Image forming apparatus and image forming method

Abstract

To achieve the mechanism of improved screen processing to provide a print image with good quality while alleviating a reduction in image resolution.SOLUTION: An image forming apparatus comprises: an acquisition unit that acquires characteristic information indicating the characteristics of a recording material; an image processing unit that performs screening of an input image by using a dither matrix selected from a plurality of dither matrices based on the characteristic information to create a screened image; and an image forming unit that forms the screened image on the recording material. The plurality of dither matrices include a first dither matrix for growing dots with a first screen ruling in a pixel value range below a first threshold and growing dots with a lower second screen ruling in a pixel value range above the first threshold, and a second dither matrix for growing dots with the first screen ruling in at least part of the pixel value range above the first threshold.SELECTED DRAWING: Figure 3

Description

[Technical field]

[0001] The present disclosure relates to an image forming apparatus and an image forming method. [Background technology]

[0002] Electrophotography is known as an image forming method used in image forming devices such as printers and copiers. In electrophotography, a latent image is formed on a photosensitive drum by exposing the photosensitive drum to a laser beam, and then a toner image is developed by supplying a charged color material (also called toner) to the photosensitive drum. The developed toner image is transferred from the photosensitive drum (directly or indirectly via a transfer belt) to a recording material and fixed to the recording material through heating and pressure. When the image to be formed is a multi-tone image, screening (also called halftone screen) is performed prior to the formation of the latent image. Screening is a process of generating a halftone image that pseudo-expresses multi-tones from a multi-tone image by converting intermediate gradations into a corresponding dot pattern.

[0003] Generally, screen processing is classified into FM (Frequency Modulated) screens and AM (Amplitude Modulated) screens. In FM screens, the higher the density of the input pixel, the more dots are placed per unit area. In AM screens, the higher the density of the input pixel, the larger the dots are placed. AM screens are further classified into high-line screens that place dots at a relatively high density (high frequency) and low-line screens that place dots at a relatively low density (low frequency). In general, low-line screens can make the dots larger and have high stability in dot generation, making it easier to adjust density and providing high gradation reproducibility. High stability in dot generation means that it is also advantageous in preventing graininess and density unevenness (e.g., banding). On the other hand, high-line screens provide high resolution and improve the resolution of image formation.

[0004] Patent Document 1 proposes a method of applying a low-line screen to low-density areas of an image and a high-line screen to high-density areas in order to reduce toner consumption. Patent Document 2 proposes a method of reducing the screen line number in high-density areas to ensure the stability of dot generation in high-density areas and suppress gradation loss, thereby making the gradient of gradation relative to density change gentler. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2017-46074 A [Patent Document 2] JP 2019-22153 A Summary of the Invention [Problem to be solved by the invention]

[0006] However, when an image is formed on a recording material with a low surface smoothness (i.e., a rough surface), the mottled pattern (also called mottle) caused by uneven grain of the paper, which tends to become apparent in high density areas, deteriorates the subjective image quality of the printed image. Lowering the screen ruling in high density areas, as in the method proposed in Patent Document 2, has the effect of suppressing mottle, but this comes at the expense of sacrificing resolution.

[0007] It would be desirable to have an improved screening scheme that provides good quality printed images while mitigating the loss of image resolution. [Means for solving the problem]

[0008] According to one aspect, a method includes the steps of: receiving a user input specifying a type related to the smoothness of a surface of a recording material; a screen processing section generating a screen image by screening an input image using a dither matrix; and an image forming section forming the screen image generated by the screen processing section on the recording material, wherein the screen processing section selects, as the dither matrix to be used for screening the input image, a first dither matrix for performing screen processing at a first screen line frequency in a range where pixel values ​​are below a first threshold value when the type of the recording material specified by the user input is a first type, and for performing screen processing at a second screen line frequency lower than the first screen line frequency in a range where pixel values ​​exceed the first threshold value when the type of the recording material specified by the user input is a second type having a smoother surface than the first type, and a second dither matrix for performing screen processing at the first screen line frequency in at least a part of the range where pixel values ​​exceed the first threshold value when the type of the recording material specified by the user input is a second type having a smoother surface than the first type. the second dither matrix is ​​a matrix for performing screen processing at the first screen line frequency when pixel values ​​are below a second threshold value that is greater than the first threshold value, and for performing screen processing at the second screen line frequency when pixel values ​​are above the second threshold value, or a matrix for performing screen processing at the first screen line frequency for all pixel values. An image forming apparatus is provided. Effect of the Invention

[0009] According to the present disclosure, it is possible to provide a good quality printed image while reducing the decrease in image resolution. [Brief description of the drawings]

[0010] [Figure 1] FIG. 1 is a block diagram showing an example of a configuration of an image forming system according to an embodiment. [Diagram 2] 2 is a schematic diagram showing an example of the structure of a print engine of the image forming apparatus shown in FIG. [Diagram 3] 2 is a block diagram showing an example of a detailed configuration of an image processing unit of the image forming apparatus shown in FIG. 1; [Figure 4] 1 is an explanatory diagram for explaining the principle of screen processing for converting a multi-tone image into a halftone image using a dither matrix. [Diagram 5] 11 is an explanatory diagram for explaining a dither matrix in which the screen ruling changes with the growth of dots; [Figure 6] FIG. 11 is an explanatory diagram showing an example of a table that associates the type of recording material with a category related to smoothness and a dither matrix to be selected. [Figure 7] 11A and 11B are explanatory diagrams showing examples of dither matrices with different threshold values ​​for transition of screen ruling; [Figure 8] FIG. 11 is an explanatory diagram for explaining evaluation results of mottle in an evaluation test. [Figure 9] 11A and 11B are explanatory diagrams for explaining evaluation results of resolution and overall evaluation results in an evaluation test. [Figure 10] FIG. 4 is a schematic diagram showing an example of the configuration of a sensor for detecting the characteristics of a recording material. [Figure 11] 1. FIG. 4 is a block diagram showing another example of the detailed configuration of the image processing unit of the image forming apparatus shown in FIG. [Figure 12] 1A and 1B are explanatory diagrams showing examples of surface images and feature amounts of a recording material with a rough surface and a recording material with a smooth surface. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] Hereinafter, the embodiments will be described in detail with reference to the attached drawings. Note that the following embodiments do not limit the invention according to the claims. Although the embodiments describe a number of features, not all of these features are essential to the invention, and the features may be combined in any manner. Furthermore, in the attached drawings, the same reference numbers are used for the same or similar configurations, and duplicated descriptions are omitted.

