Image compression apparatus, image forming apparatus, inspection apparatus, image compression method and recording medium

Abstract

An image compression apparatus includes: a compression unit configured to receive image data and a parameter for compression, and perform compression processing on the image data based on information of the image data and the parameter using a quantization table of a luminance signal and a quantization table of a color signal; and a parameter setting unit configured to set the parameter in the compression unit. The parameter setting unit sets an upper limit of the parameter of the quantization table for the color signal to a value less than or equal to alpha, 90% or more of a parameter of an AC component of the quantization table for the luminance signal is set to have a value greater than or equal to a corresponding parameter of an AC component of the quantization table for the color signal.

Description

Technical Field

[0001] The present disclosure relates to an image compression device, an image forming device, an inspection device, an image compression method, and a recording medium. Background Art

[0002] Conventionally, in order to improve the compression effect of a read image such as a document, a technology has been disclosed that compresses a text area and a background area of ​​the read image using different quantization tables.

[0003] List of citations

[0004] Patent Literature

[0005] [PTL1] Japanese Unexamined Patent Application Publication No. 2009-060474 Summary of the Invention

[0006] Technical issues

[0007] As described in the related art, using different quantization tables for the text area and the background area of ​​the image is a special form of compression. Therefore, this form of compression cannot be used by general optical character recognition / reader (OCR) processing software.

[0008] Conventional technology considers the compression of color signals (color difference), but does not consider the compression of luminance signals. Therefore, a sufficient compression effect cannot be expected.

[0009] Solutions to the Problem

[0010] According to one embodiment of the present invention, an image compression apparatus includes: a compression unit configured to receive image data and compression parameters, and compress the image data based on the image data and information about the parameters using a quantization table for a luminance signal and a quantization table for a color signal; and a parameter setting unit configured to set the parameters in the compression unit. The parameter setting unit sets an upper limit of the parameters of the quantization table for the color signal to a value less than or equal to α, and sets 90% or more of the parameters of the AC components of the quantization table for the luminance signal to have values ​​greater than or equal to the corresponding parameters of the AC components of the quantization table for the color signal.

[0011] According to an embodiment of the present invention, an image forming apparatus includes: the above-mentioned image compression apparatus, and an image forming unit configured to print image data compressed by the image compression apparatus.

[0012] According to an embodiment of the present invention, an inspection device includes the above-mentioned image compression device.

[0013] According to an embodiment of the present invention, an image compression method includes: receiving image data and parameters for compression; performing compression processing on the image data using a quantization table for a luminance signal and a quantization table for a color signal based on information about the image data and the parameters; and setting the parameters. The setting includes setting an upper limit of the parameters of the quantization table for the color signal to a value less than or equal to α, and setting 90% or more of the parameters of the AC components of the quantization table for the luminance signal to have values ​​greater than or equal to the corresponding parameters of the AC components of the quantization table for the color signal.

[0014] According to an embodiment of the present invention, a recording medium stores a program for causing a computer to execute the above-mentioned image compression method.

[0015] Effects of the present invention

[0016] According to at least one embodiment of the present disclosure, a universal compressed image that achieves both improved compression efficiency and improved OCR processing accuracy can be generated. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] A more complete understanding of the embodiments and many of their attendant advantages and features can be readily obtained and understood from the following detailed description taken in conjunction with the accompanying drawings.

[0018] Figure 1 is a diagram showing the configuration of an image forming apparatus according to the first embodiment.

[0019] Figure 2 Yes Figure 1 1 is a block diagram showing an example of the hardware configuration of the image forming apparatus shown in FIG.

[0020] Figure 3 Yes Figure 1 1 is a block diagram showing an example of the functional configuration of the image forming apparatus shown in FIG.

[0021] Figure 4 This is a block diagram showing the functional structure of an image compression device.

[0022] Figure 5A is a diagram schematically showing a quantization table.

[0023] Figure 5B is a diagram schematically showing a quantization table.

[0024] Figure 6 This figure shows an 8×8 pixel block of a black text portion.

[0025] Figure 7A This diagram shows the frequency components of the color signal of an 8×8 pixel block in the black text area.

[0026] Figure 7BThis diagram shows the red (R), green (G), and blue (B) values ​​of the color signal of an 8×8 pixel block in a black text area.

[0027] Figure 8A and Figure 8B This diagram shows an example of compression using a quantization table with a low compression ratio.

[0028] Figure 9A is a diagram schematically showing another quantization table.

[0029] Figure 9B is a diagram schematically showing another quantization table.

[0030] Figure 10A FIG. 1 is a flowchart showing the image compression process of the image compression apparatus according to the first embodiment.

[0031] Figure 10B This is a flowchart showing the parameter setting process.

[0032] Figure 11 This is a block diagram showing the functional structure of the image compression device according to the second embodiment.

[0033] 12A to 12C This is a diagram showing an example of parameter generation.

[0034] 13A to 13D This is a diagram showing an example of correcting a quantization table in parameter generation.

[0035] Figure 14 It is a diagram for explaining the subsampling process of the image compression device of the third embodiment.

[0036] Figure 15 It is a diagram for explaining the image diagnosis process in the inspection apparatus according to the fourth embodiment.

