JP2013026729A - Image processing device, image forming device, image processing method, and computer program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve gradation reproducibility by image processing for combining results of image conversion by the respective ones of a plurality of gradation reproduction methods.SOLUTION: An image processing device that performs image processing using a first gradation reproduction method and a second gradation reproduction method comprises an input gradation correction unit that performs gradation correction on an input image. The input gradation correction unit increases density of the input image so as to compensate density insufficiency in a composite image due to partial overlapping of dots in combining a first gradation reproduction image and a second gradation reproduction image.

Description

  The present invention relates to an apparatus, a method, and a computer program for performing image processing for reproducing the gradation of an image.

  In an image forming apparatus that reproduces a halftone by area gradation, image processing using a gradation reproduction method such as a dither method or an error diffusion method is performed. In the image processing, a multi-value input image is converted into a binary or multi-value output image suitable for the configuration of the image forming mechanism. In this image processing, a plurality of gradation reproduction methods can be used properly or used together. In other words, selectively using a gradation reproduction method suitable for image attributes, such as using an error diffusion method with excellent reproducibility of high-frequency components for text and a dither method with excellent graininess for photographs. Image processing is generally performed. Also, a combined image processing is known in which an input image is converted into a single input image using a plurality of gradation reproduction methods, and the obtained images are synthesized at a predetermined ratio.

  In image processing that uses multiple gradation reproduction methods, the advantage of each of the multiple gradation reproduction methods can be exploited by adjusting the composition ratio, that is, the distribution of the multiple gradation reproduction methods, according to the input image. An output image with good image quality can be generated. Regarding the adjustment of the composition ratio, in Japanese Patent Application Laid-Open No. 2004-228688, image processing for determining a distribution ratio between the error diffusion method and the dither processing method based on the maximum density and the minimum density in a block having a predetermined number of pixels in a multi-valued image. A device has been proposed. Patent Document 2 proposes an image processing apparatus that performs wavelet analysis on features of an input image and determines a composition ratio in accordance with the determined features.

JP 2010-161504 A JP 2010-74627 A

  In combined image processing that combines multiple images obtained by each of multiple tone reproduction methods, the density of the output image is lower than the density of the input image that you want to reproduce, no matter how the composition ratio is determined. It turned out that there was a problem.

  An object of the present invention is to improve the gradation reproducibility of image processing for combining the results of image conversion by each of a plurality of gradation reproduction methods.

  An image processing apparatus that achieves the above object includes: an input tone correction unit that performs tone correction for increasing density of an input image; and the input image that has been tone-corrected by the input tone correction unit. A first processing unit that converts the first gradation reproduction method into a first image representing a density according to a first ratio that determines the distribution of the first gradation reproduction method, and the input gradation correction; A second ratio for determining the distribution of the second gradation reproduction method using the second gradation reproduction method for the input image subjected to gradation correction by the unit or the input image not subjected to gradation correction; A second processing unit that converts to a second image representing a density corresponding to the first image, the first image obtained by the first processing unit, and the second image obtained by the second processing unit. And a synthesis unit for generating an output image thereby.

  According to the present invention, the gradation reduction of the input image is compensated for the decrease in density caused by overlapping non-zero pixels among the plurality of images to be combined, so that the gradation reproducibility of the image processing is improved. .

1 is a diagram illustrating a schematic configuration of an image forming apparatus according to an embodiment of the present invention. 2 is a block diagram illustrating a hardware configuration of the image forming apparatus. FIG. 3 is a flowchart illustrating an outline of processing executed by a printer controller that functions as an image processing apparatus. 2 is a block diagram illustrating a functional configuration of a printer controller. FIG. 3 is a block diagram illustrating a functional configuration of a gradation reproduction unit in a printer controller. FIG. It is a figure which shows the method of an image composition. It is a figure which shows typically the mode of the image synthesis | combination in case gradation correction is not performed. It is a figure which shows typically the mode of the image composition in the case of performing gradation correction. It is a graph which shows the relationship between the 1st synthetic | combination ratio and a gradation correction level in the case of adding a common gradation correction to the input image respectively given to two screen processing parts. Relationship between the first composition ratio R and the gradation correction level L _C when an input image with gradation correction is given to one of the two screen processing units and an input image without gradation correction is given to the other It is a graph which shows. It is a figure which shows an example of the screen which comprises a man machine interface.