[0012] <<1. First embodiment>> In this section, an example in which the technology according to the present disclosure is applied to an image forming apparatus 10 that is a printer will be mainly described. However, the technology according to the present disclosure can be applied to any type of image forming apparatus, such as a copier, a facsimile, and a multifunction machine. Unless otherwise specified, each of the components such as an apparatus, a device, a module, and a chip described below may be composed of a single entity, or may be composed of multiple physically different entities.

[0013] <1-1. Basic configuration example> 1 is a block diagram showing an example of a configuration of an image forming system 1 according to an embodiment of the present invention. Referring to FIG. 1, the image forming system 1 includes an image forming apparatus 10, a network 30, and a host computer 50.

[0014] The image forming apparatus 10 is a printer that forms an image on a recording material by an electrophotographic method. Here, the image forming apparatus 10 is assumed to be a color printer that can form a color image using toners of multiple colors. However, the present embodiment is equally applicable to a monochrome printer that can form only monochrome images. The network 30 is a network that connects the image forming apparatus 10 and the host computer 50. The network 30 may be a wired network or a wireless network. The host computer 50 is a terminal device used by a user. The host computer 50 may be, for example, a PC (Personal Computer), a workstation, or a smartphone. The host computer 50 transmits a print job for printing content (e.g., a document or an image) created using some device or application to the image forming apparatus 10 via the network 30. The print job may include input image data (e.g., PDL (Page Description Language) data) that represents an image to be printed.

[0015] 1, the image forming apparatus 10 includes a controller 21, a print engine 22, and an operation unit 23. The controller 21 includes a communication I / F 101, a CPU 102, a RAM 103, a ROM 104, an image processing unit 105, an engine I / F 106, an operation I / F 107, and an internal bus 110.

[0016] The communication I / F 101 is an interface that mediates communication between the image forming apparatus 10 and other devices. In the example of FIG. 1, only the host computer 50 is shown as a connection partner of the communication I / F 101, but the communication I / F 101 may be connected to more devices, including other terminal devices and database servers. The CPU (Central Processing Unit) 102 is a processor or processing circuit for controlling the overall functions of the image forming apparatus 10. The CPU 102 executes a computer program that is stored in advance in the ROM 104 and loaded into the RAM 103, for example, to realize a control function to be described later. The RAM (Random Access Memory) 103 is a so-called main memory, and provides a temporary storage area to the CPU 102 and the image processing unit 105. The ROM (Read Only Memory) 103 is a non-volatile memory that stores various types of data, such as program data, control data, and setting data, for the control function of the controller 21. Under the control of the CPU 102, the image processing unit 105 converts the input image data of the print job to be executed into print image data in a format suitable for processing in the print engine 22. For example, the image processing unit 105 converts the color space of the input image from RGB space to CMYK space and performs gamma correction as necessary. Furthermore, when the input image is a multi-tone image, the image processing unit 105 executes screen processing on the input image to generate a screen image (halftone image). Then, the image processing unit 105 outputs print image data including the generated screen image to the print engine 22 via the engine I / F 106. Each color component of each pixel of the multi-tone image has a pixel value range of 0 to 255 with a bit depth of, for example, 8 bits, whereas each color component of each pixel of the screen image indicates, for example, zero or one. In addition to the print image data, the image processing unit 105 may output attribute data representing the attribute of each pixel to the print engine 22. The attribute data may indicate, for example, the type of object to which each pixel belongs (for example, character, figure, or photo). The engine I / F 106 is an interface that connects the controller 21 to the print engine 22 .The print engine 22 executes printing based on print image data input from the image processing unit 105, that is, forms an image on a recording material (also called paper). The print engine 22 may be called an image forming unit 22. The operation I / F 107 is an interface that connects the controller 21 to the operation unit 23. The operation unit 23 may include an input device (e.g., a touch panel, a button, and a switch) for receiving user input, and an output device (e.g., a display) for presenting an image or information to the user. The internal bus 110 is a signal line that mutually connects the communication I / F 101, the CPU 102, the RAM 103, the ROM 104, the image processing unit 105, the engine I / F 106, and the operation I / F 107.

[0017] Fig. 2 is a schematic diagram showing an example of the structure of the print engine 22 of the image forming apparatus 10 shown in Fig. 1. Fig. 2(A) shows the overall structure of the print engine 22 in a schematic manner, and Fig. 2(B) shows the structure of an image forming station for one color (yellow in this case) in more detail. Here, the print engine 22 is a printer unit that employs an inline system and an intermediate transfer system.

[0018] The print engine 22 has image forming stations SY, SM, SC, and SK for yellow (Y), magenta (M), cyan (C), and black (K), respectively. As an example, focusing on the image forming station SY for yellow, the image forming station SY includes a process cartridge 204Y and a primary transfer roller 206Y located on the opposite side of the process cartridge 204Y across the intermediate transfer belt 205. The image forming stations SM, SC, and SK are configured in the same manner as the image forming station SY, except that the colors of the toner they handle are different. The intermediate transfer belt 205 is an intermediate transfer body that rotates counterclockwise in the figure, as indicated by the arrow A. The image forming stations SY, SM, SC, and SK are arranged side by side in the extension direction of the intermediate transfer belt 205. In the following description, when it is not necessary to distinguish between these four colors, the suffixes Y, M, C, and K at the end of the reference numerals are omitted, and a description common to the four colors is given.

[0019] Referring to FIG. 2B, the process cartridge 204 includes a developing unit 239 and a drum unit 240. The developing unit 239 has a developing roller 233, a supply roller 236, an agitator 237, and a toner regulating blade 238, and contains toner T therein. The drum unit 240 has a photosensitive drum 231, a charging roller 232, a cleaning blade 234, and a waste toner container 235. The photosensitive drum 231 is rotated clockwise (arrow B) in the figure by a driving means (not shown). The charging roller 232 uniformly charges the surface of the photosensitive drum 231 based on a high voltage applied from a high voltage power supply (not shown). Next, the scanner unit 207 irradiates a laser onto the photosensitive drum 231 according to print image data input from the controller 21, thereby forming an electrostatic latent image on the surface of the photosensitive drum 231. The developing roller 233 is rotated counterclockwise (arrow C) in the figure by a driving means (not shown). The surface of the developing roller 233 is coated with charged toner supplied from a supply roller 236 described later, and the toner adheres to the electrostatic latent image on the surface of the photosensitive drum 231, thereby developing the electrostatic latent image into a visible toner image. The base layer of the photosensitive drum 231 is grounded, while a voltage of the opposite polarity to that of the toner is applied to the primary transfer roller 206 by a high-voltage power source (not shown). Therefore, a transfer electric field is formed at the primary transfer position, which is the nip between the primary transfer roller 206 and the photosensitive drum 231, and the toner image is transferred from the photosensitive drum 231 to the intermediate transfer belt 205 at the primary transfer position. Toner remaining on the surface of the photosensitive drum 231 without being transferred is scraped off by a cleaning blade 234 and disposed of in a waste toner container 235.