[0037] The accompanying drawings are intended to illustrate embodiments of the present disclosure and should not be construed as limiting the scope thereof. Unless expressly noted otherwise, the accompanying drawings should not be considered to be drawn to scale. Furthermore, throughout the several views, the same or similar reference numerals represent the same or similar components. DETAILED DESCRIPTION

[0038] In describing the embodiments shown in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology selected, and it is to be understood that each specific component includes all technical equivalents having similar functions, operating in a similar manner, and achieving similar results.

[0039] Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings. As used herein, the singular forms "a", "an" and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise.

[0040] Hereinafter, an image compression apparatus, an image forming apparatus, an inspection apparatus, an image compression method, and a program recorded on a recording medium according to embodiments will be described in detail with reference to the accompanying drawings.

[0041] The following describes an example in which the image forming apparatus according to the embodiments of the present invention is applied to a multifunction peripheral (MFP) having at least two of the following functions: copying, printing, scanning, and faxing. However, the image forming apparatus according to the embodiments of the present invention is not limited to MFPs. For example, the image forming apparatus according to the embodiments of the present invention can also be applied to image forming apparatuses such as copiers, printers, scanners, and fax machines.

[0042] First embodiment

[0043] Figure 1 FIG. 1 is a diagram showing the structure of the image forming apparatus 1 according to the first embodiment. Figure 1 As shown, the image forming apparatus 1 includes an automatic document feeder (ADF) 100, a scanner 6, a paper feeder 2, a plotter 4, and an image compression device 5. Details of the image compression device 5 will be described later.

[0044] The paper feeder 2 includes: paper feed cassettes 21 and 22 that store recording paper of different sizes; and a paper feed device 23 that has various rollers that transport the recording paper stored in the paper feed cassettes 21 and 22 to the image forming position of the plotter 4.

[0045] The plotter 4 is an electrophotographic image forming unit including an exposure device 31, a photosensitive drum 32, a developing device 33, a transfer belt 34, and a fixing device 35. The plotter 4 is not limited to an electrophotographic plotter and may be another type of plotter such as an inkjet plotter.

[0046] The plotter 4 prints the image data output from the scanner 6 onto recording paper. Specifically, in the plotter 4, the exposure device 31 exposes the photoreceptor drum 32 based on the image data of the original document read by the image reading unit of the scanner 6, forming a latent image on the photoreceptor drum 32. In the plotter 4, the developing device 33 supplies different colors of toner to each photoreceptor drum 32 to develop the latent image. In the plotter 4, the developed image on each photoreceptor drum 32 is transferred to the recording paper supplied from the paper feed device 2 via the transfer belt 34. Then, in the plotter 4, the fixing device 35 melts the toner of the toner image transferred to the recording paper, thereby fixing the color image on the recording paper.

[0047] Figure 2 1 is a block diagram showing an example of the hardware configuration of the image forming apparatus 1. Figure 2As shown, the image forming apparatus 1 includes, for example, a controller 210 , an operation panel 220 , an FCU (facsimile control unit) 230 , a USB (Universal Serial Bus) device 240 , an MLB (Media Link Board) 250 , a scanner 6 , and a plotter 4 .

[0048] The operation panel 220 is a user interface that allows a user who uses the image forming apparatus 1 to input various settings and displays various information to be presented to the user.

[0049] The FCU 230 is a control unit that controls the fax function of the image forming apparatus 1. The USB device 240 is connected to the image forming apparatus 1 via USB. The MLB 250 is a conversion board that converts the format of image data. The scanner 6 is an engine that reads documents. The plotter 4 is an engine that performs printing. The scanner 6 reads documents to obtain images to be processed.

[0050] The controller 210 is a control device that controls the operation of the image forming apparatus 1. Figure 2 As shown, the controller 210 includes a central processing unit (CPU) 211, a system memory 212, a hard disk drive (HDD) 3, a physical layer (PHY) 214 of a communication circuit, and an application-specific integrated circuit (ASIC) 215. The operation panel 220 is connected to the ASIC 215 of the controller 210. The FCU 230, the USB device 240, the MLB 250, the scanner 6, and the plotter 4 are connected to the ASIC 215 of the controller 210 via a data transmission bus 280.

[0051] Figure 3 1 is a block diagram showing an example of the functional configuration of the image forming apparatus 1. Figure 3 As shown, the image forming apparatus 1 includes an image processor 20 and an image compression device 5 in addition to a scanner 6 , an HDD 3 , and a plotter 4 .

[0052] Part or all of the functional components of the image forming apparatus are primarily implemented by the controller 210 of the image forming apparatus 1. Specifically, the image processor 20 is implemented, for example, by the ASIC 215 of the controller 210. The image compression device 5 is implemented, for example, by the CPU 211 of the controller 210 executing a predetermined program (software) stored in the system memory 212 or the HDD 3.

[0053] Alternatively, the program may be provided as a file that can be installed in the image forming apparatus 1 and recorded on a computer-readable recording medium such as a CD-ROM (Compact Disc Read Only Memory), a floppy disk (FD), a CD-R (Compact Disc Recordable), or a DVD (Digital Versatile Disc). Alternatively, the program may be stored on a computer connected to a network such as the Internet, downloaded via the network, and provided to the image forming apparatus 1. Alternatively, the program may be provided or distributed via a network such as the Internet. For example, the program may be provided after being pre-installed in the system memory 212, HDD 3, or the like of the image forming apparatus 1.

[0054] The scanner 6 reads the document fed by the ADF 100 to obtain image data. The scanner 6 sends the obtained image data to the image processor 20.