An example will be given in which the image forming apparatus performs image processing for gradation reproduction by area gradation. However,
Image processing can be performed in an apparatus separate from the image forming apparatus, and the obtained image can be input to the image forming apparatus and printed. Further, the use of the result of the image processing is not limited to image formation, and image processing may be performed for image display on a screen constituted by binary display elements, for example.

  Hereinafter, the configuration of the image forming apparatus will be described first, and then image processing performed in the image forming apparatus 1 will be described.

  An image forming apparatus 1 illustrated in FIG. 1 is an MFP (Multi-functional Peripheral) having various uses including copying and network printing. The image forming apparatus 1 includes a tandem printer engine 10 that forms a color or monochrome image by electrophotography. The printer engine 10 uses four imaging units 11, 12, 13, and 14 to form toner images of four colors of yellow (Y), magenta (M), cyan (C), and black (K) in parallel. can do. The imaging units 11 to 14 each have one photosensitive drum 15, 16, 17, and 18 as an image carrier. In each of the imaging units 11 to 14, components necessary for forming a toner image such as a developing device, a print head, a charging charger, and a cleaner are arranged around a cylindrical photosensitive member. The image forming apparatus 1 is given a print job by a direct operation using the operation panel 35 or an access from an external device (typically a personal computer) (not shown) connected via a network. The print job includes a copy job in which an original image is read by the image scanner 30 and copied. For example, when a print job for forming a color image is given, the image forming apparatus 1 operates as follows.

  Based on image data to be printed supplied from the image scanner 30 or an external device, the electric circuit 100 mounted on the image forming apparatus 1 generates a raster image that is exposure pattern data of each color of Y, M, C, and K. Generate. In a state where the photoconductor drums 15, 16, 17, and 18 are rotating, the print heads 22 of the imaging units 11 to 14 irradiate laser beams toward the charged photoconductor. At this time, the print head controls the light source according to the raster image, thereby performing main scanning for each color of the photosensitive member. An electrostatic latent image is formed on the photoconductor by main scanning and sub-scanning (rotation of the photoconductor). Since the development format is reversal development, in the formation of the electrostatic latent image, the dot portion to which the toner is to be attached is exposed, and the charge in that portion is less than the charge in the other portion. The developing units of the imaging units 11 to 14 adsorb the toner to the photoconductor to form a toner image corresponding to the electrostatic latent image. The formed toner image is primarily transferred to the intermediate transfer belt 25 which is an endless transfer body that moves along the arrangement of the photoreceptors. The order of toner image formation and the order of primary transfer are determined by the arrangement of the imaging units 11 to 14, and in this example, the order is Y, M, C, K from the upstream side.

  In parallel with the operation of forming four toner images per sheet surface, a sheet (not shown) is taken out from the multi-stage sheet stacker 40 by the pickup roller 45 and conveyed to the secondary transfer position. The secondary transfer position is a position where the intermediate transfer belt 25 and the pressure roller 26 face each other so as to form a nip portion. Toner images of four colors overlapped from the intermediate transfer belt 25 are secondarily transferred to a sheet passing through the secondary transfer position. Thereafter, the sheet passes through the inside of the fixing unit 29 and is discharged onto the discharge tray 48. When the paper passes through the fixing unit 29, the toner is melted by heating and pressurization, and the toner image is fixed on the paper as a print image.

  As shown in FIG. 2, the image forming apparatus 1 includes a main controller 60 that controls the entire control. The main controller 20 performs predetermined control in response to a user instruction from the operation panel 35 and an operation request from an external device communicating via the communication interface 66. In the copying operation, the image scanner 30 is controlled. In various printing operations including copying, an instruction is given from the main controller 20 to the engine controller 80 that controls the printer engine 10. Further, a hard disk drive (HDD) as the built-in storage 68 is accessed by the main controller 20 as necessary. The main controller 20 includes a central processing unit (CPU) that executes a control program and various applications, a read only memory (ROM) that stores the control program, and a random access memory (RAM) that is a work area for executing the program. Have. The main controller 20 includes a printer controller 70 that functions as an image processing apparatus. The printer controller 70 is a functional element realized by executing a control program in the electric circuit 100. In other words, the electric circuit 100 including a CPU, a RAM, a ROM, and other circuit components includes an image processing circuit that realizes the function of the printer controller 70.