[0020] The supply roller 236 is driven to rotate clockwise (arrow D) in the figure, and supplies the toner contained in the development unit 239 to the development roller 233. The agitator 237 is driven to rotate clockwise (arrow E) in the figure, and supplies the toner to the supply roller 236. The toner regulating blade 238 is fixedly attached to the development unit 239, and the development roller 233 rubs against the toner regulating blade 238 as it rotates. The toner supplied from the supply roller 236 to the development roller 233 is regulated to a certain amount while being charged by the friction between the development roller 233 and the toner regulating blade 238, and the concentration of the toner adhering to the photosensitive drum 231 during development is stabilized.

[0021] 2A, the toner images of each color developed at the image forming stations SY, SM, SC, and SK are transferred in order onto the intermediate transfer belt 205 rotating in the direction of arrow A to form a color image. The color image on the intermediate transfer belt 205 is transported to a secondary transfer position, which is the nip between a secondary transfer roller 211 and a secondary transfer opposing roller 212.

[0022] The recording material 203 is stacked and stored in a cassette 208. When a print job is started, a feed roller 209 is rotated and the recording material 203 is fed from the cassette 208 to a conveying path. The recording material 203 is conveyed by a pair of registration rollers 210 and reaches a secondary transfer position between a secondary transfer roller 211 and a secondary transfer counter roller 212. Typically, the timing at which the leading edge of the recording material 203 reaches the secondary transfer position is synchronized with the timing at which the leading edge of a color image on the intermediate transfer belt 205 reaches the secondary transfer position. The recording material 203 is sandwiched and conveyed by the secondary transfer roller 211 and the secondary transfer counter roller 212, during which a voltage of the opposite polarity to that of the toner is applied to the secondary transfer roller 211 from a power source (not shown). The secondary transfer counter roller 212 is grounded, and a transfer electric field is formed at the secondary transfer position. This transfer electric field transfers the color image from the intermediate transfer belt 205 to the recording material 203. After passing through the secondary transfer position, the recording material 203 is heated and pressed by a fixing device 213. As a result, the color image is fixed to the recording material 203. Thereafter, the recording material 203 is discharged from a paper discharge port 214 to a paper discharge tray 215, and the image forming operation is completed. Meanwhile, toner remaining on the intermediate transfer belt 205 without being transferred to the recording material 203 at the secondary transfer position is removed from the intermediate transfer belt 205 by a cleaning member 216. The sensor unit 250 is a sensor for measuring the characteristics of the recording material. However, in this embodiment, the sensor unit 250 does not need to be used.

[0023] <1-2. Detailed configuration example of image processing unit> Fig. 3 is a block diagram showing an example of a detailed configuration of the image processing unit 105 of the image forming apparatus 10 shown in Fig. 1. Referring to Fig. 3, the image processing unit 105 includes a color conversion unit 301, a gamma correction unit 302, a screen processing unit 303, a matrix storage unit 304, and a characteristic information acquisition unit 305.

[0024] The image processing unit 105 receives input image data of a print job received from the host computer 50. The CPU 102 may rasterize PDL data included in the print job to convert it into raster data, and input the raster data to the image processing unit 105 as input image data. Data may be transferred to the image processing unit 105 via the RAM 103. The color conversion unit 301 of the image processing unit 105 converts the color space of the input image data from the RGB space to the CMYK space. The gamma correction unit 302 executes gamma correction for pixel values ​​of each color component of CMYK using a lookup table that associates pixel values ​​before and after correction so that the density characteristics of the image transferred to the recording material match the desired characteristics. The CPU 102 may update this lookup table in response to changes in the state of the print engine 22 so that the effects of changes in the state of the print engine 22 are offset.

[0025] The screen processing unit 303 generates a screen image suitable for processing by the print engine 22 by screening the input image using a dither matrix stored in advance in the matrix storage unit 304. The input image is a multi-tone image having density gradations for each of the four color components, CMYK, whereas the screen image is a halftone image that artificially expresses the gradations of each color component by the density of dots. The screen processing unit 303 outputs the screen image generated by the screen processing to the print engine 22 via the engine I / F 106. This screen image is formed on a recording material by the print engine 22 described above.

[0026] FIG. 4 is an explanatory diagram for explaining the principle of the screen processing using a dither matrix. The upper part of FIG. 4 shows an input image 401, the middle part shows a dither matrix 402, and the lower part shows a screen image 403 generated as a result of the screen processing. For simplicity of explanation, attention is paid to one color component of a small area of ​​6×6 pixels that occupies a part of the input image. In the example of FIG. 4, the pixel values ​​of all pixels in the input image 401 are equal to 130. The dither matrix 402 is a 6×6 matrix, and the value of each element of the matrix is ​​compared with the pixel value of the corresponding pixel position. For pixels in the input image that show a pixel value larger than the element value of the corresponding position of the dither matrix 402, the pixel value of the screen image is set to a maximum value (e.g., 255). Conversely, for pixels in the input image that show a pixel value equal to or smaller than the element value of the corresponding position of the dither matrix 402, the pixel value of the screen image is set to a minimum value (e.g., zero). When the input image 401 is input, the screen processing unit 303 sets each pixel of the input image 401 as a pixel of interest, and sequentially determines the pixel value of the pixel of interest in the screen image 403 by comparing the pixel value of the pixel of interest with the element value of the corresponding position in the dither matrix 402. In the example of FIG. 4, the pixel values ​​of the upper left 3×3 pixel and the lower right 3×3 pixel of the input image 401 are greater than the corresponding element value of the dither matrix 402, so the pixel values ​​of those pixel positions in the screen image 403 are set to 255 (shaded in the figure). On the other hand, the pixel values ​​of the lower left 3×3 pixel and the upper right 3×3 pixel of the input image 401 are greater than the corresponding element value of the dither matrix 402, so the pixel values ​​of those pixel positions in the screen image 403 are set to zero (blank in the figure). As a result, two dots each having a size of 3×3 pixels appear in the screen image 403.