[0055] The image processor 20 performs predetermined image processing on the image data acquired by the scanner 6. The image processor 20 transmits the data subjected to the predetermined image processing to the image compression device 5.

[0056] The image processor 20 includes a gamma correction unit 26 , an image area separation unit 27 , a data interface unit 28 , a color processing / UCR (Under Color Removal) unit 24 , and a printer correction unit 25 .

[0057] The gamma correction unit 26 performs one-dimensional conversion (gamma correction processing) on ​​the image data acquired by the scanner 6 (e.g., 8-bit signals for each color of red (R), green (G), and blue (B) after analog-to-digital conversion) to adjust the tonal balance of each color. The density linear signals (R, G, and B signals with a signal value of 0 indicating white) generated by the one-dimensional conversion are sent to the image area separation unit 27 and the data interface unit 28.

[0058] Image region separation unit 27 separates input image data (input image) into multiple regions corresponding to multiple attributes. For example, for each pixel in the image data, image region separation unit 27 outputs a text determination result (comprehensive determination result) X1 and a color determination result X2. The text determination result X1 indicates whether the pixel represents "text" or "non-text." The color determination result X2 indicates whether the pixel is chromatic or achromatic.

[0059] A text determination result X1 of "text" means, for example, that the pixel is a text pixel. Since the determination related to text is performed independently of the determination related to color, both colored text and non-colored text are determined to be "text." Colored text primarily refers to text represented by a color (e.g., colored text). Achromatic text primarily refers to text represented by an achromatic color (e.g., black text). A text determination result X1 of "non-text" means, for example, that the pixel is a non-text portion (such as a seal portion or a photograph). In the following description, colored text and non-colored text may be referred to as colored text and black text, respectively.

[0060] The unit for outputting the determination result may be a unit of one pixel or a unit of a pixel block including a plurality of pixels. A description will be given below mainly of a case where the determination result is output in units of one pixel as an example.

[0061] The data interface unit 28 is an interface used when temporarily storing the text determination result X1 and the color determination result X2 output from the image area separation unit 27 and the image data (density linear signal) output from the gamma correction unit 26 in the HDD 3. The data interface unit 28 outputs the gamma-corrected image data and the text determination result X1 and the color determination result X2 received from the image area separation unit 27 to the color processing / UCR unit 24.

[0062] The color processing / UCR unit 24 selectively performs color processing and UCR processing based on the text determination result X1 and color determination result X2 for each pixel. For example, the color processing / UCR unit 24 converts the 8-bit R, G, and B signals into an image signal (8-bit signals of cyan (C), magenta (M), yellow (Y), and black (Bk)) and outputs the image signal. The image signal serves as a control signal for the plotter 4.

[0063] The printer correction unit 25 performs gamma correction processing and dither processing (digital halftone processing) on ​​the C, M, Y, and Bk image signals reflecting the grayscale characteristics of the plotter 4 , and outputs the signals to the plotter 4 .

[0064] The image compression device 5 performs image compression processing on the image data acquired by the scanner 6 and image-processed by the image processor 20 .

[0065] Figure 4This is a block diagram showing the structure of the image compression device 5. In order to prevent color mixing during compression and to compress the color signal with high image quality, the image compression device 5 of this embodiment sets a table that can remove unintentional coloring caused by optical factors through compression. In this way, the image compression device 5 can take into account both the improvement of OCR processing accuracy and the reduction of document size. Specifically, the image compression device 5 sets the upper limit of the parameters (coefficients) of the color signal to less than α, thereby reducing the compression rate. As a result, the image compression device 5 suppresses the color mixing of the black part and the color part, thereby improving the OCR accuracy. The image compression device 5 sets the parameters (coefficients) of the quantization table of the brightness signal and the color signal to a size relationship. In this way, the image compression device 5 reduces the document size.

[0066] like Figure 4 As shown, the image compression device 5 includes a parameter setting unit 51 and a JPEG (Joint Photographic Experts Group) compression unit 52.

[0067] The parameter setting unit 51 sets parameters (coefficients) in the JPEG compression unit 52 .

[0068] The JPEG compression unit 52 receives image data subjected to prescribed image processing from the image processor 20 and parameters (coefficients) from the parameter setting unit 51. Based on the text determination result X1 and color determination result X2 output from the image region separation unit 27 and temporarily stored in the HDD 3, the JPEG compression unit 52 determines two regions, described later, within the processed image data: a "text portion" and a "text portion overlapping with a seal." The JPEG compression unit 52 performs JPEG compression processing on the image data identified as the "text portion" and the "text portion overlapping with a seal," based on the parameters. While the compression process in this example is JPEG compression, any other pre-prepared image compression process is also applicable.

[0069] Figure 5A and Figure 5B : is a diagram schematically showing a quantization table used when the JPEG compression unit 52 performs JPEG compression. Figure 5A This is a diagram showing an example of a quantization table used for compressing a luminance signal.

[0070] Figure 5B An example of a quantization table used in compressing a color signal (color difference) is shown. The quantization table includes parameters (coefficients) used in compressing frequency components (components). There is a quantization table for a luminance signal ( Figure 5A ) and for color signals ( Figure 5B ) two quantization tables. Figure 5A and Figure 5BThe values ​​of the parameters (coefficients) of each of the illustrated quantization tables are examples.

