles and when the ink droplets are ejected independently from each other. Furthermore, after a droplet lands, if there are droplets nearby that have already landed but have not yet fully penetrated, the two droplets may attract each other or combine.

[0333] In such a case, if the gradation data acquisition area 25036 is formed using an HT process that is different from the HT process for the input image data, there is a risk of a discrepancy occurring between the amount of correction obtained from the acquired gradation data and the unevenness or streaks that appear on the input image data. Therefore, if such a discrepancy is expected, it is preferable to use the same process for both HT processes.

[0334] For example, in the case of dither processing, it is preferable to use the same dither mask, and in the case of error diffusion processing, it is preferable to use the same diffusion coefficient, threshold value, and initial error.

[0335] Furthermore, in this embodiment, it is preferable to perform HT processing so that the number of dots in the transport direction is uniform for uniform input signal values. In other words, it is preferable to determine the dot arrangement so that the usage rate of each nozzle is approximately the same for uniform input signal values.

[0336] Specifically, when dithering is used as HT processing, pixels with small threshold values ​​in the dither matrix are more likely to become dot-on, while pixels with large threshold values ​​are more likely to become dot-off. Therefore, if the threshold values ​​of small pixels that are more likely to become dot-on are biased toward a particular nozzle row, some nozzles will frequently eject ink and others will not. As a result, nozzles with low ejection frequency are more likely to experience defects due to ink concentration.

[0337] Furthermore, if the dot positions of the HT pattern added as the gradation data acquisition area 25036 are biased toward a particular nozzle, the number of ink droplets ejected from each nozzle will differ. As a result, the read values ​​for the same input signal value may not match between nozzles with approximately the same characteristics.

[0338] For example, there is a risk that the read values ​​corresponding to nozzles with many dots allocated on the HT pattern may differ from the read values ​​corresponding to nozzles with few dots allocated due to the number of dots allocated.

[0339] It is difficult to obtain the ejection characteristics of each nozzle from read values ​​that do not substantially match due to the number of dots assigned rather than the nozzle characteristics, and as a result, the effectiveness of the correction process may be reduced.Furthermore, if there is a difference in the number of output dots for the same color signal value for each nozzle position, even if the density fluctuations caused by the ejection characteristics of each nozzle can be correctly corrected, the effectiveness of the correction (number of output dots) may differ for each nozzle, and the effectiveness of the density unevenness correction may be reduced.

[0340] In view of the above, in this embodiment, it is effective to use a dither matrix in which the arrangement of threshold values ​​is adjusted so that the frequency of dots formed by each nozzle for the same color signal value is uniform.

[0341] In this case, it is preferable that the difference in the total number of dots for each nozzle be less than 1 dot. Furthermore, if the size of the dither matrix is ​​256px x 256px, it is preferable that the total number of dots for each nozzle be less than 1 dot for a height of 256px.

[0342] For example, if the height of the gradation data acquisition area 25036 is 130 px, it is preferable that the total number of dots for each nozzle is approximately the same even within a limited area obtained by dividing the matrix by a height of 130 px.

[0343] [Number of shots in the preliminary ejection area] In the explanation up to this point, the preliminary ejection areas 25031 to 25034 have been described as so-called solid images. At this time, before the gradation data acquisition area 25036 is formed, as shown in Figure 25(d), each nozzle corresponding to that ink performs preliminary ejection with approximately the same number of shots as the other inks. Therefore, the gradation data acquisition area 25036 is unlikely to contain defects due to ink concentration.

[0344] On the other hand, the number of dots ejected into the preliminary ejection area affects the running costs of the image forming system, so it is preferable to eject as few dots as possible into the preliminary ejection area, but in an amount sufficient to discharge concentrated ink.

[0345] 25(d), when a line pattern is output between the preliminary ejection area and the gradation data acquisition area, ink is also ejected by outputting the line pattern. In other words, the nozzles can perform preliminary ejection to the same extent as the dots in the line pattern without ejecting dots in the preliminary ejection area.

[0346] At this time, the amount sufficient to discharge the concentrated ink refers to the number of dots discharged by other inks in the preliminary discharge regions 25031 to 25034, for example.

[0347] Specifically, it is assumed that the line pattern shown in Fig. 30(a) is formed by nozzle numbers x=1 to 8. Also, Fig. 30(a) is a so-called solid pattern in which dots are formed (ON) in all pixels.

[0348] In this case, the amount of ink can be reduced by using a pattern in which the same number of dots as the line pattern shown in Figure 30(b) is turned off instead of the solid pattern shown in Figure 30(a) as the HT pattern in the preliminary ejection area.

[0349] Specifically, in the example shown in Figure 30(b), only four dots are turned off for the nozzles corresponding to nozzle numbers 1, 5, 9, etc. In this case, the dots to be turned off can be any number, and it is also possible to use a pattern such as that shown in Figure 30(c), for example.

[0350] 25(d) can additionally hold patterns used for alignment, etc. For example, as shown in FIG. 31, position detection patterns 25037 to 25039 and a module transition pattern 25040 may be additionally held.

[0351] The position detection patterns 25037 to 25039 are rectangular areas formed only with K ink. In this case, each area may be, for example, a square area of ​​2.54 mm x 2.54 mm, and the signal value of all pixels may be 1 (ON). Note that circular areas may be used instead of rectangular areas, or a predetermined pattern may be used.

[0352] By providing the position detection patterns 25037 to 25039, it becomes possible to more easily acquire the read value corresponding to the gradation data acquisition area 25036 in the above-mentioned S2701. In other words, by searching for the position detection patterns in the scanned image and correcting the position, size, and angle of the rectangular area containing them, it is possible to easily acquire the two-dimensional data corresponding to the header image 2503.