[0027] Here, assuming an input image in which all pixels have pixel values ​​equal to, for example, 10, two dots each having a size of 1×1 pixel will appear in the screen image generated by screening such an input image with the dither matrix 402. Assuming an input image in which all pixels have pixel values ​​equal to, for example, 80, two dots each having a size of 2×3 pixels will appear in the screen image generated by screening such an input image with the dither matrix 402. In this way, the dither matrix 402 can be said to be a matrix for growing dots (increasing dot size) according to an increase in pixel value (density) using two points as reference points. The reference points for such dot growth are called growth points, and the density of the growth points corresponds to the screen ruling. This method of gradation expression is also called area gradation.

[0028] 4 is merely an example for explaining the principles of screen processing, and is not necessarily the same as the dither matrix actually used by the screen processing unit 303. The dither matrix 402 is designed so that the screen ruling is constant throughout the entire pixel value range from 0 to 255. In contrast, this embodiment employs a dither matrix for growing dots at a first screen ruling in a pixel value range below a certain threshold, and at a second screen ruling in a pixel value range above the threshold. The second screen ruling is lower than the first screen ruling.

[0029] FIG. 5 is an explanatory diagram for explaining a dither matrix in which the screen ruling changes with the growth of dots. The upper part of FIG. 5 shows the arrangement of two types of growth points in an image area of ​​15×15 pixels. Growth points 501 shown by hollow circles are regularly arranged at a density corresponding to a first screen ruling (e.g., 141 lines). On the other hand, growth points 502 shown by solid circles are regularly arranged at a density corresponding to a second screen ruling (e.g., 100 lines) lower than the first screen ruling. The dither matrix 510 shown on the lower right of FIG. 5 is an example of a matrix for growing dots at the first screen ruling throughout the entire pixel value range, with the growth point 501 as the reference point. The dither matrix 402 shown in FIG. 4 is essentially equivalent to the repetition of two dither matrices 510 (while shifting the positions by half). Dither matrix 511 shown in the lower left of Figure 5 is an example of a matrix for growing dots at a first screen ruling using growth point 501 as a reference point in low-density regions, and for growing dots at a second screen ruling using growth point 502 as a reference point in high-density regions. The dashed arrows in the figure indicate the direction of dot growth. The growth of dots in the shaded areas of dither matrix 511 is the same as the growth of dots using dither matrix 510. By using dither matrix 511, the screen ruling is reduced in high-density regions, which may improve the quality of printed images by suppressing mottle that is easily evident in high-density regions.

[0030] When the dither matrix 511 is used, the screen ruling is equal to the first screen ruling in the pixel value range where the pixel value is equal to or less than a threshold value T1 (for example, T1=127), and is equal to the second screen ruling in the pixel value range where the pixel value is greater than the threshold value T1. If the threshold value is low, a relatively low screen ruling is used in a wider pixel value range on the high density side, but excessive use of a low screen ruling may result in significant subjective deterioration of image quality due to a decrease in resolution. For example, when an image is formed on a recording material with a high surface smoothness, mottle tends not to be so noticeable, and image quality deterioration due to a decrease in resolution may be dominant over the improvement in image quality due to the reduction in screen ruling. Therefore, in this embodiment, the screen processing unit 303 uses a dither matrix selected from a plurality of dither matrices based on the characteristics of the recording material in the screen processing. One of the plurality of dither matrices is a matrix that reduces the screen ruling from the first screen ruling to the second screen ruling at the threshold value T1 as the dots grow, as in the above-mentioned dither matrix 511. This matrix is ​​also referred to as the first dither matrix in the following description. Another one of the dither matrices is a matrix that grows dots at the first screen ruling in at least a part of the pixel value range above the threshold T1. This matrix is ​​also referred to as the second dither matrix in the following description. Some examples of dither matrices will be further described later.

[0031] Returning to Fig. 3, the matrix storage unit 304 is a memory that stores multiple dither matrices in advance. The matrix storage unit 304 may store multiple dither matrix candidates for each of the four color components, CMYK. For example, separate dither matrices with different screen angles may be defined for different color components even if the screen ruling is the same.

[0032] The characteristic information acquisition unit 305 acquires characteristic information indicating the characteristics of the recording material supplied to the print engine 22, and outputs the acquired characteristic information to the screen processing unit 303. The screen processing unit 303 selects a dither matrix to be used from a plurality of dither matrices stored in the matrix storage unit 304 based on the characteristic information input from the characteristic information acquisition unit 305. For example, the characteristics of the recording material include the surface smoothness of the recording material. In this example, the screen processing unit 303 selects the above-mentioned first dither matrix for a recording material with a relatively low smoothness and the above-mentioned second dither matrix for a recording material with a relatively high smoothness based on the characteristic information. As a result, the pixel value range to which a low screen ruling is applied is reduced or eliminated for a recording material with a relatively high smoothness, and therefore excessive use of a low screen ruling can be avoided.

[0033] More specifically, the recording material may be classified into one of two or more categories depending on the smoothness of the recording material's surface. In this case, the characteristic information indicates the category into which the recording material is classified. FIG. 6 shows an example of a table that associates the type of recording material with a category related to smoothness and a dither matrix to be selected. In the example of FIG. 6, there are ten types of recording materials, namely, bond paper, rough paper, plain paper, recycled paper, light paper, medium paper, medium-weight paper, heavy paper, extra-heavy paper, and glossy paper. Of these recording materials, the surface of bond paper is the roughest, and bond paper is classified into the category "bond". The surface of rough paper is the next roughest, and rough paper is classified into the category "rough". The surface of glossy paper is the smoothest, and is classified into the category "smooth". The remaining types of recording material are classified into the category "standard". The categories "bond", "rough", "standard" and "smooth" are then associated with dither matrices 511, 512, 513 and 514, respectively.

[0034] Fig. 7 shows examples of four dither matrices with different thresholds for screen line transition. The matrix shown on the far left is the dither matrix 511 described with reference to Fig. 5. The threshold T1 for screen line transition (e.g., from 141 lines to 100 lines) of the dither matrix 511 is, for example, equal to 127 as described above. According to the table in Fig. 6, the screen processing unit 303 selects the dither matrix 511 when the category of the recording material indicated by the characteristic information is "bond".