[0071] Parameters (coefficients) refer to the elements of the 8×8 quantization table. Let (x, y) represent the coordinates in the quantization table. "x" and "y" have any value among 0, 1, 2, 3, 4, 5, 6, and 7. The coordinates of the parameter (coefficient) at the upper left corner of the 8×8 quantization table are defined as (0, 0). Figure 5A In the example of the quantization table shown, the parameter (coefficient) at (1,0) is equal to "14".

[0072] In each quantization table, the pixel at (0,0) is called a DC component, and the pixels at coordinates other than (0,0) are called AC components.

[0073] As the parameters (coefficients) of the quantization table increase, the compression rate increases. That is, as the parameters (coefficients) of the quantization table increase, the image quality decreases and color mixing occurs.

[0074] An example of a document that can be processed by OCR is an accounting form. The OCR targets for accounting forms include two areas: the "text area" and the "text area overlapping a seal." Since the only obstacles hindering OCR processing in the "text area" are noise and other factors, the OCR process is relatively easy. Therefore, when processing the "text area" by OCR, the luminance signal, which does not require high image quality, is compressed more than the color signal, thereby improving the compression ratio.

[0075] On the other hand, in the "text portion overlapping with the seal" area, the text and seal portions are close to each other, so color mixing occurs between the text and seal portions during compression. Consequently, during binarization or color dropout processing, which is a pre-process for OCR, text may be lost due to erroneous processing such as removal of the text portion. This increases the probability of misrecognition. Therefore, in OCR processing of the "text portion overlapping with the seal" area, the difficulty of OCR processing text with a seal or other material superimposed on it is significantly higher.

[0076] During compression, when the parameters of the quantization table used for the color signal are set to small values—that is, when the color signal is set to high image quality—color mixing between black and colored text is successfully suppressed, preventing loss during dropout color processing. Consequently, OCR processing accuracy is successfully improved for the "text portion overlapping the seal" area.

[0077] On the other hand, when a quantization table for high image quality is simply used in the compression of the color signal, the document size of the "text portion" may increase. This is because when a quantization table for high image quality is used for the color signal, the color signal (unexpected coloration) generated in the "text portion" by optical factors such as noise, lens aberration, and misalignment caused by the sensor of the image reader inside the scanner 6 still exists after compression. Unexpected coloration caused by optical factors tends to occur in low-frequency areas. By increasing the compression rate of the low-frequency area of ​​the color signal, the unexpected coloration caused by optical factors generated in the "text portion" is successfully reduced. Therefore, compression with high image quality and an improved compression rate are successfully achieved.

[0078] An example of unintended coloration remaining due to optical factors will be described. Figure 6 This diagram shows an 8×8 pixel block (the minimum unit in JPEG compression processing) of a black text portion. Figure 7A and Figure 7B This diagram shows the frequency components of the color signal and the R, G, and B values ​​of an 8×8 pixel block in the black text area. Figure 8A and Figure 8B This is a diagram showing an example of compression using a quantization table with a low compression ratio.

[0079] Figure 6 This is an example of an image having a large document size because color signals (unintended coloring) are not removed by compression. Figure 6 The example shown is an 8×8 pixel image extracted from a “text portion” that is a black text portion of the image data of the original obtained by the scanner 6 and image-processed in the image processor 20 .

[0080] Figure 7A This represents the frequency components of the color signal of the 8×8 pixel block in the black text area. Figure 7B The R, G, and B values ​​of the 8×8 pixel block in the text area. Figure 7A and Figure 7B As shown, if Figure 6 If all pixels of the image shown have the same R, G, and B values, then all AC components are equal to "0". Figure 6 In the image shown, the AC component includes a non-zero value due to unintended coloring in the "text portion" caused by optical factors such as noise, lens aberration, and misalignment caused by the sensor of the image reader inside the scanner 6. Figure 7B As shown by the shading in , there are four pixels with the same R, G, and B values. This indicates that although the pixel block appears black, it includes slight unintended coloring.

[0081] Figure 8A and Figure 8BExpress Figure 7A The example of the frequency components shown is an example of performing JPEG compression with a low compression ratio. When compression is performed using a quantization table with a low compression ratio, for example, Figure 8A and Figure 8B As shown in the frequency components of the compressed color signal, coefficients at (1, 0) and (2, 0) remain. When encoding the quantized frequency components, if "0" elements continue, the file size can be reduced. However, when using a quantization table with a low compression ratio, elements (indicated by circle B) remain. This hinders file size reduction.

[0082] Optical factors such as misalignment can cause unexpected optical color shifts, particularly at (1, 0) and (0, 1). Since text often uses many vertical and horizontal lines, easily visible text is relatively large and, therefore, falls on the low-frequency side. In this embodiment, the areas prone to optical color shifts, namely, the (1, 0) and (0, 1) areas in the AC component, are defined as low-frequency regions, while areas outside the AC component are defined as high-frequency regions.

[0083] As mentioned above, when performing OCR processing on the area where the seal and text overlap, color signal compression has a greater impact than luminance signal compression. Therefore, it is desirable to use a high-quality quantization table when compressing the color signal, which affects the accuracy of OCR processing in the area where the seal overlaps the text.

[0084] For the "text portion" area, which is less difficult to process with OCR, the compression rate of the low-frequency region of the color signal can be increased, resulting in lower image quality. Specifically, during compression, the AC component coefficients of the quantization table used for the luminance signal are set larger than those of the color signal, resulting in lower image quality and a successful reduction in document size.

[0085] By using such a relationship between the quantization table for the luminance signal and the quantization table for the color signal, a compression effect of the file size can be achieved.

