JP2010157982A - Image processing apparatus, image forming apparatus, image processing method, image processing program, and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the consumption quantity of color material to be used for print processing, while maintaining gradation reproducibility. <P>SOLUTION: An image processing apparatus 10 includes a gradation reproduction processing section 19 and a mode switching section 31. The gradation reproduction processing section 19 outputs a dither image by applying multi-level dither processing to an input image while using a dither matrix 2. The mode switching section 31 switches a normal mode and a toner save mode. Furthermore, the gradation reproduction processing section 19 includes a threshold selector 52. In the case that a pixel of the dither image corresponding to a density adjustment position in the dither matrix 2 is defined as a specific pixel, the threshold selector 52 adjusts the number of gradations of the specific pixel and a gradation value indicative of a peak density in such a way that the specific pixel becomes multi-gradation even either under the normal mode or under the toner save mode, and the peak density of the specific pixel under the toner save mode is suppressed rather than under the normal mode. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an image processing apparatus and an image processing method for performing halftone processing, particularly multilevel dither processing, on input image data.

  Conventionally, as a method for outputting an image to a recording medium such as paper, various recording methods such as thermal transfer, electrophotography, or an ink jet method have been adopted. In these recording methods, so-called halftone processing is often used in which a dot pattern corresponding to the gradation of an input image is generated and formed on a print medium.

  In dither processing, which is one of the halftone processes, a dither matrix arranged in a matrix of n × m threshold values for each pixel is used, and multi-value image data input for each pixel is set to each threshold value of the dither matrix. A halftone image is obtained by comparing pixel values and quantizing multi-value image data into binary or multi-value data. Examples of the dither matrix used for creating the dot pattern include a dot dispersion type and a dot concentration type dither matrix.

  On the other hand, with the recent increase in awareness of environmental problems, there is a growing need to reduce the amount of paper and toner used in multifunction devices installed in offices and the like as much as possible. On the other hand, Patent Document 1 discloses a method for realizing toner saving with a simple configuration while minimizing color change. Patent Document 1 discloses that when the toner save mode is selected, white mask processing for replacing output data at a predetermined position of the dither matrix with 0 is performed.

Japanese Patent Laying-Open No. 2005-86719 (Publication date: March 31, 2005)

  However, in the configuration of Patent Document 1, when white mask processing is performed, the amount of toner used (output area) can be suppressed. However, white mask processing may be performed even on a high density portion, and the reproduction is reproduced. There is a problem that the gray level becomes unnatural.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the conventional problems, and image processing capable of suppressing consumption of color materials (toner, ink, etc.) used in printing processing while maintaining gradation reproducibility. An apparatus and an image processing method are provided.

  In order to achieve the above object, the present invention corresponds to each pixel of a multi-tone input image and each pixel of a multi-tone dither image obtained by performing multi-value dither processing on the input image. Based on the dither matrix in which the relationship between the gradation value of each pixel of the input image and the gradation value of each pixel of the dither image is determined for each pixel corresponding position that is a position, In an image processing apparatus including a dither processing unit that performs multi-value dither processing for converting a gradation value into a gradation value of each pixel of the dither image, the normal mode and the normal mode when the dither image is printed. A switching unit for switching between the save mode in which the multi-value dither processing is performed so that the consumption of the color material for printing is suppressed more than the time, and the dither processing unit corresponds to all pixel corresponding positions in the dither matrix. Out of When the pixel of the dither image corresponding to a specific position is a specific pixel, the specific pixel has multiple gradations regardless of whether the specific mode is the normal mode or the save mode. And an adjustment unit that adjusts the number of gradations of the specific pixel and the gradation value indicating the maximum density so that the maximum density of the specific pixel is suppressed more than in the normal mode. To do.

  According to the configuration of the present invention, in the save mode, for some pixels included in the dithered image to be printed, the maximum density level is suppressed while maintaining a multi-gradation (the number of gradations is 3 or more). Therefore, it is possible to suppress consumption of the color material during printing while suppressing unnatural gradation reproducibility in the entire printed dither image.

  In addition, when the image handled by the image processing apparatus of the present invention is a color image, multi-value dither processing is performed on an input image with a different color component when multi-value dither processing is performed on an input image with a certain color component. If the specific position (that is, the position where the density is suppressed) is the same as when processing is performed, the density of the region corresponding to the specific position in the printed color image may be extremely reduced. If this happens, the gradation reproducibility may become unnatural. Therefore, when the image processing apparatus according to the present invention is a color image processing apparatus that handles the input image and the dither image for each color component, the adjustment unit sets the specific position in the dither matrix for each color component. It is preferable to change the location. According to this configuration, the specific position differs between when multi-value dither processing is performed on an input image of a certain color component and when multi-value dither processing is performed on an input image of another color component. Therefore, it is possible to suppress a situation in which the density of the region corresponding to the specific position in the printed color image is extremely reduced, and it is possible to suppress the gradation reproducibility from being deteriorated.