[0035] The second matrix from the left in Fig. 7 is a dither matrix 512. A threshold T2 for transition of the screen ruling of the dither matrix 512 is greater than the threshold T1 and is equal to, for example, 172. The dither matrix 512 grows dots at a first screen ruling (141 lines) in a pixel value range (0 to 172) equal to or less than the threshold T2, and grows dots at a second screen ruling (100 lines) in a pixel value range (173 to 255) exceeding the threshold T2. According to the table in Fig. 6, the screen processing unit 303 selects the dither matrix 512 when the category of the recording material indicated by the characteristic information is "coarse."

[0036] The third matrix from the left in Fig. 7 is dither matrix 513. A threshold T3 for transition of screen ruling in dither matrix 513 is greater than thresholds T1 and T2, and is equal to, for example, 202. Dither matrix 512 grows dots with a first screen ruling in a pixel value range (0 to 202) equal to or less than threshold T3, and grows dots with a second screen ruling in a pixel value range (203 to 255) exceeding threshold T3. According to the table in Fig. 6, screen processing unit 303 selects dither matrix 513 when the category of the recording material indicated by the characteristic information is "standard."

[0037] The matrix shown on the far right of Fig. 7 is the dither matrix 510 described with reference to Fig. 5. In the dither matrix 510, the screen ruling does not change, and dots grow at the first screen ruling throughout the entire pixel value range. According to the table in Fig. 6, the screen processing unit 303 selects the dither matrix 510 when the category of the recording material indicated by the characteristic information is "smooth".

[0038] The characteristic information acquisition unit 305 may acquire characteristic information based on a user input. For example, the ROM 104 stores in advance a list of types of recording materials that can be handled by the image forming apparatus 10 (for example, the 10 types shown in FIG. 6). The characteristic information acquisition unit 305 may present the list of types of recording materials to the user via a user interface (for example, a control panel displayed on a screen) provided by the operation unit 23, and allow the user to specify the type of recording material to be used. The list of types of recording materials that can be handled by the image forming apparatus 10 may be set in a printer driver that is pre-installed in the host computer 50. In this case, the user can specify the type of recording material to be used for printing in the image forming apparatus 10 on the screen of the host computer 50. The characteristic information acquisition unit 305 thus accepts a user input that specifies the type of recording material. In addition, a database that associates the type of recording material with a category as illustrated in FIG. 6 is provided inside (for example, the ROM 104) or outside the image forming apparatus 10. The characteristic information acquiring unit 305 can acquire, by referring to the database, a category related to the smoothness corresponding to the type designated by the received user input, as characteristic information.

[0039] The print engine 22 may select appropriate process conditions (e.g., rotation speeds of the drum, belt and rollers, voltages applied for exposure, charging, development and transfer, and fixing temperature) according to the type of recording material specified to perform the image forming operation. However, the selection of process conditions is not related to the subject of this embodiment, and will not be described in detail here.

[0040] <1-3. Image quality evaluation> The inventors conducted tests to evaluate the degree to which subjective image quality is improved by the above-mentioned method of switching the dither matrix used in screen processing according to the table in Fig. 6. As comparative examples, a first comparative example (i.e., a uniform high-line screen) using dither matrix 510 regardless of the type of recording material, and a second comparative example (i.e., switching of screen lines at a uniform threshold value) using dither matrix 511 regardless of the type of recording material were considered.

[0041] In the evaluation test, first, five patches with total pixel values ​​equal to 127, 163, 191, 227, and 255 were arranged side by side on an evaluation image, and a portrait image (photograph) of a black-haired person was also arranged on the same evaluation image. The patch with total pixel values ​​equal to 255 was a small solid image, while the other patches were small uniform halftone images. These were primarily used to evaluate mottle. The portrait image of the person was primarily used to evaluate resolution based on the reproducibility of hair. In addition, the following four types of recording materials were prepared as representative recording materials for each category: "Bond" ... NEENAH BOND 60g "Coarse" ... Xerox Vitality 75g ·“Standard” ... Canon CS-680 ·"Smooth" ... HP Premium Presentation Paper 120g

[0042] In the first comparative example, the evaluation image was uniformly screened using the dither matrix 510, and the screening image was formed on the four types of recording material described above. In the second comparative example, the evaluation image was uniformly screened using the dither matrix 511, and the screening image was formed on the four types of recording material described above. In the embodiment, the evaluation image was screened using one of the dither matrices 510 to 513 corresponding to the category of the recording material, and the screening image was formed on each of the four types of recording material. Then, five subjects visually inspected each printed image in an environment with a color temperature of 5000K and an illuminance of 300lx, and evaluated the degree of mottle and resolution.

[0043] In the Mottle evaluation, subjects assigned five patches a score out of five, as follows: '1' ... mottle not visible '2' ... Mottle is visible but not noticeable '3' ... Mottle is noticeable but tolerable '4' ... mottle exceeds acceptable levels '5' ... The original image is unrecognizable due to the severe mottle.

[0044] FIG. 8A shows the evaluation results of mottle for an image printed on a recording material (Xerox Vitality 75g) of the category "coarse". The horizontal axis of FIG. 8A represents pixel values, and the vertical axis represents mottle evaluation scores. Hollow circles are plots of evaluation scores for each patch in the first comparative example. Solid circles are plots of evaluation scores for each patch in the embodiment. A dashed line 803 represents a threshold for the transition of the screen ruling of the dither matrix 512 applied to the recording material of the category "coarse". From this evaluation result, it can be seen that a significant effect of improving image quality by suppressing mottle is obtained in a high density area (also called a shadow area) with a density higher than the threshold for the transition of the screen ruling. However, in both the first comparative example and the embodiment, there is a tendency for mottle to worsen as the density increases (the pixel value increases), and the evaluation score is maximum (worst) for a patch with a pixel value equal to 227 (see plots 801 and 802). In the low density area (also called the highlight area) and solid area, the evaluation scores of the first comparative example and the embodiment are almost the same.

[0045] FIG. 8B shows a bar graph comparing the maximum evaluation score of mottle for each category of "bond", "coarse" and "standard" between the first comparative example and the embodiment. The height of the shaded bar indicates the maximum evaluation score in the first comparative example, and the height of the open bar indicates the maximum evaluation score in the embodiment. In the evaluation results in FIG. 8B, the gap between the first comparative example and the embodiment becomes smaller as the smoothness of the recording material increases, which is due to the fact that the smoother the surface of the recording material, the less likely mottle is to become apparent. In addition, the absolute value of the evaluation score becomes smaller as the smoothness of the recording material increases (the image quality is evaluated as being better). Therefore, it can be seen that for recording materials with higher smoothness, it is easier to obtain better subjective image quality by placing emphasis on resolution rather than suppressing the originally slight mottle.