  FIG. 3 shows an outline of processing executed by the printer controller 70. The printer controller 70 receives image data to be printed (S71), and executes rasterizing including color conversion processing from RGB to YMCK (S72). Then, the printer controller 70 uses a plurality of gradation reproduction methods to output multi-value (for example, 8-bit, 256 gradation) raster image data as a rasterization result, and output image data that is raster data for exposure control. (S73).

  As shown in FIG. 4, the printer controller 70 includes an analysis unit 71, a rasterization unit 72, and a gradation reproduction unit 73. The printer controller 70 generates output image data D4y, D4m, D4c, and D4k corresponding to each of the four color toner images for printing the print target data D1. Scan image data is input as print target data D1 from the image scanner 30 in the copy operation to the printer controller 70, and print target data D1 is input from the communication interface 66 in network printing and facsimile reception. When printing a document stored in the image forming apparatus 1, print target data D <b> 1 is input from the storage 68 to the printer controller 70.

  When the print job is a network printing job, the analysis unit 71 interprets print target data D1 described in a page description language (PDL), generates intermediate data representing the contents to be rendered, and rasterizes the data. Delivered to part 72. That is, the intermediate data is the data D2 output by the analysis unit 71 in this case. The contents to be drawn described in PDL are roughly divided into three attributes (types) of “text” representing characters, “graphics” representing figures and lines, and “image” representing raster images. The intermediate data includes information indicating the attribute of the area in the page. On the other hand, when image data D1 not described in PDL is input as in a copy job or a facsimile reception job, the analysis unit 71 hands over the print target data D1 to the rasterization unit 72 as data D2 substantially as it is. At the same time, the analysis unit 71 extracts a text region and a graphic region from the print target data D1 using a known region discrimination technique, and gives region attribute information indicating the result to the rasterization unit 72 and the gradation reproduction unit 73.

  The rasterizing unit 72 performs color conversion processing from RGB to YMCK on the data D2 delivered from the analyzing unit 71, and multi-value raster image data (frames) D3y, D3m corresponding to each of the four colors of YMCK. , D3c, D3k.

  The gradation reproduction unit 73 converts the raster image data D3y, D3m, D3c, and D3k into raster image data D4y, D4m, D4c, and D4k for exposure control using a plurality of gradation reproduction methods. The raster image data D4y, D4m, D4c, and D4k generated by the gradation reproduction unit 73 is sent to the print head corresponding to each of the four colors via the engine controller 80 to control exposure using a semiconductor laser as a light source. Used.

  FIG. 5 shows a functional configuration of the gradation reproduction unit 73. In FIG. 5, the processing for each of the four colors of YMCK is shown collectively. The tone reproduction unit 73 receives input image data D3 (a general term for the above-described raster image data D3y, D3m, D3c, and D3k), and outputs image data D4 (the above-described raster image data D4y, D4m) from the tone reproduction unit 73. , D4c, D4k).

  The input image data D3 includes a first processing block 731 that uses the first gradation reproduction method, a second processing block 732 that uses the second gradation reproduction method, and a distribution relating to the application of the plurality of gradation reproduction methods. This is input to the ratio determining unit 733 to be determined. The ratio determining unit 733 determines the distribution for each predetermined section in the input image, and determines the first combination ratio (R) and the second combination ratio (1-R) representing the determined distribution as the first processing block 731 and Output to the second processing block 732. In the present embodiment, the first gradation reproduction method is a dither method, and the second gradation reproduction method is an error diffusion method.

  Here, the distribution is determined based on the density information of the input image data D3 and the region attribute information from the analysis unit 71 described above. For example, by increasing the distribution of the dither method for the photographic area and increasing the distribution of the error diffusion method for the character area, the gradation of the photographic area is good and the resolution of the character area is high. Reproducibility can be realized.