[0353] Furthermore, the module transition pattern 25040 is a so-called solid pattern formed with the same ink color as the ink forming the gradation data acquisition area 25036. In this case, the module transition pattern 25040 is formed only in one of the head modules, and by referencing the edge of the pattern in the nozzle row direction (x direction), it is possible to estimate the module switching position on the read image.

[0354] For example, when using a configuration in which multiple head modules are combined to cover the paper width, such as the recording head shown by reference numeral 2201 in Figure 22(c), the dot spacing in the nozzle direction at the joints of the modules may become larger or smaller than the nozzle resolution due to mounting errors of the head modules. In such cases, the head modules may be arranged in a staggered pattern, as shown by reference numeral 2201 in Figure 22(c), and the ends of the modules may overlap. In this case, by distributing and forming the overlapping portions to both head modules, the joints described above can be made less visible.

[0355] In such a case, it is difficult to estimate from the read image which head formed the overlapping portion of the line pattern area 25035 and the gradation data acquisition area 25036.

[0356] At this time, by including a module transition pattern 25040 in the header image 2503, it becomes possible to easily estimate the joints of the head modules on the read image.

[0357] As described above, it is preferable to eject a sufficient amount of concentrated ink into the preliminary ejection area and to eject as few droplets as possible into the preliminary ejection area.

[0358] Therefore, if the header image 2503 separately holds patterns used for alignment, etc., it is preferable to adjust the number of dots in the preliminary ejection area so that a sufficient number of dots are ejected in combination with the number of dots ejected to form those patterns.

[0359] Specifically, it is assumed that a solid image of C ink is formed in the preliminary ejection area 25031. Similarly, it is assumed that the preliminary ejection area 25034, line patterns 25035, position detection patterns 25037 to 25039, module transition pattern 25040, and gradation data acquisition area 25036 are formed with K ink.

[0360] At this time, the number of dots ejected for each nozzle in the preliminary ejection area 25034 formed prior to the gradation data area 25036, the position detection patterns 25037 and 25038, and the module transition pattern 25040 is totaled. It is preferable that the number of dots ejected by each nozzle be equal to or greater than the number of dots ejected by each nozzle in the preliminary ejection area 25031 for C ink.

[0361] That is, in the above example, it is preferable that 120 droplets of K ink are distributed and ejected from each nozzle to the preliminary ejection area 25034, the position detection patterns 25037 and 25038, and the module transition pattern 25040.

[0362] [Fifth embodiment] In the fourth embodiment described above, the correction process is performed after one set of predetermined signal values ​​is acquired.

[0363] However, it is also possible to change the correction table and perform the correction process as soon as the readings for each signal value are obtained.

[0364] In this way, by performing correction immediately without waiting for one set to be acquired, it is possible to handle cases where the nozzle characteristics change rapidly, or cases where the header image holds the HT pattern intermittently.

[0365] The flow of printing a user image and correction processing based on the formed image in this embodiment will be described below using the flow diagram shown in Fig. 32. Note that the same configurations and processes as in the first embodiment are given the same reference numerals, and descriptions thereof will be omitted or simplified.

[0366] First, the image processing unit 2106 acquires input image data stored in the external storage device 2105 (S2601), and the input color conversion processing unit 21061 performs input color conversion on the acquired data (S2602).

[0367] Next, in S3201, the header image adding unit 21064 determines the ink colors and color signal values ​​that form the gradation data acquisition area 25036. For example, as in the first embodiment described above, HT patterns with different ink colors and color signal values ​​are selected in a predetermined order based on the counter cnt. Alternatively, HT patterns may be selected randomly from all combinations of ink colors and color signals. By selecting randomly in this way, there is no need to store and manage the counter cnt. There is also no need to determine and store the order in advance.

[0368] At this time, it is possible to randomly select one of all combinations of ink colors and color signals each time. Alternatively, the number of selections for each combination can be stored in RAM 2101, and a combination with the fewest selections can be randomly selected. Furthermore, color signals can be randomly selected from only HT patterns corresponding to, for example, five predetermined values ​​(0, 64, 128, 192, 255). By limiting the color signal values ​​in this way, the frequency with which HT patterns are reselected increases. As a result, the frequency with which characteristics are acquired for each signal value also increases. Therefore, by limiting the color signal values, it is possible to immediately acquire changes even if the nozzle characteristics fluctuate suddenly. Alternatively, all possible values ​​of the color signal value can be randomly selected from 0 to 255, for example, if the color signal value is 8 bits. By selecting from all possible values ​​of the color signal value in this way, it is possible to acquire and correct changes, even if a fluctuation occurs only in a specific color signal value.

[0369] In the following S3202, the header image adding unit 21064 adds a header image 2502 to the image data after the HT process. Note that the header image 2502 added in this embodiment includes the HT pattern selected in S3201 as a gradation data acquisition area 25036.

[0370] Next, in S2604, the correction processing unit 21062 performs correction processing on the image data after color conversion based on the correction table 21066. Furthermore, in S2605, the HT processing unit 21603 performs HT processing on the image data after correction processing.

[0371] Next, in S2607, the image forming unit 2107 forms an image on paper based on the HT image to which the header image has been added, thereby creating a formed image 2400.

[0372] In the following S2608, the image acquisition unit 2108 captures the formed image 2400 and acquires two-dimensional captured image data.

[0373] In the next step S3203, a correction table is created and updated based on the acquired gradation data by the correction table creation unit 21065. Details of the process will be described later.

[0374] In the next step S2611, the image processing unit 2106 determines whether the submitted print job has been completed. If it is determined that the print job has been completed, the flow ends. On the other hand, if it is determined that the print job has not been completed, the flow returns to S2604, where correction processing is performed using the updated correction table, and printing continues.