[0046] To evaluate the resolution, participants were asked to give each portrait image a score on a 5-point scale, with the average of the scores given being the evaluation score: '1' ... resolution is good '2' ... Resolution loss is visible but not noticeable '3' ... The loss of resolution is noticeable, but acceptable '4' ... Resolution loss exceeds acceptable level '5' ... The resolution is so poor that the original image cannot be recognized.

[0047] FIG. 9A shows a table listing the evaluation scores of the resolution in the second comparative example and the embodiment. The evaluation score in the second comparative example was 4 in all four categories. On the other hand, the evaluation scores in the embodiment were '3' for 'rough', '2' for 'standard', and '1' for 'smooth', which shows that the improvement in the resolution in the shadow area contributed to the improvement of the subjective image quality compared to the second comparative example. In particular, the dither matrix 510 was selected for the recording material in the 'smooth' category, and a high-line screen was performed over the entire pixel value range, resulting in the best evaluation score of '1'. For the recording material in the 'bond' category, the dither matrix 511 was selected for both the second comparative example and the embodiment, so the evaluation scores of the resolution were naturally the same.

[0048] Finally, the first comparative example, the second comparative example, and the embodiment were evaluated for overall image quality. In the overall evaluation, the subjects gave a score on a 5-point scale to a portrait image of a person, taking into consideration both mottle and resolution, and the average of the given scores was used as the evaluation score: '1' ... image quality is good '2' ... The factors that negatively impact the impression are visible, but do not bother me '3' ... I am concerned about factors that may negatively impact my impression, but I can tolerate them. '4' ... Factors that negatively impact the impression exceed acceptable levels

[0049] FIG. 9B shows a table listing the evaluation scores obtained in the overall evaluation. In the first comparative example, the lower the smoothness of the recording material surface, the higher the overall evaluation score, and it can be seen that mottle becomes more apparent in rougher recording materials, worsening the impression of image quality. In the second comparative example, the overall evaluation score is '3' in every category, and it can be seen that the use of a low-line screen does not adequately contribute to improving the overall image quality. In contrast, in the working example, the overall evaluation scores are improved compared to the first comparative example in the categories "bond" and "rough," and the overall evaluation scores remain good in the categories "standard" and "smooth."

[0050] The results of the evaluation tests described above show that the technique of selectively switching dither matrices described in this section can provide good image quality by suppressing mottle while reducing the decrease in image resolution.

[0051] <1-4. Modifications> Various modifications of the above-described embodiment can be envisioned. In the above, an example in which the recording material is classified into one of four categories according to the smoothness of the surface of the recording material has been described, but the number of category candidates may be more than four or less than four. In addition, although four dither matrix examples are shown in FIG. 7, the contents of the dither matrix are not limited to the illustrated example. For example, although the dither matrix 511 is shown as an example of the first dither matrix in FIG. 5, the dither matrix 512 or 513, or another dither matrix, may be treated as the first dither matrix. The other dither matrix refers to a dither matrix that differs in at least one of the following: at least one element value, matrix size, dot growth direction, and number of growth points. In addition, although the above examples of 141 lines and 100 lines have been described as examples of screen ruling, a higher or lower screen ruling may be used.

[0052] Also, instead of using categories related to the smoothness of the surface of the recording material, a physical property value indicating the smoothness itself may be used. For example, Beck smoothness may be used as a physical property value indicating the smoothness. Beck smoothness is the surface smoothness of a 10 cm2 effective area optically finished with a circular hole in the center through which air leaks in. 2 A glass standard surface is placed on the surface of the sample at 1 kg / cm 2 The Beck smoothness is measured as the time required for 10 ml of air to pass between the two surfaces at a predetermined pressure difference when the two surfaces are pressed against each other with a pressure of 100 MPa. The unit of Beck smoothness is seconds. The rougher the surface of the sample (here, the recording material), the lower the Beck smoothness. For example, the Beck smoothness of each of a plurality of candidates for the type of recording material is stored in advance in a database provided inside or outside the image forming apparatus 10. The characteristic information acquisition unit 305 acquires the Beck smoothness corresponding to the type designated by the user input as characteristic information by referring to the database. Then, the screen processing unit 303 selects a dither matrix to be used from a plurality of dither matrices based on the Beck smoothness indicated by the characteristic information acquired by the characteristic information acquisition unit 305. In this case, too, by selecting the first dither matrix described above for a recording material with a relatively low smoothness and the second dither matrix described above for a recording material with a relatively high smoothness, the image quality of the printed image can be improved by avoiding excessive use of a low screen ruling.

[0053] When the inventors examined the occurrence of mottle for several types of recording materials with different Beck smoothness, they found that mottle is likely to become evident in recording materials with a Beck smoothness of 45 seconds or less. Therefore, as a condition for switching between the first dither matrix and the second dither matrix, it is considered to select the first dither matrix when the Beck smoothness of the recording material is 45 seconds or less, and the second dither matrix when the Beck smoothness exceeds 45 seconds.

[0054] Also, mottle tends to be evident in a single color, especially black, and is less evident in process black or other secondary colors with a large amount of toner. Mottles can occur in single colors other than black, but are not as evident as in pure black. Therefore, the screen processing unit 303 may selectively switch the dither matrix to be used for screen processing based on the characteristic information only when the input image is a single-color black image or when the print engine 22 forms an image in monochrome mode. In this case, the screen ruling is lowered so that mottle is suppressed only in cases where mottle is likely to be evident, and a high screen ruling is maintained in other cases to avoid a decrease in resolution.

[0055] As seen from the evaluation results in FIG. 8A, selective switching of dither matrices based on the characteristic information is particularly effective in improving the image quality of high-density regions (shadow regions). Therefore, even if the characteristic information indicates that the first dither matrix should be selected, the screen processing unit 303 may select the second dither matrix for screen processing if the area ratio of the high-density region is below a predetermined reference value. The area ratio of the high-density region may be calculated, for example, as the ratio of pixels having pixel values ​​exceeding a threshold T1 to all pixels in the input image. The reference value of the area ratio may be, for example, 10%. In this case, it is possible to avoid a decrease in resolution for an input image that has only a small number of high-density regions to the extent that mottle is not noticeable, and to obtain a higher image quality of the printed image.