In the first processing block 731, the correction level determination unit 771 determines the gradation correction level (L _C ) based on the first composition ratio (R) and the second composition ratio (1-R), and the gradation correction. The unit 761 performs gradation correction according to the gradation correction level (L _C ) on the input image data D3. Then, the screen processing unit 751 performs gradation reproduction processing (first screen processing) by a dither method on the input image data D3 subjected to gradation correction. The first processing block 731 outputs the first image data D31 obtained by the screen processing unit 751 to the image composition unit 734. The method for determining the gradation correction level and the gradation correction process will be described later.

In the second processing block 732, as in the first processing block 731, the correction level determination unit 772 determines the gradation correction level (L) based on the first combination ratio (R) and the second combination ratio (1-R). _C ) is determined, and the gradation correction unit 762 performs gradation correction corresponding to the gradation correction level (L _C ) on the input image data D3. In the second processing block 732, the screen processing unit 752 performs gradation reproduction processing (second screen processing) by the error diffusion method on the input image data D3 subjected to gradation correction. The second processing block 732 outputs the second image data D32 obtained by the screen processing unit 752 to the image composition unit 734.

  The image synthesis unit 734 synthesizes the first image data D31 and the second image data D32, thereby generating output image data D4. The image synthesis referred to here is an overlay process for obtaining a logical sum (OR) of both image data to be synthesized in units of pixels. That is, as shown in FIG. 6, each pixel of the composite image is a non-zero colored pixel having a density corresponding to a dot of the toner image (at least one of the corresponding pixels in the two images to be combined is filled in the drawing). Pixel), it becomes a colored pixel.

  In the example of FIG. 5, the input tone correction unit 730 that performs tone correction on the input image data D3 includes two correction level determination units 771 and 772 and two tone correction units 761 and 762. In this configuration, for the dither method and the error diffusion method, it is possible to perform gradation correction with different correction amounts between each other, and it is also possible to perform common gradation correction. However, when the correction is limited to the common gradation correction, the input gradation correction unit 730 is configured by one correction level determination unit 771 and one gradation correction unit 761, and the input image corrected by the gradation correction unit 761. The data D3 may be input to the two screen processing units 751 and 752 in common.

  Next, the function of the input tone correction unit 730 will be described in more detail. The input tone correction unit 730 is provided to improve tone reproducibility of image processing using a plurality of tone reproduction methods. That is, when the input image data D3 is directly input to the screen processing units 751 and 752 without performing gradation correction on the input image data D3 in the configuration of FIG. The density indicated by D4 is lower than the density to be reproduced indicated by the input image data D3. The reason is that overlapping of colored pixels corresponding to dots occurs between the image data D31 output from one screen processing unit 751 and the image data D32 output from the other screen processing unit 752. FIG. 7 schematically shows the state of image composition when gradation correction is not performed.

  In FIG. 7, the target gradation (L) that is the density of a certain area in the input image is set to 25% for the first synthesis ratio (R) that is the distribution of the dither method, and the second that is the distribution of the error diffusion method. A case where the synthesis ratio (1-R) is reproduced as 75% is illustrated. In the illustrated assumption, the target gradation (L) is a gradation level of 80% of the maximum gradation value (maximum density). As shown in FIG. 7A, for the dither method, a screen MA20 having an area ratio of 20% determined by the product (80% × 25%) of the target gradation (L) and the first synthesis ratio (R) is applied. Is done. As for the error diffusion method, as shown in FIG. 7B, a screen with an area ratio of 60% determined by the product (80% × 75%) of the target gradation (L) and the second synthesis ratio (1-R). MA60 is applied. When the screen MA20 and the screen MA60 are combined (superposed) as shown in FIG. 7C, some dots overlap between the two screens, so the dot area in the combined image M4z is equal to the dot area of the screen MA20. It becomes smaller than the sum of the dot area of MA60. The overlapping area ratio depends on the dot arrangement pattern, but can be regarded as a product of the area ratio of the screen MA20 and the area ratio of the screen MA60 in terms of probability calculation. The overlapping area ratio represents the magnitude of the gradation reproduction error due to the overlapping of dots.