[0375] [Update correction table] The correction table update process in S3203 described above will be described below.

[0376] In this embodiment, the correction process (S2604) and HT process (S2605) are performed including the header image added in S3202. Therefore, the read value obtained in S2608 includes the result of correction using the current correction table. Therefore, it is sufficient to reduce the color signal value in the dark area of ​​the read value, and similarly increase the color signal value in the light area of ​​the read value.

[0377] Specifically, the data is one-dimensional line data in the nozzle row direction, and for nozzles with a value greater than the average, the signal value for the color signal value is decreased by 1. For nozzles with a value smaller than the average, the signal value is increased by 1.

[0378] At this time, each read value contains measurement errors due to dark current of the sensor and dust or scratches on the formed image. To suppress correction for such errors, a threshold value for correction may be set. That is, if the value is greater than the average value by more than the threshold value, the signal value may be reduced. Similarly, if the value is less than the average value by more than the threshold value, the signal value may be increased.

[0379] Alternatively, the greater the difference from the average value, the greater the amount of change to be applied to the color signal.

[0380] Alternatively, in addition to the reading value acquired this time, it is also possible to estimate the measurement curve shown by curve 2802 in Figure 28 from, for example, the most recent five reading values ​​using a known interpolation method, and calculate the corrected color signal value based on the measurement curve as in the above-mentioned embodiment 4.

[0381] [Sixth embodiment] By adding a header image according to the above-described embodiment, concentrated ink is discharged by preliminary ejection, and the characteristics of each nozzle, head, and module can be obtained from the read values, and the correction table can be updated.

[0382] At this time, if the amount of ink ejected in the preliminary ejection region is small, concentrated ink may remain in the nozzles, causing defects such as non-ejection, unevenness, and streaks in the gradation data acquisition region and image data formation region.

[0383] On the other hand, if a large amount of ink is ejected onto the preliminary ejection area, the running costs and productivity may deteriorate due to increased ink consumption and replenishment frequency.

[0384] In view of the above, it is preferable to eject ink into the preliminary ejection area in an amount sufficient to expel the concentrated ink and in as few shots as possible.

[0385] However, the amount of preliminary ejection that is sufficient also depends on the input image data. For example, if all nozzles eject a sufficient number of ink droplets when forming the image data, preliminary ejection may not be necessary. On the other hand, the fewer the number of ejection droplets in the image data, the more dots need to be ejected in the preliminary ejection area.

[0386] Alternatively, the amount of sufficient preliminary ejection also depends on the printing environment, such as humidity, temperature, etc. Therefore, a header image set under certain conditions may be too large or too small for different image data or printing environments.

[0387] In this case, if the amount of preliminary ejection is insufficient and the above-mentioned defects occur, particularly in the input image data area, the value of the resulting printout may be significantly diminished. Therefore, it is necessary to eject a sufficient amount of ink before generating the input image data.

[0388] On the other hand, there may be cases where a defect is included in the gradation data acquisition area, but the number of dots in that area is sufficient for preliminary ejection, and the input image data area does not include any defect.

[0389] In such cases, although gradation data cannot be acquired, the value of the print is unlikely to be significantly diminished. However, if a correction table is created using data that includes defects, the color signal may be corrected to cancel out streaks that should have been eliminated by preliminary ejection, which could instead result in inverted streaks, thereby diminishing the value of the print.

[0390] In view of the above, in this embodiment, an embodiment will be described in which a defect determination unit that determines defects caused by concentrated ink is provided.

[0391] The same configurations and processes as those in the fourth and fifth embodiments are denoted by the same reference numerals, and the description thereof will be omitted or simplified.

[0392] [Functional configuration of the image processing unit] Fig. 33 shows an example of the configuration of the image processing unit 2106 in this embodiment. As shown in Fig. 33, the image processing unit 2106 in this embodiment includes a defect determination unit 21067 in addition to the configurations of the fourth and fifth embodiments shown in Fig. 3 above.

[0393] The defect determination unit 21067 receives two-dimensional image data as read data from the image reading unit 2108 and determines whether or not each region contains defects caused by concentrated ink, or whether a sufficient number of dots have been ejected to discharge concentrated ink and each nozzle has been refreshed before image formation in each region.

[0394] The above determination can be made, for example, by focusing on the read values ​​of portions of the area that are located at different positions in the y direction and determining whether there is a significant difference in the read values. In other words, if there is a significant variation in the read values ​​depending on the formation time between areas formed with approximately the same color signal values, it can be assumed that concentrated ink has been discharged and refreshing has not been completed. Alternatively, it can be assumed that there is a defect caused by concentrated ink, such as a density variation.

[0395] [Determination process in the defect determination unit 21067] Fig. 34 is an enlarged view of a portion of the schematic diagram of the header image shown in Fig. 25(d). The above-mentioned determination process will be explained in more detail below with reference to Fig. 34. Note that the following description will focus on the process of determining whether or not a defect is included in the preliminary ejection area 25032 in Fig. 34.

[0396] First, the defect determination unit 21067 refers to the area corresponding to the preliminary ejection area 25032 in the read data obtained from the image data reading unit, and obtains read values ​​corresponding to two portions 25032a and 25032b formed at different times. In this embodiment, a 16-bit value linear with respect to Y in the CIEXYZ color space is obtained as the read value.

[0397] The defect determination unit 21067 averages the obtained read values ​​for both parts in the conveying direction (y direction) to obtain a one-dimensional line profile.

[0398] An example of a line profile obtained by the above averaging process is shown in Fig. 35. In Fig. 35, a solid line 3501a is an example of a line profile obtained from a portion 35032a. A dotted line 3501b is an example of a line profile obtained from a portion 35032b.