[0056] <<2. Second embodiment>> In the first embodiment described in the previous section, an example was described in which the characteristic information acquisition unit 305 acquires characteristic information based on user input. In contrast, in the second embodiment described in this section, the characteristic information acquisition unit 305 acquires characteristic information based on sensor data from a sensor that detects the characteristics of the recording material supplied to the print engine 22.

[0057] The basic configuration of the image forming apparatus 10 according to the second embodiment may be the same as the configuration described with reference to FIGS. 1 and 2. In this embodiment, the sensor unit 250 includes an image sensor (also called a media sensor) that captures an image of the surface of the recording material conveyed along the conveying path of the print engine 22 and outputs surface image data. The sensor unit 250 is located, for example, between the pair of registration rollers 210 and the secondary transfer position on the conveying path (see FIG. 2(A)). FIG. 10 shows an example of the configuration of the sensor unit 250. In the example of FIG. 10, the sensor unit 250 is composed of an LED 251, an imaging lens 252, a CMOS line sensor 253, a protective member 254, and a pressing member 255. The LED 251 irradiates light 256 toward the surface of the recording material S on the conveying path. As an example, the LED 251 is a white LED, and the incident angle of the light 256 with respect to the recording material S is 80 degrees (however, the color and incident angle of the light 256 are not limited to these examples). The imaging lens 252 forms an image of the light 256, which is irradiated from the LED 251 and reflected by the surface of the recording material S, on the light receiving surface of the CMOS line sensor 253. The CMOS line sensor 253 captures the image of the light 256 (i.e., the image of the surface of the recording material) formed on the light receiving surface via the imaging lens 252. Note that an area sensor having two-dimensionally arranged light receiving elements may be used instead of the CMOS line sensor 253. The protective member 254 protects the imaging lens 252 and the LED 251. The pressing member 255 is disposed at a position facing the protective member 254, and presses the recording material S conveyed along the conveying path against the protective member 254. The image captured by the CMOS line sensor 253 becomes captured image data through AD conversion (not shown), and a collection of captured image data for a plurality of lines constitutes the surface image data of one recording material.

[0058] In this embodiment, the characteristic information acquisition unit 305 acquires characteristic information related to the smoothness of the surface of the recording material based on the above-mentioned surface image data input from the sensor unit 250. Fig. 11 shows an example of a more detailed configuration of the characteristic information acquisition unit 305 of the image processing unit 105 according to this embodiment. With reference to Fig. 11, the characteristic information acquisition unit 305 includes an image extraction unit 311, a feature amount calculation unit 312, and a characteristic determination unit 313.

[0059] The image extraction unit 311 extracts data within an effective range to be used for determining the characteristics of the surface of the recording material from the surface image data input from the sensor unit 250. The image extraction unit 311 may perform shading correction in addition to extracting data. The feature amount calculation unit 312 calculates a feature amount representing the characteristics of the surface of the recording material based on the surface image data within the effective range extracted by the image extraction unit 311. The feature amount calculated by the feature amount calculation unit 312 may be, for example, an integrated value of the differences in pixel values ​​between adjacent pixels in the surface image data. The feature amount calculated by the feature amount calculation unit 312 may also be a representative value (for example, a maximum value, an average value, or a median value) of the differences in pixel values ​​between adjacent pixels. The larger the value of such a feature amount, the rougher the surface of the recording material, and the smaller the value, the smoother the surface of the recording material.

[0060] FIG. 12 shows examples of surface images and feature values ​​(surface profiles) of a recording material with a rough surface and a recording material with a smooth surface. An image 1201 shown on the left in FIG. 12 is an example of a surface image of a recording material with a relatively low surface smoothness (rough). An image 1202 shown on the right is an example of a surface image of a recording material with a relatively high surface smoothness (smooth). When pixel values ​​for one line are extracted from the surface image 1201 after shading correction, a surface profile curve with a large amplitude or containing more high-frequency components is obtained, as shown in the middle left of FIG. 12. On the other hand, when pixel values ​​for one line are extracted from the surface image 1202 after shading correction, a surface profile curve with a small amplitude or containing not so many high-frequency components is obtained, as shown in the middle right of FIG. 12. When the above-mentioned feature values ​​are calculated for such a surface profile, the feature value of the surface image 1201 will be greater than the feature value of the surface image 1202.

[0061] As an example, for four representative brands of recording material in the four categories of smoothness described above, the integrated value of the difference between adjacent pixels in each line of the surface image was calculated, and the integrated values ​​were averaged over all the lines to obtain the following average integrated difference value: Bond (NEENAH BOND 60g) ... 4.5 · Coarse (Xerox Vitality 75g) ... 3.0 ·Standard (CANON CS-680) ... 1.9 ·Smooth (CANON GF-C081) ... 0.9

[0062] The characteristic determination unit 313 may classify the recording material into one of two or more categories based on the above-mentioned feature amount calculated by the feature amount calculation unit 312. For example, the characteristic determination unit 313 may classify the recording material into the category "bond" when the average difference integrated value calculated from the surface image data of the recording material is 4.0 or more. The characteristic determination unit 313 may classify the recording material into the category "coarse" when the average difference integrated value is 2.5 or more and less than 4.0. The characteristic determination unit 313 may classify the recording material into the category "normal" when the average difference integrated value is 1.5 or more and less than 2.5. The characteristic determination unit 313 may classify the recording material into the category "smooth" when the average difference integrated value is less than 1.5. The screen processing unit 303 selects a dither matrix to be used for screen processing from a plurality of dither matrices based on the category of the recording material determined by the characteristic determination unit 313 in this way, as in the first embodiment.

[0063] In this embodiment, the characteristics related to the surface smoothness of the recording material are actually measured during image formation, and the dither matrix for screen processing is selectively switched based on the measurement results. Therefore, without requiring the user to input the type of recording material, or even if the user mistakenly specifies the type of recording material, a dither matrix suited to the characteristics of the recording material is appropriately used to provide the user with a printed image with improved image quality.