  The gradation reproducibility of the image processing for converting the input image data D3 to the output image data D4 is improved by performing gradation correction for increasing the density prior to the screen processing in anticipation of the density reduction due to such overlapping of dots. Can be made. FIG. 8 schematically shows a state of image composition when gradation correction is performed.

In FIG. 8, similarly to the example of FIG. 7, the first combination ratio (R) is set to 25%, the second combination ratio (1-R) is set to 75%, and the target gradation (L) at the gradation level of 80% is set. The case of reproduction is illustrated. The gradation correction level L _C in the example of FIG. 8 is common to the dither method and the error diffusion method. As shown in FIG. 8A, in the dither method, the gradation level (L + L _C ) obtained by adding the gradation correction level (L _C ) to the target gradation (L) and the first synthesis ratio (R) A screen MA23 having a predetermined area ratio determined by the product is applied. As for the error diffusion method, as shown in FIG. 7B, the gradation level (L + L _C ) obtained by adding the gradation correction level (L _C ) to the target gradation (L) and the second synthesis ratio (1-R The screen MA63 having a predetermined area ratio determined by the product of As a result, as shown in FIG. 8C, a composite image M4 having an area ratio of 80% that matches the target gradation (L) is generated.

The gradation correction level (L _C ) is determined by the first synthesis ratio (R) and the second synthesis ratio (1-R) as follows.
here,
Maximum gradation value: L ₀
Target gradation: L
Tone correction level (common to the first screen process and the second screen process): L _C
First synthesis ratio: R
Second synthesis ratio: 1-R
Area ratio of target gradation L: S _L = L / L ₀
Area ratio of gradation level after correction: S _C = (L + L _C ) / L ₀
Area ratio of first screen processing: S _C × R
Area ratio of second screen processing: S _C × (1-R)
Overlap area ratio: S _C × R × S _C × (1−R) = S _C ² × R × (1−R)
And Since the area ratio S _C of the gradation level after the gradation correction may if equal to the area ratio S _L of target gradation L, that, first, overlap area ratio of the sum of the area ratio of the second screen processing the value obtained by subtracting may be consistent with the value of the area ratio S _L,
S _C · R + S _C · (1-R) -S _C ² · R · (1-R) = S _L
L _C that satisfies the above may be calculated. The gradation correction level L _C is expressed by the following equation (1) (description of derivation is omitted).

FIG. 9 is a graph showing the relationship between the first composition ratio R and the gradation correction level L _C when common gradation correction is applied to the input images given to the two screen processing sections. FIG. 9 shows the gradation correction level L _C calculated by the above equation (1) when the target gradation L is 64, 126, and 192, respectively. As shown in the figure, when the first composition ratio R is 50%, that is, when the first composition ratio R and the second composition ratio (1-R) are equal, the gradation correction level L _C is the largest. The synthesis ratio R is small gradation correction level L _C smaller than 50%, the gradation correction level L _C is smaller gradation correction level L _C larger than 50%. That is, the smaller the difference between the first synthesis ratio R and the second synthesis ratio (1-R), the greater the gradation correction level L _C.

Based on the above equation (1) for specifying such correction characteristics, the correction level determination units 771 and 772 (see FIG. 5) determine the gradation correction level L _C by calculation or lookup table lookup, The correction units 761 and 762 perform gradation correction by adding the gradation correction level L _C to the target gradation L. The screen processing unit 751 performs screen processing according to the corrected gradation level (L + L _C ) and the first composition ratio R, and the screen processing unit 752 performs the correction gradation level (L + L _C ) and the first level. The screen processing according to the synthesis ratio (1-R) of 2 is performed.