[0399] Furthermore, the defect determination unit 21067 compares the two line profiles, calculates the difference for each pixel, and obtains a differential profile. An example of the differential profile is shown by the dashed line 3502 in Figure 35. The defect determination unit 21067 then focuses on the maximum value of the differential profile, and if the maximum value is equal to or greater than a predetermined threshold, determines that the region contains a defect.

[0400] In this case, instead of a predetermined threshold value, it is also possible to use a value dynamically generated from each line profile. For example, it is possible to calculate the variance σ of the line profile dotted line 3501b corresponding to the part formed at a later time, and use three times that σ, i.e., 3σ, as the threshold value.

[0401] It should be noted that, rather than making a judgment based on the difference between the line profiles obtained from two areas formed at different times as described above, it is also possible to make a judgment by comparing a line profile obtained in advance with the line profile of the area to be judged.

[0402] For example, after sufficient preliminary ejection, a uniform area is formed for each color signal value, and one-dimensional line profiles are calculated and stored from the read data. Then, in the judgment, the corresponding line profile is obtained based on the color signal value of the judgment area, and the maximum value of the difference is compared with a threshold value.

[0403] In this case, the line profile acquired after sufficient preliminary ejection is unlikely to contain defects caused by concentrated ink. In other words, if the difference between the line profile and the line profile of the judgment area is small, it can be determined that the area is unlikely to contain defects. In this case, it can be determined that preliminary ejection was completed before the difference judgment area.

[0404] In this way, by making a determination based on a line profile acquired in advance, it is possible to determine whether or not preliminary ejection has been completed up to the determination portion.

[0405] For example, suppose that a portion 25036b located at the end of writing in the gradation acquisition area 25036 in FIG. 34 is compared with a line profile acquired in advance, and the maximum value of the difference is equal to or less than a threshold value.

[0406] At this time, the preliminary ejection is completed before the portion 25036b, and it can be determined that there is a possibility that defects caused by concentrated ink may occur in the image area formed after this portion.

[0407] Instead of using a line profile acquired in advance as described above for the judgment, it is also possible to make the judgment based on the difference from a line profile that is approximately constant regardless of the x position. For example, at the time of judgment, the average value of the line profiles of the judgment area is calculated. Then, it is also possible to determine a constant profile using the calculated average value regardless of the x position, and make the judgment based on the maximum value of the difference from that profile and a threshold value.

[0408] [User image printing flow] The flow of printing a user image and correction processing based on the formed image in this embodiment will be described below with reference to the flowchart shown in FIG.

[0409] First, the image processing unit 2106 acquires (S3601) input image data stored in the external storage device 2105. Then, the input color conversion processing unit 21061 performs input color conversion processing on the acquired data (S3602).

[0410] Next, in S3603, the correction processing unit 21062 performs correction processing based on the correction table 21066 on the image data after color conversion.

[0411] Furthermore, in S3604, the HT processing unit 21603 performs HT processing on the image data after the correction processing.

[0412] Next, in S3605, the header image adding unit 21064 determines the ink color and HT pattern that form the gradation data acquisition area 25036. In this step, the ink color and color signal value are determined in the same manner as in S2201 described above. Furthermore, based on the determined ink color and color signal value, a previously stored HT pattern is acquired or determined.

[0413] In the following S3606, the header image adding unit 21064 adds a header image 2502 to the image data after the HT process. Note that the header image 2502 added in this embodiment includes the HT pattern selected in S3604 above as the gradation data acquisition area 25036.

[0414] Furthermore, in this embodiment, among the preliminary ejection regions 25031 to 25034, in regions corresponding to the same ink color as the ink color forming the gradation data acquisition region 25036, an HT pattern is formed instead of a solid pattern.

[0415] At this time, the header image adding unit 21064 adds an inverted HT pattern, which is an inverted version of the HT pattern added to the gradation data acquisition region, to the corresponding preliminary ejection region.

[0416] More specifically, when the HT pattern shown in FIG. 37(a) is added to the gradation data acquisition area and formed with C ink, the inverted HT pattern shown in FIG. 37(b) is used in the preliminary ejection area 25031 corresponding to C ink.

[0417] On the other hand, solid images are formed in the other preliminary ejection regions in this embodiment as well. That is, when an HT pattern is formed in the preliminary ejection region 25031, solid patterns are formed in the other preliminary ejection regions 25032 to 25034.

[0418] In this embodiment, the inversion pattern is a pattern in which the ON / OFF of each pixel is inverted. That is, pixel positions where a black circle exists (ON) in the pattern shown in Fig. 37(a) are not present (OFF) in the inversion pattern shown in Fig. 37(b). Similarly, pixel positions where a black circle does not exist (OFF) in the pattern shown in Fig. 37(a) are not present (ON) in the inversion pattern shown in Fig. 37(b).

[0419] Returning to the flowchart shown in Fig. 36, in the following step S3607, the image forming unit 2107 forms an image on paper based on the HT image to which the header image has been added, thereby creating a formed image 2400.

[0420] In the following S3608, the image acquisition unit 2108 captures the formed image 2400 and acquires two-dimensional captured image data.

[0421] In the next step S3609, the defect determination unit 21067 determines whether or not the area using the HT pattern among the preliminary ejection areas includes a defect caused by concentrated ink, i.e., whether or not preliminary ejection has been completed before the preliminary ejection area.

[0422] For example, suppose the header image shown in Fig. 34 has been added in S3606 described above, and furthermore, of the four preliminary ejection regions, an HT pattern is used in region 25032. In this case, a determination can be made based on the difference between the line profiles obtained from two different regions 25032a and 25032b within region 25032 shown in Fig. 34.