  Furthermore, if the specific pixels corresponding to the specific position are concentrated in a certain area in the dither image, there is a possibility that the density of the area will be extremely lower than the density of other areas. If this situation occurs, the gradation reproducibility may become unnatural. Therefore, in the image processing apparatus of the present invention, a plurality of specific positions are not continuous along the row direction of the dither matrix, and the specific positions are not continuous along the column direction of the dither matrix. The specific position is preferably arranged in the dither matrix. According to this configuration, it is possible to suppress the specific pixels corresponding to the specific position from being concentrated on a certain area in the dither image, and it is possible to further suppress the gradation reproducibility from becoming unnatural.

  Further, in the image processing apparatus of the present invention, in addition to the above configuration, the dither matrix has a plurality of specific positions, and the plurality of specific positions include at least first and second specific positions, The adjustment unit causes the gradation value indicating the highest density of the first specific pixel corresponding to the first specific position to be different from the gradation value indicating the highest density of the second specific pixel corresponding to the second specific position. Alternatively, the adjustment may be performed. Furthermore, in the image processing apparatus of the present invention, in addition to the above-described configuration, the dither matrix includes a plurality of sub-matrices, each sub-matrix has the specific position, and the adjustment unit is a specific unit belonging to different sub-matrices The configuration may be such that the adjustment is performed so that the gradation values indicating the highest density of the specific pixel corresponding to the specific position are different from each other at the positions. With the above configuration, it is possible to obtain an image in which more gradation levels are maintained, and it is possible to contribute to a certain degree of image quality improvement even when toner save is performed.

  In the image processing apparatus of the present invention, for each pixel-corresponding position of the dither matrix, a threshold value for determining the gradation value of the dither image corresponding to the gradation value of the input image is determined. The dither processing unit is configured to compare the gradation value of the input image with a threshold value, and to determine the gradation value of the dither image to be converted from the gradation value of the input image according to the comparison result. May be. Furthermore, in the image processing apparatus of the present invention, the dither processing unit is configured such that the correspondence between the gradation value of each pixel of the input image and the gradation value of each pixel of the dither image defined by the dither matrix. The gradation value of each pixel of the input image may be converted to the gradation value of each pixel of the dither image using a table showing the above. The configuration using the table has an advantage that the multi-value dither processing can be speeded up.

  Furthermore, the image processing apparatus of the present invention uses the gradation correction table showing the relationship between the gradation value before correction and the gradation value after correction, and the input image before the multi-value dither processing is executed. There may be provided an output gradation correction unit for performing gradation correction processing. Further, in this case, the output gradation correction unit performs the gradation correction processing using the gradation correction table set for the normal mode in the normal mode, and for the normal mode in the save mode. The gradation correction table is changed by multiplying each corrected gradation value included in the gradation correction table set to a constant coefficient, and the gradation correction is performed using the gradation correction table after the change. It is preferable that the processing is performed. Further, the output gradation correction unit performs the gradation correction processing using the gradation correction table set for the save mode in the save mode, and is set for the normal mode in the normal mode. The gradation correction table is corrected by multiplying each corrected gradation value included in the gradation correction table by a constant coefficient, and gradation correction processing is performed using the corrected gradation correction table. It is preferable that it is the structure which performs. In this way, the gradation correction table set for one mode can be used in the other mode. Therefore, if only the gradation correction table set for one mode is stored as a default, it is not necessary to store separate tables for the normal mode and the save mode, and the storage capacity of the image processing apparatus is reduced. It becomes possible to save.

  An image forming apparatus including the above-described image processing apparatus is also included in the scope of the present invention. Here, the image forming apparatus means an electrophotographic or inkjet printer, a copier having the printer, and a multifunction machine having the printer.

  Furthermore, the present invention provides each pixel corresponding position which is a position corresponding to each pixel of the multi-tone input image and each pixel of the multi-tone dither image obtained by performing multi-value dither processing on the input image. The dither image is obtained from the tone value of each pixel of the input image based on a dither matrix in which the relationship between the tone value of each pixel of the input image and the tone value of each pixel of the dither image is defined. In an image processing method including a dither processing step of performing multi-value dither processing for converting to a gradation value of each pixel of the normal mode, and when the dither image is printed, the color material for printing is more than in the normal mode. A save mode for performing the multi-value dither processing so that the consumption amount of the dither is reduced, and in the dither processing step, the dither corresponding to some specific positions of all the pixel corresponding positions of the dither matrix Picture When the specific pixel is a specific pixel, the specific pixel has multiple gradations regardless of whether the specific mode is the normal mode or the save mode, and the specific pixel is more specific in the save mode than in the normal mode. The number of gradations of the specific pixel and the gradation value indicating the maximum density may be adjusted so that the maximum density of the pixel is suppressed.

  Furthermore, the image processing apparatus may be realized by a computer. In this case, an image processing program that causes the computer to function as each unit described above, and a computer-readable recording medium that records the image processing program are also provided. It falls into the category of the invention.