In the above, an example in which the common gradation correction is performed in the first processing block 731 and the second processing block 732 has been described, but a different amount of gradation correction may be performed for each gradation reproduction method. For example, gradation correction may be performed only on the input image data D3 input to the first processing block 731. here,
Maximum gradation value: L ₀
Target gradation: L
Tone correction level (first screen processing only): L _C
First synthesis ratio: R
Second synthesis ratio: 1-R
Area ratio of target gradation L: S _L = L / L ₀
Area ratio of gradation level after correction: S _C = (L + L _C ) / L ₀
Area ratio of first screen processing: S _C × R
Area ratio of second screen processing: S _L × (1-R)
Overlap area ratio: S _C × R × S _L × (1-R)
And Also in this case, the area ratio S _C of the gradation level after gradation correction should be equal to the area ratio S _L of the target gradation L.
S _C · R + S _L · (1−R) −S _C · S _L · R (1−R) = S _L
L _C can be calculated as follows. The gradation correction level L _C is expressed by the following equation (1) (description of derivation is omitted).

FIG. 10 shows the first composition ratio R and the gradation correction level L _C when an input image with gradation correction is given to one of the two screen processing units and an input image without gradation correction is given to the other. It is a graph which shows the relationship. FIG. 10 shows the gradation correction level L _C calculated by the above equation (2) when the target gradation L is 64, 126, and 192, respectively. As shown in the figure, the correction characteristic in this case is such that the gradation correction level L _C becomes smaller as the composition ratio R corresponding to the input image subjected to gradation correction is larger.

  As a modification of the above embodiment, for example, when the area ratio of the first screen process or the area ratio of the second screen process exceeds a predetermined threshold, the input image of the screen process that exceeds the threshold is used. May not perform gradation correction, and may perform gradation correction only for the input image of the other screen processing. This is to prevent the disadvantage that the gradation on the high density side is saturated due to the gradation correction.

  Further, a man-machine interface may be provided for the user of the image forming apparatus 1 or a service person to specify conditions for performing gradation correction. For example, by displaying the operation screen Q10 as shown in FIG. 11 on the display 351 of the operation panel 35, an operation for designating the upper limit of the gradation level range to be subjected to gradation correction becomes possible. The operation screen Q10 includes areas 903 and 904 for numerically inputting a gradation level or a composition ratio that the operator wants to set as an upper limit.

According to the above-described embodiment, gradation correction for increasing the density of the input image is performed prior to the screen processing, so that there is insufficient output image density due to overlapping of dots when combining the results of a plurality of screen processing. It is reduced. Since the gradation correction level L _C is determined based on the expression (1) or (2), gradation reproduction with a small error between the input and the output is realized.

  In the above-described embodiment, the gradation reproduction method to be applied is not limited to the combination of the dither method and the error diffusion method. For example, the present invention can be applied to a case where two types of dither methods having different screen patterns are used in combination. The function of the gradation correction unit 73 may be realized by software as illustrated, or all or part of the function may be realized by hardware.

1 Image forming device 70 Printer controller (image processing device)
100 Electrical circuit (image processing circuit)
D3 Input image data (input image)
730 Input tone correction unit R Composition ratio (first ratio)
D31 First image 751 Screen processing unit (first processing unit)
(1-R) Composition ratio (second ratio)
D32 Second image 752 Screen processing unit (second processing unit)
D4 output image 734 Image composition unit (composition unit)
10 Printer engine (imaging mechanism)
Q10 screen (man-machine interface)

Claims (13)