[0423] Specifically, the readings from both parts are averaged in the transport direction to obtain two line profiles. Also, a threshold value h, which is determined based on the fluctuation specifications of each nozzle or module at the time of factory shipment, is obtained.

[0424] At this time, if the differences between the two line profiles for each x position are all equal to or less than the threshold value h, it can be determined that preliminary ejection was completed before forming the area 25032. At this time, there is a low possibility that the area contains a defect.

[0425] On the other hand, if either difference is greater than the threshold, it can be determined that preliminary ejection was not completed before forming the area 25032. In this case, there is a possibility that the area contains a defect.

[0426] It is also possible to make a determination based on the average read value of each portion, obtained by averaging the read values ​​within each portion, rather than the line profile of each portion. In other words, if the difference between the average read values ​​is greater than a threshold value, it may be determined that preliminary ejection is not complete.

[0427] If it is determined in step S3609 that the preliminary discharge has already been completed, the process proceeds to step S3610.

[0428] In S3610, the correction table creation unit 21065 updates the correction table based on the read values ​​of the preliminary ejection area and the gradation data acquisition area using the HT pattern. Details of the processing in S3610 will be described later. After updating the correction table, the process proceeds to S3611.

[0429] In S3611, the image processing unit 2106 determines whether the submitted print job is complete. If it is determined that the print job is complete, the flow ends. On the other hand, if it is determined that the print job is not complete, the flow returns to S3603, where correction processing is performed using the updated correction table, and printing continues.

[0430] Incidentally, if it is determined in step S3609 that preliminary ejection has not been completed before the preliminary ejection region, the process proceeds to step S3612.

[0431] In S1612, the defect determination unit 1067 determines whether or not there is a defect caused by concentrated ink in the gradation data acquisition area 25036. In other words, it determines whether or not the preliminary ejection is completed before the gradation data area.

[0432] For example, the determination may be made based on the difference between two different areas 25036a and 25036b in the gradation data acquisition area shown in FIG.

[0433] Alternatively, it can be determined from a preliminary ejection area formed with the same ink color as the gradation data acquisition area 25036 and an HT pattern.

[0434] Specifically, it is assumed that an HT pattern is formed in the preliminary ejection region 25032 in Fig. 34. At this time, the defect determination unit 21067 acquires the line profile of a portion 25032b located at the end of writing in that region.

[0435] Then, the difference between the line profile obtained from the portion 25032b and a line profile formed with the same ink color and the same color signal value after sufficient preliminary ejection is performed in advance is calculated for each pixel.

[0436] At this time, if the maximum value of the differences is smaller than a predetermined threshold value, for example, it may be determined that the preliminary ejection is completed before the gradation data acquisition region 25036 is formed.

[0437] That is, it may be judged whether the line profile of the portion 25032b formed immediately before forming the gradation data acquisition region 25036 substantially matches the line profile obtained by sufficient preliminary ejection.

[0438] If preliminary ejection is completed in the portion 25032b, concentrated ink will remain in the nozzle at the start of writing the gradation data acquisition region 25036, and there is a low possibility that defects caused by concentrated ink will be present in that region.

[0439] If it is determined in S3612 that the preliminary ejection has been completed before the gradation data acquisition region is formed, the process proceeds to S3613.

[0440] In S3613, the correction table is updated based on the read values ​​acquired from the gradation data acquisition area by the correction table creation unit 21065. Details of the processing in S3613 will be described later.

[0441] After updating the correction table in S3613, the process proceeds to S3611, where it is determined whether the print job is completed.

[0442] On the other hand, if it is determined in step S3612 that preliminary ejection has not been completed, the process proceeds to step S3614.

[0443] In S3614, the defect determination unit 21067 determines whether or not the area where the input image data is to be formed contains defects caused by concentrated ink, i.e., whether or not the preliminary ejection is completed before the area where the input image data is to be formed.

[0444] Specifically, when the header image 2502 shown in FIG. 34 is used, the determination is made from the difference between the line profile of a portion 25036b located at the end of writing in the preliminary ejection region 25032 and the line profile acquired in advance.

[0445] That is, if the line profile of the portion 25036b located at the end of writing of the header image 2502 substantially matches the line profile of a line obtained by sufficient preliminary ejection, it can be determined that preliminary ejection is complete.

[0446] If it is determined in S3614 that preliminary discharge has been completed before the input image data formation area, the flow proceeds to S3611, where it is determined whether the print job has been completed.

[0447] On the other hand, if it is determined that the preliminary ejection is not complete, a reprint process is instructed. Specifically, of the image data formed in the most recent S3607, all preliminary ejection within the header image is switched to a solid pattern. Then, the process returns to S3607, and the image data switched to a solid pattern is printed again.

[0448] By following the flow shown in FIG. 36 described above, it is possible to print a user image and perform correction processing based on the formed image.

[0449] In this embodiment, the preliminary ejection region and the gradation data acquisition region are formed using an HT pattern. By forming an HT pattern in the preliminary ejection region as well, gradation data can be acquired from the preliminary ejection region as well. This makes it possible to increase the amount of gradation data that can be acquired while suppressing an increase in ink ejected within the header image. Therefore, even when nozzle characteristics change more rapidly, it is possible to suppress an increase in ink volume and to suppress unevenness and streaks that accompany the changes.

[0450] At this time, it is preferable to determine the HT pattern so that the total number of dots ejected into both the preliminary ejection region and the gradation data acquisition region is the same as the number of dots ejected into the preliminary ejection region of the other ink colors.

[0451] 37(a) and (b), by forming an inverted HT pattern in the gradation data acquisition area and the preliminary ejection area, the number of dots from each nozzle in both areas combined will match the number of dots ejected in the preliminary ejection area of ​​the other ink. Therefore, even if there is a bias in the threshold value of the HT matrix, the number of ejections in the header image can be guaranteed.