[0086] If at least 90% of the coefficients of the quantization table satisfy the relationship that the coefficient value of the AC component of the quantization table of the luminance signal is greater than or equal to the coefficient value of the AC component of the quantization table of the color signal, sufficient compression effect is provided. Figure 5A and Figure 5B As shown, for example, if the ratio of the coefficients (represented by circle A) having an opposite magnitude relationship between the values ​​of the coefficients of the AC component of the quantization table for the luminance signal and the values ​​of the coefficients of the AC component of the quantization table for the color signal is approximately 10%, the increase in the accuracy of the OCR processing and the document size compression effect can be sufficiently provided.

[0087] That is, parameter setting section 51 sets 90% or more of the AC component coefficients of the quantization table for the luminance signal to values ​​greater than or equal to the AC component coefficients of the quantization table for the color signal. This successfully improves the accuracy of the OCR process and the compression rate.

[0088] Figure 9A and Figure 9B 2 is a diagram schematically showing another quantization table used when the JPEG compression unit 52 performs JPEG compression. Figure 9A is an example of a quantization table representing a luminance signal. Figure 9B This shows an example of a quantization table for a color signal (color difference).

[0089] exist Figure 9A and Figure 9B In the example shown, the parameter setting unit 51 sets the values ​​of the coefficients of all AC components of the quantization table for the luminance signal to be greater than or equal to the values ​​of the corresponding coefficients of all AC components of the quantization table for the color signal. In this case, the compression effect can be further improved.

[0090] The low-frequency region and the high-frequency region of the color signal of the coefficients (parameters) of the quantization table for the color signal have the following relationship.

[0091] In order to achieve both improved accuracy in OCR processing and reduced document size, the parameter setting unit 51 sets the coefficients in the low-frequency region to a value greater than or equal to α / 2 and less than or equal to α, where α represents the upper limit of the coefficients in the high-frequency region in the quantization table for color signals. The reason why the coefficients in the quantization table for color signals have such characteristics will be described below.

[0092] The "text portion overlapping the seal" described above is a typical example of an obstacle in OCR processing. When the text portion and the seal portion are close to each other, the color signal's frequency components appear in both low-frequency and high-frequency regions. Therefore, when compression is performed on the "text portion overlapping the seal" area, color mixing occurs.

[0093] As the compression rate increases, document size decreases, but the side effect of color mixing increases. Color mixing within black, or black mixing within colors, can negatively impact pre-processing for OCR (OCR) or binarization. As mentioned above, as the coefficients in the quantization table increase, the compression rate increases. That is, as the coefficients in the quantization table increase, image quality decreases and color mixing occurs. Therefore, to reduce the side effects of unintended color mixing caused by compression, it is desirable to set an upper limit for the coefficients in the quantization table.

[0094] On the other hand, the influence of unexpected coloration due to optical factors on the frequency components of the color signal is less than that caused by compression when the text portion and the seal portion overlap, etc. Therefore, by setting the coefficients of the quantization table to values ​​greater than or equal to a specific value, the frequency components of unexpected coloration due to optical factors are successfully reduced to zero through compression, successfully reducing the document size.

[0095] In the compression processing of the prior art, human vision is sensitive to low-frequency areas, and therefore, the low-frequency areas are not compressed.

[0096] In contrast, in this embodiment, the compression rate is increased until the image quality satisfies human visual perception, and unintended coloring due to optical factors can be removed, so that the document size can be reduced.

[0097] There is a sufficient difference in the level of the color signal between the frequency component of the unexpected coloring due to optical factors and the frequency component that occurs when the text portion and the seal portion overlap. Therefore, if the coefficients of the high-frequency area of ​​the quantization table and the coefficients of the low-frequency area of ​​the quantization table have a doubling size relationship, a reduction in document size is achieved. Specifically, when α represents the upper limit of the coefficient on the high-frequency area side that can suppress the unexpected coloring of the frequency component when the text portion and the seal portion overlap, the coefficient of the low-frequency area is preferably greater than or equal to α / 2. In order to achieve both improved OCR processing accuracy and reduced document size, the following relationship is required: the coefficient of the high-frequency area of ​​the quantization table for the color signal is less than or equal to α, and the coefficient of the low-frequency area of ​​the quantization table for the color signal is greater than or equal to α / 2 and less than or equal to α.

[0098] As described above, in this embodiment, the upper limit of the parameters (coefficients) in the quantization table for the color signal is set to be less than or equal to α, resulting in a low compression rate. This successfully suppresses color mixing between black and color areas, thereby successfully improving OCR accuracy. Furthermore, by setting the parameters (coefficients) in the quantization tables for the luminance and color signals to a large-small relationship, document size can be reduced. In other words, this embodiment can generate a universal compressed image that achieves both improved compression rate and improved OCR processing accuracy.

[0099] In this embodiment, the coefficients of the low-frequency region of the quantization table are set to be greater than or equal to α / 2, successfully removing the color components of the unexpected coloration caused by optical factors of the text portion, and successfully reducing the document size.

[0100] FIG10 is a flowchart showing the image compression process of the image compression apparatus according to the first embodiment.

[0101] The image compression device 5 receives the image data image-processed by the image processor 20 and the parameters (quantization table) stored in the HDD 3 (S10). The parameters to be used may be a quantization table defined by default, or may be an original quantization table.