An image processing apparatus that performs image processing using a first gradation reproduction method and a second gradation reproduction method,
An input tone correction unit that performs tone correction to increase the density of the input image;
Using the first tone reproduction method, the input image that has been tone-corrected by the input tone correction unit has a density corresponding to a first ratio that determines the distribution of the first tone reproduction method. A first processing unit for converting into a first image to be represented;
Using the second gradation reproduction method, the second gradation reproduction method is used to distribute the input image that has undergone gradation correction by the input gradation correction unit or the input image that has not undergone gradation correction. A second processing unit for converting to a second image representing a density according to a second ratio to be determined;
A synthesis unit that synthesizes the first image obtained by the first processing unit and the second image obtained by the second processing unit, thereby generating an output image. An image processing apparatus. The input gradation correction unit, as the gradation correction, the first ratio and the second ratio so that the amount of increase in density increases as the difference between the first ratio and the second ratio decreases. The image processing apparatus according to claim 1, wherein the correction is performed to increase the density in accordance with the ratio. The input gradation correction unit performs first gradation correction and second gradation correction having a correction amount different from that of the first gradation correction as the gradation correction,
The first processing unit converts the input image to which the first gradation correction has been applied into the first image;
The image processing apparatus according to claim 1, wherein the second processing unit converts the input image to which the second gradation correction has been added into the second image. An image forming apparatus comprising: an image processing circuit that performs image processing using a first gradation reproduction method and a second gradation reproduction method; and an image forming mechanism that prints an image generated by the image processing circuit. ,
The image processing circuit includes:
An input tone correction unit that performs tone correction to increase the density of the input image;
Using the first tone reproduction method, the input image that has been tone-corrected by the input tone correction unit has a density corresponding to a first ratio that determines the distribution of the first tone reproduction method. A first processing unit for converting into a first image to be represented;
Using the second gradation reproduction method, the second gradation reproduction method is used to distribute the input image that has undergone gradation correction by the input gradation correction unit or the input image that has not undergone gradation correction. A second processing unit for converting to a second image representing a density according to a second ratio to be determined;
A synthesis unit that synthesizes the first image obtained by the first processing unit and the second image obtained by the second processing unit, thereby generating an output image. Image forming apparatus. The input gradation correction unit, as the gradation correction, the first ratio and the second ratio so that the amount of increase in density increases as the difference between the first ratio and the second ratio decreases. The image forming apparatus according to claim 4, wherein the correction is performed to increase the density in accordance with the ratio. The input gradation correction unit performs first gradation correction and second gradation correction having a correction amount different from that of the first gradation correction as the gradation correction,
The first processing unit converts the input image to which the first gradation correction has been applied into the first image;
The image forming apparatus according to claim 4, wherein the second processing unit converts the input image to which the second gradation correction has been applied into the second image. The image forming apparatus according to claim 4, wherein the input tone correction unit increases the density of only a portion of the input image that is equal to or lower than a set density. The image forming apparatus according to claim 7, further comprising a man-machine interface for an operation of designating the set density. The image forming apparatus according to claim 4, wherein one of the first gradation reproduction method and the second gradation reproduction method is a dither method and the other is an error diffusion method. An image processing method using a first gradation reproduction method and a second gradation reproduction method,
Perform tone correction to increase the density of the input image,
The gradation-corrected input image is converted into a first image representing a density corresponding to a first ratio that determines the distribution of the first gradation reproduction method, using the first gradation reproduction method. And
The input image that has been subjected to gradation correction or the input image that has not been subjected to gradation correction is used in accordance with a second ratio that determines the distribution of the second gradation reproduction method using the second gradation reproduction method. Converted into a second image representing the measured density,
An image processing method comprising: synthesizing the first image and the second image, thereby generating an output image. Density reduction due to overlapping of dots, determined by applying a product of the first ratio and the second ratio as the degree of overlap between the dots of the first image and the dots of the second image The image processing method according to claim 10, wherein the correction amount for the gradation correction is determined based on an arithmetic expression that determines a correction amount that compensates for the minute. In the gradation correction, the density is increased in accordance with the first ratio and the second ratio so that the amount of increase in density increases as the difference between the first ratio and the second ratio decreases. The image processing method according to claim 11, wherein the correction is performed. A computer program executed by a computer for realizing image processing using a first gradation reproduction method and a second gradation reproduction method,
An input tone correction process for performing tone correction to increase the density of the input image;
Using the first gradation reproduction method, the input image that has undergone gradation correction by the input gradation correction processing has a density corresponding to a first ratio that determines the distribution of the first gradation reproduction method. A first process for converting to a first image to be represented;
Using the second tone reproduction method, the second tone reproduction method is used to distribute the input image that has been tone-corrected by the input tone correction process or the input image that has not been tone-corrected. A second process for converting to a second image representing a density according to a second ratio to be determined;
Causing the computer to execute a synthesis process for synthesizing the first image obtained by the first process and the second image obtained by the second process, thereby generating an output image. A featured computer program.

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