[0452] At this time, in the gradation data acquisition area that will be printed later, the preliminary ejection will have been completed, and there is a high possibility that gradation data that does not contain defects can be acquired. Therefore, it is preferable to form in the gradation data acquisition area a pattern obtained by the same process as the HT process in S3604, rather than an inverted pattern.

[0453] Instead of using the above inverted pattern, the color signal values ​​may be distributed to both areas so that the total color signal value is 255 or 256. In this case, the header image determination (S3605) and addition (S3606) processes are performed prior to the correction process (S3603) and HT process (S3604).

[0454] At this time, in the HT processing, the HT processing unit 21063 performs the HT processing on the gradation data acquisition region and the preliminary ejection region in the same manner as the input image data.

[0455] In this way, by distributing color signal values ​​instead of HT patterns, it is possible to form the same HT pattern as the HT processing used in S3604 even in the preliminary ejection area, and in some cases correction can be performed with higher accuracy.

[0456] In particular, when the ink ejection frequency is high relative to the ink concentration and it is often determined in S3609 that preliminary ejection is completed before the preliminary ejection area, it is often more effective to distribute color signal values ​​rather than the HT pattern.

[0457] As described above, in this embodiment, the failure determination unit 21067 can determine whether the preliminary ejection is complete or incomplete, as shown in FIG.

[0458] For example, if there is a defect in the data acquisition area (n in S3612), the correction table is not updated, and it is possible to suppress updating of the correction table based on errors caused by concentrated ink.

[0459] Furthermore, if there is a low possibility that the preliminary ejection area contains a defect (y in S3609), it is possible to refer to the read value of that area when updating the correction table. Alternatively, if there is a possibility that preliminary ejection will not be completed by the time the input image area is reached (n in S3614), reprinting can be performed. Furthermore, at this time, the amount of preliminary ejection can also be increased.

[0460] In this embodiment, the determination in S3609 or S3614 is not essential. That is, after reading in S3608, the defect determination unit 21067 may determine in S3612 only whether preliminary discharge has been completed in the data acquisition area. In that case, it is also possible to proceed to S3613 to determine whether to update the correction table using the read value of that area, or to proceed to S3611 without updating.

[0461] In this case, in addition to the above, it may be determined whether or not the preliminary ejection area contains a defect, or it may be determined additionally whether or not the preliminary ejection has been completed up to the input image area, without determining whether or not the preliminary ejection area contains a defect.

[0462] [Correction table update process] The correction table update processing in S3610 and S3613 will be described below.

[0463] In this embodiment, the correction table can also be updated in the same manner as in S2610 or S3203 described above.

[0464] That is, as explained above using Figures 28(a) and (b), it is possible to calculate a correction curve 2802 for each print head, module, or nozzle, and determine corrected color signal values ​​2806 so as to reproduce target characteristics 2803.

[0465] At this time, in this embodiment, in addition to the newly obtained read values ​​in S3608, the correction curve 2802 is estimated from, for example, the five most recent read values. In this way, the correction curve is changed based on the read values ​​obtained for each print. Furthermore, by overwriting the correction table with the newly obtained color signal values, even if the characteristics of the print head, module, or nozzles have changed over time, the occurrence of unevenness and streaks can be suppressed by updating the correction table.

[0466] At this time, in S3612, the correction curve is changed based on the read value obtained from the gradation data acquisition area 25036.

[0467] On the other hand, in S3610, the correction curve is changed using the read values ​​obtained from the preliminary ejection area in addition to the gradation data acquisition area 25036. Therefore, it is possible to obtain a correction curve that is more responsive to changes in characteristics over time.

[0468] Note that the above-described correction table may be updated by updating only values ​​corresponding to some of the input color signal values. For example, by focusing only on "16" as the input signal value 2804, only the corrected color signal value 2806 corresponding to the color signal value "16" may be calculated in S3610 or S3613.

[0469] At this time, the color signal values ​​to be updated may be determined based on the color signal values ​​corresponding to the HT pattern formed in the gradation data acquisition area 25036 or the preliminary ejection area.

[0470] More specifically, only the color signal values ​​corresponding to the HT pattern may be updated after correction.

[0471] For example, it is assumed that the gradation data acquisition area 25036 is formed using an HT pattern corresponding to a color signal value of "16" and K ink, and that the table shown in Fig. 24 is used as the correction table.

[0472] In this case, in S3613, first, a correction curve corresponding to the head module 2201a is calculated based on the read values ​​obtained from the gradation data acquisition area 25036.

[0473] Furthermore, using the obtained correction curve and "16" as the input signal value 2804, a corrected color signal value 2806 is calculated. Then, the obtained color signal value 2806 is overwritten on the corrected color signal "14" stored in the row of the input color signal "16" and in column 2201a.

[0474] Similarly, for head modules 2201b and 2201c, corrected color signal values ​​for the input color signal "16" are calculated and overwritten.

[0475] In addition to the above, it is preferable to update the color signal values ​​corresponding to the HT pattern in the preliminary ejection area in S3610.

[0476] Furthermore, not only the color signal values ​​corresponding to the HT pattern but also all color signal values ​​affected by the HT pattern may be updated. For example, if interval linear interpolation is used to estimate the correction curve, the color signal values ​​in the interval including the newly added color signal value may be updated.

[0477] [Variations in defect detection processing] 35, the read values ​​for each pixel position x fluctuate more significantly than those for the dotted line 3501b, with peaks at some pixel positions.