[0102] The parameter setting unit 51 sets parameters (quantization table) (S11) for the JPEG compression unit 52. The parameter setting unit 51 sets the parameters so that the upper limit value of the coefficient of the quantization table for the color signal is less than or equal to α, and 90% or more of the coefficients of the AC components of the quantization table for the luminance signal have values ​​greater than or equal to the coefficient values ​​of the corresponding AC components of the quantization table for the color signal.

[0103] The JPEG compression unit 52 compresses the image data using the set parameters (quantization table) ( S12 ).

[0104] Figure 10B It is a flowchart showing the sub-steps of step S11.

[0105] In the sub-step of step S11, the parameter setting unit 51 compares the value of each AC component included in the quantization table for the color signal with a predetermined upper limit α, and if the value of the AC component exceeds α, the AC component is corrected to have a value less than or equal to α (S111). α may be a predetermined value or the value of the AC component in the quantization table for the luminance signal (e.g., Figure 5A When the AC component of the color signal is corrected to have a value less than or equal to α, the AC component may be corrected to have a value of α or a value less than α.

[0106] The parameter setting unit 51 compares the coefficient value qY(x, y) of the coordinate (x, y) of the quantization table of the luminance signal and the coefficient value qC(x, y) of the coordinate (x, y) of the quantization table of the color signal for each AC component, and counts the number of AC components that satisfy qY(x, y)<qC(x, y) (S112).

[0107] If the count result obtained in S112 is less than 90% of the number of AC components in the quantization table for the luminance signal, parameter setting unit 51 corrects the values ​​of the AC components in the quantization table for the luminance signal so that the count result is greater than 90% of the number of AC components in the quantization table for the luminance signal (S113). For example, when correcting n AC components in the quantization table for the luminance signal, parameter setting unit 51 selects n AC components from the AC components in the quantization table for the luminance signal that satisfy qY(x, y) < qC(x, y) and corrects the values ​​of the selected AC components to be equal to qC(x, y). This correction improves the compression rate of the luminance signal. However, selecting n AC components in descending order of x+y can suppress image quality degradation compared to selecting n AC components in ascending order of x+y from the AC components to be corrected in the quantization table for the luminance signal. When qY(x, y) is corrected, it may be corrected to be equal to qC(x, y) as described above, or it may be corrected to a value larger than qC(x, y).

[0108] The parameter setting unit 51 sets the parameters (quantization table) of the JPEG compression unit 52 using the quantization table of the color signal corrected in S111 and the quantization table of the luminance signal corrected in S113 ( S114 ).

[0109] In the processing steps of the image compression device 5 of the first embodiment, the values ​​of the AC components in the quantization table for the luminance signal may be corrected in S113 so that the count result obtained in S112 represents all the AC components in the quantization table for the luminance signal (100% of the number of AC components). Furthermore, if the values ​​of the AC components in the low-frequency region of the quantization table for the color signal are less than or equal to α / 2, a step of correcting the values ​​to be greater than or equal to α / 2 may be added in S111.

[0110] Second embodiment

[0111] The second embodiment will be described.

[0112] The second embodiment differs from the first embodiment in that parameters can be generated. In the following description of the second embodiment, description of the same parts as the first embodiment will be omitted, and the differences from the first embodiment will be described.

[0113] Since the color signal caused in the text portion by optical factors such as misalignment depends on the performance or variation of the hardware, the coefficient suitable for compression varies slightly. This embodiment allows the optimal setting of the low-frequency region to be set for each hardware model.

[0114] Figure 111 is a block diagram showing the configuration of the image compression apparatus 5 according to the second embodiment. The image compression apparatus 5 according to this embodiment further includes a parameter generating unit 53 and a parameter synthesizing unit 54 in addition to the configuration of the image compression apparatus 5 according to the first embodiment.

[0115] The parameter generation unit 53 generates coefficients (parameters) of the quantization table according to the set compression rate. That is, the parameter generation unit 53 generates two quantization tables having different compression rates for the low-frequency region and the high-frequency region according to the set compression rate.

[0116] The parameter synthesis unit 54 generates a new quantization table based on the quantization table generated by the parameter generation unit 53. That is, the parameter synthesis unit 54 synthesizes two quantization tables having different compression rates generated by the parameter generation unit 53 to generate a new quantization table for compression.

[0117] The “quantization table with different compression rates” generated by the parameter generation unit 53 may be a quantization table defined by default. Using the quantization table defined by default makes it easy to generate the quantization table.

[0118] The parameter setting unit 51 sets the parameters of the new quantization table generated by the parameter synthesis unit 54 in the JPEG compression unit 52 .

[0119] An example of generation of parameters will be described below.

[0120] 12A to 12C This is a diagram showing an example of parameter generation. Figure 12A The quantization table for "Image Quality 70" is shown and Figure 12B The quantization table for "image quality 90" shown is a quantization table obtained based on the quantization table defined by default. In this embodiment, the quantization table defined by default is used. However, the quantization table to be used is not limited to these quantization tables, and the original quantization table may be used.

[0121] The parameter generating unit 53 sets a table for the low frequency region using the quantization table of “image quality 70” and sets a table for the high frequency region using the quantization table of “image quality 90” having higher image quality than “image quality 70”.

[0122] The parameter is generated by the parameter generating unit 53 and Figure 12C In the quantization table represented in Figure 12C When viewing the quantization table downward or to the right, some coefficients of the quantization table for the high-frequency region are smaller than the coefficients of the frequency components at (1, 0) and (0, 1) in the low-frequency region. In other words, the coefficients of the frequency components in the high-frequency region achieve higher image quality than the coefficients of the frequency components in the low-frequency region.