[0064] <<3. Summary>> Up to this point, several embodiments of the present disclosure have been described in detail with reference to FIGS. 1 to 12. In the above-described embodiment, one of a plurality of dither matrices is selected based on characteristic information indicating the characteristics of the recording material, and an image screened using the selected dither matrix is ​​formed on the recording material. The plurality of dither matrices include at least a first dither matrix and a second dither matrix. The first dither matrix is ​​a matrix for growing dots at a first screen line frequency in a pixel value range below a first threshold value, and for growing dots at a second screen line frequency lower than the first screen line frequency in a pixel value range above the first threshold value. The second dither matrix is ​​a matrix for growing dots at the first screen line frequency in at least a part of the pixel value range above the first threshold value. With this configuration, flexible screen processing is possible, such as using a low screen line frequency in a wide pixel value range on the high density side for a certain type of recording material, and suppressing or avoiding the use of a low screen line frequency for another type of recording material. As a result, it is possible to effectively suppress mottle that is easily manifested in a high density region while reducing the decrease in image resolution, thereby providing a good quality printed image.

[0065] As one example, the second dither matrix may be a matrix for growing dots at a first screen line ruling in a pixel value range below a second threshold value that is greater than the first threshold value, and for growing dots at a second screen line ruling in a pixel value range above the second threshold value. In this case, in the pixel value range between the first and second threshold values, the first dither matrix grows dots at a low screen line ruling (second screen line ruling), whereas the second dither matrix grows dots at a high screen line ruling (first screen line ruling). As another example, the second dither matrix may be a matrix for growing dots at a first screen line ruling throughout the entire pixel value range. In this case, in the first dither matrix, dots grow at a low screen line ruling, whereas the second dither matrix grows dots at a high screen line ruling throughout the entire pixel value range above the first threshold value. In any of these examples, the width of the pixel value range on the high density side where the low screen line ruling is used can be selectively switched according to the characteristics of the recording material through the selection of the dither matrix.

[0066] In the above-described embodiment, the characteristics of the recording material include the smoothness of the surface of the recording material. By selecting a dither matrix taking into consideration the smoothness of the surface of the recording material, it is possible to perform screen processing at a wider pixel value range and a lower screen ruling only for recording materials in which mottle is likely to become apparent. Specifically, a first dither matrix may be selected for screen processing for recording materials with relatively low smoothness, and a second dither matrix may be selected for screen processing for recording materials with relatively high smoothness. In this case, a low ruling screen with a wider pixel value range on the high density side is applied to a recording material with a rough surface in which mottle is likely to become apparent, thereby enhancing the effect of mottle suppression, while excessive application of a low ruling screen to a recording material with a smooth surface in which mottle is unlikely to become apparent can be avoided. For example, the recording material may be classified into one of two or more categories according to the smoothness of the surface, and the characteristic information may indicate the one category. In this case, by defining in advance the dither matrix to be selected for each of the candidate categories, it is possible to easily determine which dither matrix to select.

[0067] In one embodiment, the characteristic information is obtained based on a user input. For example, characteristic information corresponding to the type of recording material specified by the user input (e.g., a category related to smoothness, or the smoothness itself) may be obtained from a database provided inside or outside the image forming apparatus. In this case, a dither matrix suited to the characteristics of the recording material can be selected without providing an additional device for detecting the characteristics of the recording material in the image forming apparatus. In another embodiment, the characteristic information is obtained based on sensor data from a sensor for detecting the characteristics of the recording material supplied to the image forming unit. In this case, a dither matrix suited to the characteristics of the recording material can be appropriately selected without imposing the burden of user input on the user.

[0068] <<4. Other embodiments>> The above-mentioned embodiment can also be realized in the form of a process in which a program for realizing one or more functions is supplied to a system or device via a network or a storage medium, and one or more processors in a computer of the system or device read and execute the program. It can also be realized by a circuit (e.g., ASIC) that realizes one or more functions.

[0069] The invention is not limited to the above-described embodiments, and various modifications and variations are possible 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]

[0070] 10: image forming apparatus, 22: print engine (image forming section), 250: sensor unit, 303: screen processing section, 305: characteristic information acquisition section, 510, 511, 512, 513: dither matrix

Claims

1. a receiving unit that receives a user input specifying a type related to the smoothness of the surface of the recording material; a screen processing unit that generates a screen image by screening an input image using a dither matrix; an image forming section that forms the screen image generated by the screen processing section on the recording material; Equipped with The screen processing unit sets the dither matrix used for screening the input image as when the type of the recording material specified by the user input is a first type, a first dither matrix is ​​selected for performing screen processing at a first screen line frequency within a range where pixel values ​​are below a first threshold value, and for performing screen processing at a second screen line frequency lower than the first screen line frequency within a range where pixel values ​​exceed the first threshold value; selects a second dither matrix for performing screen processing with the first screen ruling in at least a portion of a range in which pixel values ​​exceed the first threshold value when the type of the recording material specified by the user input is a second type having a smoothness higher than that of the first type; the second dither matrix is ​​a matrix for performing screen processing at the first screen line frequency when pixel values ​​are below a second threshold value that is greater than the first threshold value, and for performing screen processing at the second screen line frequency when pixel values ​​are above the second threshold value, or a matrix for performing screen processing at the first screen line frequency for all pixel values. Image forming device.

2. 2. The image forming apparatus according to claim 1, the screen processing unit screens the input image using the second dither matrix when a ratio of pixels having pixel values ​​exceeding the first threshold in the input image falls below a predetermined reference value even if the type of the recording material specified by the user input is the first type. Image forming device.

3. 3. The image forming apparatus according to claim 1, the screen processing unit screens the input image using a dither matrix selected from the first dither matrix and the second dither matrix only when the input image is a black monochrome image or the image forming unit forms an image in a monochrome mode. Image forming device.

4. An image forming method performed by an image forming apparatus, comprising: accepting a user input specifying a type related to the smoothness of the surface of the recording material; generating a screen image by screening an input image using a dither matrix; forming the generated screen image on the recording material; Including, The dither matrix used to screen the input image is a first dither matrix for performing screen processing at a first screen line frequency within a range where pixel values ​​are below a first threshold value and for performing screen processing at a second screen line frequency lower than the first screen line frequency within a range where pixel values ​​exceed the first threshold value when the type of the recording material specified by the user input is a first type; a second dither matrix for performing screen processing with the first screen ruling in at least a portion of a range in which pixel values ​​exceed the first threshold value when the type of the recording material specified by the user input is a second type having a smoothness higher than that of the first type, the second dither matrix is ​​a matrix for performing screen processing at the first screen line frequency when pixel values ​​are below a second threshold value that is greater than the first threshold value, and for performing screen processing at the second screen line frequency when pixel values ​​are above the second threshold value, or a matrix for performing screen processing at the first screen line frequency for all pixel values. Image forming method.

Images