[0478] These variations in position and the average readings are thought to be due to ink concentration within the nozzles. For example, if the frequency of use of each nozzle differed in the most recent image formation, the degree of concentration that occurs in each nozzle will differ. In such a case, if a uniform pattern is formed, the degree of concentration will appear as differences in density on the image. Furthermore, these are obtained as variations in the readings for each pixel position x on the line profile of the readings.

[0479] Furthermore, the more concentrated the ink, the higher the ratio of pigments and resins contained in the ink, which can result in higher ink viscosity. In such cases, a nozzle may not eject droplets in response to an ejection signal, resulting in a blank shot. When such a blank shot occurs, the number of dots ejected at the x position corresponding to the blank shot nozzle decreases, and blank spots may appear on the image. In this case, blank spots on the image are captured as peaks in the read values ​​on the line profile of the read values.

[0480] Furthermore, even if the frequency of use of each nozzle is approximately the same, if that frequency is low, the ratio of pigment contained per unit number of shots will be high, and as a result, the density of the formed image may be high. Such fluctuations in density are captured as fluctuations in the average value of the line profile on the line profile of the read values.

[0481] 34, the portion 25032a is located at the beginning of the preliminary ejection region 25032, and is the portion most likely to be affected by ink concentrated in the nozzle. On the other hand, the portion 25032b is located at the end of the preliminary ejection region 25032, and is likely to be unaffected by ink concentration if the height of the preliminary ejection region 25032 is sufficient.

[0482] Therefore, for example, by referring to the line profile for the portion 25032a shown as dotted line 3501a in Figure 35, if the fluctuations in the read values ​​for each pixel position x, as well as the fluctuations in the peak and average values, are greater than a certain level, it can be determined that the area in question contains the effects of defects due to ink concentration.

[0483] On the other hand, by referring to the line profile in the area 25032b shown as dotted line 3501b in Figure 35, if there are no fluctuations in the read values ​​for each pixel position x, peaks, or average values, it can be determined that refreshing has been completed in that area.

[0484] In this case, whether the variation at each position is above a certain level can be determined, for example, by checking whether the variance σ obtained from each line profile is greater than a predetermined threshold. Furthermore, a peak in the readout value can be determined by generating a differential profile by subtracting the average value from each line profile and then judging the peak based on the maximum value in the differential profile. That is, whether a peak exists can be determined by checking whether the maximum value obtained from the differential profile is greater than, for example, a predetermined threshold. Furthermore, the variation in the average value can be determined by checking whether the average value obtained from each line profile is within a predetermined range.

[0485] Note that peaks may be determined based on the maximum read value of a high-frequency profile obtained by applying a high-pass filter to a line profile, rather than using a differential profile from the average value. Furthermore, the threshold value may be dynamically generated from each line profile, rather than being a predetermined value. For example, the variance of each line profile may be calculated, and the smallest value among them may be set as σ, and three times that value may be used as the threshold.

[0486] The defect determination unit 21067 may make a determination based on the number of ejections rather than the read value. That is, in addition to the additional area 2502, the number of ejections in the image data 2501 is counted and stored for each nozzle. Then, when the count number for all nozzles within a predetermined time reaches a specified number, it may be determined that refreshing is complete.

[0487] Furthermore, the judgment in the defect judgment unit 21067 may be made by focusing on only one of the viewpoints explained so far, or may be made comprehensively based on the results of several judgments.

[0488] <Other embodiments> The resolution in the transport direction (y direction) of the two-dimensional scanned image data acquired by the image reading unit 2108 described above depends on the sensor's reading frequency and the paper transport speed. The acquired image is also affected by lens blur and the sensor's Bayer pattern. Therefore, particularly when the paper transport speed is high, these factors, rather than density fluctuations caused by concentrated ink, may prevent accurate acquisition of the read value.

[0489] In such cases, updating the correction table based on the read value of the HT pattern contained in the header image may result in unevenness or streaks.

[0490] In this case, the image processing unit 2106 may not include the correction table creating unit 21065, but may include a time-dependent change determining unit that determines changes over time.

[0491] In this case, it is preferable to use a solid pattern in each preliminary ejection region in the header image 2502 so that preliminary ejection is completed as far as possible by the time the gradation data acquisition unit 25036 is reached.

[0492] The time-varying change determining unit compares the line profile acquired by the gradation data acquiring unit 25036 with the line profile before the time-varying change occurred, and determines that the time-varying change has occurred if the difference is large.

[0493] At this time, the difference may be calculated for each pixel, or may be calculated based on an average read value obtained by averaging the line profiles.

[0494] In this case, the line profile before the time-dependent change occurs may be acquired in advance, or may be acquired when the image forming apparatus is turned on. Alternatively, a line profile that is constant regardless of the x position may be used.

[0495] The aging determination unit constantly monitors the header image in each print 2400, and if it determines that the header image has aged, it issues a warning to the user via the display unit 2104.

[0496] Alternatively, printing may be stopped, or a calibration chart having a sufficient height to withstand the effects of the above-mentioned sensors, lenses, and conveying speed may be acquired separately instead of the header image, and the correction table may be updated based on the readings of that chart.

[0497] In the embodiments described so far, so-called cut paper that has been cut in accordance with a dimensional standard such as A3, which is an international standard, can be used as the recording paper 2206. Alternatively, so-called roll paper, which is wound around a support without being cut in the transport direction, can also be used.

[0498] 38 shows an example of conveyance when cut paper is used as the recording paper 2206. As shown in the figure, a header image 2503 is formed in a position 2502 that is printed on the single printing medium 2206 prior to an input image 2501.

[0499] On the other hand, Fig. 39 shows an example of conveyance when roll paper is used as the recording paper. As shown in the figure, a header image 2503 is formed on the same recording medium as an input image 2501, and is added to a position 2502 where it is printed before the input image 2501.