[0123] On the other hand, human vision is more sensitive to changes in low-frequency components and less so to changes in high-frequency components. When image quality improves from low-frequency to high-frequency areas, the low-frequency components may be altered by the high-frequency components. This can create images that appear strange to human vision.

[0124] In order to overcome the above-mentioned problem, the parameter synthesis unit 54 corrects the quantization table under the following conditions.

[0125] 13A to 13D : is a diagram showing an example of correcting the quantization table in parameter generation. 13A to 13C As shown in FIG. 5 , when the quantization table includes a combination of one coefficient of the AC component and another coefficient located immediately to the right or immediately below the one coefficient and smaller than the one coefficient, the parameter synthesis unit 54 corrects the value of the smaller coefficient to the value of the coefficient of the quantization table with a high compression rate (low image quality). When the correction value of the coefficient exceeds the upper limit α, the parameter synthesis unit 54 corrects the correction value of the coefficient to the upper limit α. Figure 13D As shown, the shaded portion C of the corrected quantization table is the corrected area.

[0126] This generates an image that is insensitive to changes in high frequencies but sensitive to changes in low frequencies, and conforms to human visual characteristics without causing a sense of discomfort.

[0127] As described above, in this embodiment, different quantization tables are independently provided for the low-frequency region and the high-frequency region, and these quantization tables are synthesized to generate a new quantization table, so that the optimal settings for the low-frequency region can be successfully set for each hardware model. Therefore, it is possible to generate parameters (coefficients) for the low-frequency region at a desired compression rate, thereby providing a quantization table that achieves both reduced document size and improved OCR processing accuracy, and that can cope with unexpected color variations caused by lens aberrations or misalignment due to individual differences in the image reader of the scanner 6.

[0128] Third embodiment

[0129] The third embodiment will be described.

[0130] The third embodiment differs from the first and second embodiments in that the JPEG compression unit 52 selectively performs subsampling processing (color component reduction processing). In the following description of the third embodiment, the description of the same parts as the first and second embodiments is omitted, and the differences from the first and second embodiments are described.

[0131] Figure 14This diagram illustrates the subsampling process of the image compression device 5 of the third embodiment. The JPEG compression unit 52 selectively performs subsampling. Subsampling is a method of reducing color components, reducing the amount of information and thus reducing the file size. Conventional technology uses color component reduction because human vision is insensitive to changes in color components.

[0132] Figure 14 The subsampling process shown is a method of reducing the color components of 2×2 pixels to the color components of one pixel. The method of reducing the color components of 2×2 pixels to the color components of one pixel is just an example. The color components can be reduced in the horizontal direction or vertical direction, or the color components can be reduced in a wider range.

[0133] The JPEG compression unit 52 can disable subsampling processing. When the JPEG compression unit 52 disables subsampling processing, color mixing between black text and colored text due to the reduction of color components can be suppressed, and text loss in dropout color processing can be suppressed. Therefore, the accuracy of OCR processing is successfully improved.

[0134] The JPEG compression unit 52 can enable subsampling. When subsampling is enabled, color mixing occurs due to thinning of color components. However, thinning becomes a general compression process, making it compatible with various general-purpose OCR software.

[0135] Fourth embodiment

[0136] A fourth embodiment will be described.

[0137] The fourth embodiment differs from the first to third embodiments in that the compression technology described in the first to third embodiments is used in diagnostic imaging using color images. In the following description of the fourth embodiment, descriptions of portions identical to those in the first to third embodiments will be omitted, and differences from the first to third embodiments will be described.

[0138] Figure 15 It is a diagram for explaining the image diagnosis process in the inspection apparatus according to the fourth embodiment. Figure 15 This is an example of a color image of a human organ captured by an inspection device such as an endoscope.

[0139] like Figure 15 As shown, in an image captured by an inspection device such as an endoscope, "parts not reached by the light source" are dark because they are not illuminated by light, and "parts reached by the light source" are bright because they are illuminated by light, so the color component becomes larger.

[0140] As described in the first through third embodiments, the compression technology of the present invention is used to significantly compress the luminance component while minimizing the compression effect on the color components, maintaining high image quality. Therefore, in this embodiment, in diagnostic imaging performed using an inspection device such as an endoscope, the compression technology described herein is used to generate results obtained by the inspection device. High image quality is maintained in image areas where the light source reaches, which are desirable for diagnosis, while high compression rates are achieved in image areas where the light source does not reach, which are not useful for diagnosis. Consequently, a compressed image with a high compression effect is successfully obtained.

[0141] The embodiments of the present invention have been described above, but the above embodiments are provided as examples and are not intended to limit the scope of the present invention. Such new embodiments can be implemented in various other modified forms. Various omissions, substitutions, and changes can be made without departing from the scope of the present disclosure. Such new embodiments and their modifications are included in the scope and spirit of the present disclosure and are included in the scope of the claims and their equivalents. The constituent elements of different embodiments or modifications may also be appropriately combined.