[0500] It should be noted that the width of the header image 2503 does not need to match that of the input image 2501. For example, as shown in Fig. 40, it is also possible to always add a header image 2503 that is equal to the paper width, regardless of the width of the input image 2501.

[0501] Furthermore, the positional relationship between the preliminary ejection regions 25031 to 25034, the line pattern region 25035, and the gradation data acquisition region 25036 is not limited to the positions shown in FIG. 25(d) and FIG.

[0502] For example, as shown in Figures 41(a) to 41(c), a line pattern region 25035 and a gradation data acquisition region 25036 may be formed at a position that intersects with the preliminary ejection regions 25031 to 25034. Alternatively, as shown in Figure 41(d), it is also possible to intersect only the gradation data acquisition region 25036.

[0503] In either case, however, it is preferable to arrange the line pattern area and gradation data acquisition area so that they are formed after the preliminary ejection area of ​​the ink color that forms both areas, in order to suppress the effects of concentrated ink.

[0504] For example, when the line pattern region 25035 and the gradation data acquisition region 25036 are formed in the same color as the preliminary ejection region 25031, the gradation data after preliminary ejection can be acquired using any of the header images shown in Figures 41(a) to 41(c). Alternatively, it is also possible to acquire the data using the header images shown in Figures 25(d) and 31.

[0505] On the other hand, when forming the area with the same ink color as the preliminary ejection area 25033, it is preferable to use the header image shown in FIG. 41(c) or FIG. 25(d) and FIG.

[0506] In this way, when the header images shown in Figures 41(a) to (d) are used, the gradation data acquisition area is formed immediately after the preliminary ejection, so the influence of more concentrated ink can be eliminated. On the other hand, with the header images shown in Figures 25(d) and 31, there is no need to change the layout of the header image for each ink formed, which makes it particularly easy to acquire characteristic data from the scanned image.

[0507] 25(d), 31, and 41(a) to (c), the line pattern region 25035 and the gradation data acquisition region 25036 are adjacent to each other, which is preferable because it allows for accurate estimation of the correspondence between the reading position and the nozzle position, especially when there is a large paper transport error. In this case, it is preferable to make the widths and heights on the paper surface of the preliminary ejection regions 25031 to 25034, the line pattern region 25035, and the gradation data acquisition region 25036 approximately the same, because this allows for common processing when cutting out each region from the read image.

[0508] The invention is not limited to the above-described embodiments, and various changes and modifications can be made without departing from the spirit and scope of the invention. Accordingly, the following claims are appended to apprise the public of the scope of the invention. [Explanation of symbols]

[0509] 1...image forming system, 11...image processing device 11, 12...image forming device, 100...CPU, 101...RAM, 102...ROM, 106...image processing unit, 110...I / F unit, 107...image forming unit, 108...image sensor, 109...maintenance unit, 111...I / F unit, 114...RAM, 103...operation unit, 104...display unit, 105...external storage device

Claims

1. image acquisition means for acquiring an output image; an image adding means for adding an additional image to the output image; an image forming unit that includes a plurality of nozzles for ejecting a recording material, and ejects the recording material onto a recording medium to form the output image to which the additional image is added; a reading means for reading the formed image as a read image; Equipped with the additional image includes an acquisition area for acquiring characteristics of the image forming means based on the read image, The acquisition area includes a preliminary ejection area for performing preliminary ejection of the image forming means. An image forming apparatus characterized by:

2. 2. The image forming apparatus according to claim 1, wherein the preliminary ejection area is formed prior to the acquisition area in the additional image, and the acquisition area is formed prior to the output image.

3. 3. The image forming apparatus according to claim 1, wherein the acquisition area is formed by an HT pattern corresponding to a color signal value determined for each output.

4. 2. The image forming apparatus according to claim 1, further comprising: an acquisition unit that acquires characteristics in the nozzle row direction based on a read value obtained from the preliminary ejection area.

5. 5. The image forming apparatus according to claim 4, further comprising: a non-uniformity correcting unit for suppressing variations in the characteristics based on the characteristics in the nozzle row direction.

6. An image forming apparatus according to any one of claims 3 to 5, further comprising an HT processing means for performing HT processing on the output image, wherein the HT pattern of the acquisition area is an HT pattern obtained for a predetermined color signal by the same processing as the HT processing used by the HT processing means.

7. 7. The image forming apparatus according to claim 1, further comprising a determining unit for determining whether or not preliminary ejection has been completed in each region of the read image.

8. 8. The image forming apparatus according to claim 7, wherein only when the determination means determines that preliminary ejection is completed before forming the preliminary ejection area, the characteristics in the nozzle array direction are obtained by further using the read value obtained from the preliminary ejection area.

9. 9. The image forming apparatus according to claim 7, wherein the characteristics in the nozzle row direction are acquired only when the determining unit determines that preliminary ejection is completed before the acquisition area is formed.

10. 10. The image forming apparatus according to claim 1, further comprising: a step of: based on the characteristics obtained from the acquired area, warning a user that a change over time is occurring; stopping image formation; or outputting a dedicated chart for new correction.

11. A program for causing a computer to function as each of the means of the image forming apparatus according to any one of claims 1 to 10.

12. an image acquisition step of acquiring an output image; an image adding step of adding an additional image to the output image; an image forming step in which a plurality of nozzles for ejecting a recording material are provided, and the recording material is ejected onto a recording medium to form the output image to which the additional image is added; a reading step of reading the formed image as a read image; Equipped with the additional image includes an acquisition area for acquiring characteristics of the image forming process based on the read image, The acquisition area includes a preliminary ejection area for performing preliminary ejection in the image forming process. An image forming method comprising:

Images