[0142] The present invention can be implemented in any convenient form, such as using dedicated hardware, or a mixture of dedicated hardware and software. The present invention can be implemented as computer software implemented by one or more networked processing devices. The processing device includes any appropriately programmed device, such as a general-purpose computer, a personal digital assistant, a wireless application protocol (WAP) or a third generation (3G) compatible mobile phone, etc. Since the present invention can be implemented as software, each aspect of the present invention includes computer software that can be implemented on a programmable device. Any traditional carrier medium (carrier device) can be used to provide the computer software to the programmable device. The carrier medium includes a transient carrier medium, such as an electrical, optical, microwave, acoustic or radio frequency signal that carries the computer code. An example of such a transient medium is a transmission control protocol / internet protocol (TCP / IP) signal that carries the computer code over an IP network such as the Internet. The carrier medium also includes a storage medium for storing processor-readable code, such as a floppy disk, a hard disk, a compact disc read-only memory (CD-ROM), a magnetic tape device or a solid-state memory device.

[0143] The functions of the elements disclosed herein can be implemented using circuits or processing circuits, which include general-purpose processors, special-purpose processors, integrated circuits, application-specific integrated circuits (ASICs), digital signal processors (DSPs), field-programmable gate arrays (FPGAs), conventional circuits, and / or combinations thereof, configured or programmed to perform the disclosed functions. Processors are considered processing circuits or circuits because they include transistors and other circuits therein. In this disclosure, a circuit, unit, or device is hardware that performs or is programmed to perform the described functions. The hardware can be any hardware disclosed herein or otherwise known that is programmed or configured to perform the described functions. When the hardware is a processor, it can be considered a type of circuit, and the circuit, device, or unit is a combination of hardware and software, with the software being used to configure the hardware and / or processor.

[0144] This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2023-013552 filed with the Japan Patent Office on January 31, 2023, and Japanese Patent Application No. 2023-205681 filed with the Japan Patent Office on December 5, 2023, the disclosures of which are incorporated herein by reference in their entirety.

[0145] Reference Signs List

[0146] 1 Image forming device

[0147] 4 Plotter (image forming unit)

[0148] 5 Image compression device

[0149] 51 parameter setting unit

[0150] 52 JPEG compression units (compression units)

[0151] 53 parameter generation unit

[0152] 54 parameter synthesis unit

Claims

1. An image compression device, comprising: a compression unit configured to receive image data and parameters for compression, and perform compression processing on the image data using a quantization table for a luminance signal and a quantization table for a color signal based on information of the image data and the parameters; and a parameter setting unit configured to set the parameters in the compression unit, in, The parameter setting unit sets the upper limit of the parameters of the quantization table for the color signal to a value less than or equal to α, and sets 90% or more of the parameters of the AC component of the quantization table for the luminance signal to values ​​greater than or equal to the corresponding parameters of the AC component of the quantization table for the color signal.

2. The image compression device according to claim 1, wherein The parameter setting unit is configured to set a value of each parameter of all AC components of the quantization table for the luminance signal to be greater than or equal to a value of a corresponding parameter of all AC components of the quantization table for the color signal.

3. The image compression device according to claim 1, wherein: The parameter setting unit is configured to set the parameter of the low-frequency region of the quantization table for the color signal to a value greater than or equal to α / 2 and less than or equal to α, where α represents an upper limit value of the parameter of the high-frequency region of the quantization table for the color signal.

4. The image compression device according to claim 1, further comprising: a parameter generating unit configured to generate two quantization tables with different compression rates for a low-frequency region and a high-frequency region according to a set compression rate; and a parameter synthesis unit configured to synthesize the two quantization tables generated by the parameter generation unit to generate a new quantization table for compression, in, The parameter setting unit sets the parameters of the new quantization table generated by the parameter synthesis unit in the compression unit.

5. The image compression device according to claim 4, wherein: In a case where the new quantization table includes a synthesis of one coefficient of the AC component and another coefficient, where the other coefficient is located immediately to the right or immediately below the one coefficient and is smaller than the one coefficient, the parameter synthesis unit corrects the value of the other coefficient to the value of the coefficient in the quantization table with the higher compression rate among the two quantization tables, and The parameter synthesis unit corrects the correction value of the other coefficient to the upper limit α when the correction value of the other coefficient exceeds the upper limit α.

6. The image compression device according to claim 1, wherein The compression unit selectively performs a subsampling process for reducing color components.

7. An image forming apparatus comprising: The image compression device according to any one of claims 1 to 6; and An image forming unit is configured to print the image data compressed by the image compressing device.

8. An inspection device comprising: An image compression device according to any one of claims 1 to 6.

9. An image compression method, comprising: receiving image data and parameters for compression; Based on the image data and the parameter information, compressing the image data using a quantization table for a luminance signal and a quantization table for a color signal; as well as Set the parameters, Wherein, the setting includes: setting an upper limit of the parameter of the quantization table for the color signal to a value less than or equal to α, and 90% or more of the parameters of the AC components of the quantization table for the luminance signal are set to have values ​​greater than or equal to the corresponding parameters of the AC components of the quantization table for the color signal.

10. A recording medium storing a program for causing a computer to execute an image compression method, the image compression method comprising: receiving image data and parameters for compression; Based on the image data and the parameter information, compressing the image data using a quantization table for a luminance signal and a quantization table for a color signal; as well as Set the parameters, Wherein, the setting includes: setting an upper limit of the parameter of the quantization table for the color signal to a value less than or equal to α, and 90% or more of the parameters of the AC components of the quantization table for the luminance signal are set to have values ​​greater than or equal to the corresponding parameters of the AC components of the quantization table for the color signal.

Images