  As described above, in the image processing apparatus according to the present invention, when the pixels of the dither image corresponding to some specific positions among all the pixel corresponding positions of the dither matrix are set as specific pixels, the specific pixels are In any of the normal mode and the save mode, the specific pixel has multiple gradations, and the maximum density of the specific pixel is suppressed more in the save mode than in the normal mode. And an adjustment section for adjusting the number of gradations and the gradation value indicating the maximum density. Therefore, it is possible to suppress consumption of the color material during printing while suppressing unnatural gradation reproducibility in the entire printed dither image.

1 is a block diagram showing a schematic configuration of a digital color copying machine (image forming apparatus) according to an embodiment of the present invention. (A) is a diagram showing a dither matrix used in the multi-value dither processing of the present embodiment, and (b) is a density adjustment in the dither matrix used in the multi-value dither processing of the present embodiment. It is the figure which showed the position. (A) shows the threshold value and output value associated with the normal position among the pixel corresponding positions in the dither matrix and the normal mode associated with the density adjustment position among the pixel corresponding positions. It is the figure which showed the example of a threshold value and an output value. (B) is a diagram showing an example of a threshold value and an output value for toner save mode associated with a density adjustment position among pixel corresponding positions in the dither matrix. It is explanatory drawing which showed the application method of the dither matrix with respect to an input image. It is a block diagram which shows schematic structure of the gradation reproduction process part shown by FIG. It is the flowchart which showed the flow of the process performed in a gradation reproduction process part. FIG. 10 is a diagram illustrating a dither pattern growth process (dot growth process) when the density of an input image is uniformly increased in a normal mode. FIG. 7 is a diagram illustrating a dither pattern growth process (dot growth process) when the density of an input image is uniformly increased in the toner save mode. (A) is a graph showing the relationship between the gradation value (input value) for each pixel of the input image in the normal mode and the total output value in one dither matrix, and (b) is in the toner save mode. 5 is a graph showing the relationship between the gradation value for each pixel of the input image and the total output value in one dither matrix. It is the block diagram which showed schematic structure of the gradation reproduction process part of the form which performs a multi-value dither process using a dither output conversion table. 1 is a block diagram illustrating a configuration when a printer driver to which an image processing apparatus according to an embodiment is applied is realized by a computer. FIG. 10 is a diagram showing a dither pattern growth process (dot growth process) when the maximum output value is made different between density adjustment positions belonging to different sub-matrices in the toner save mode. FIG. 13A is an example of an output image in which the highest output values of all density adjustment positions in the dither matrix are made the same. FIG. 13B shows the highest density adjustment positions belonging to different sub-matrices. It is an example of the output image of the form which makes an output value different.

[Configuration of image processing apparatus]
Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing a schematic configuration of a digital color copying machine (image forming apparatus) 40 including a color image processing apparatus (image processing apparatus) 10 according to an embodiment of the present invention.

  As shown in FIG. 1, the digital color copying machine 40 includes an image processing device 10, a color image input device (image input device) 20, and a color image output device (image output device) 30. The image processing apparatus 10 and the image output apparatus 30 are connected to the image processing apparatus 10. The digital color copying machine 40 includes an operation panel 41.

  The image processing apparatus 10 includes an A / D conversion unit (analog / digital) 11, a shading correction unit 12, an input tone correction unit 13, a region separation processing unit 14, a color correction unit 15, a black generation and under color removal unit 16, and a space. A filter processing unit 17, an output tone correction unit 18, and a tone reproduction processing unit 19 are provided.

  The image input device 20 includes, for example, a scanner unit including a charge coupled device (hereinafter referred to as a CCD), irradiates light on a document on which an image is recorded, and the reflected light image is applied to the CCD. Are read as RGB analog signals, and the analog signals are input to the image processing apparatus 10.

  An analog signal input to the image processing apparatus 10 includes an A / D conversion unit 11, a shading correction unit 12, an input tone correction unit 13, a region separation processing unit 14, a color correction unit 15, a black generation and under color removal unit 16, The spatial filter processing unit 17, the output tone correction unit 18, and the tone reproduction processing unit 19 are sent in this order, and are output to the color image output device 30 as CMYK digital color signals.

  The A / D converter 11 converts the input RGB analog signal into a digital signal. The shading correction unit 12 performs processing for removing various distortions generated in the illumination system, the imaging system, and the imaging system of the image input device 20 on the RGB digital signals output from the A / D conversion unit 11.

  The input tone correction unit 13 performs a process for adjusting the color balance on the RGB signal (RGB reflectance signal) from which various distortions have been removed by the shading correction unit 12 and is employed in the color image processing apparatus 10. It is converted into a signal (for example, a density signal) that is easy to handle in the image processing method.

  The region separation processing unit 14 separates each pixel in the input image indicated by the RGB signal output from the input tone correction unit 13 into a plurality of regions such as a character edge region, a halftone dot region, and a photo region. Further, the region separation processing unit 14 generates a region identification signal indicating which region the pixel belongs to based on the separation result, and generates a black generation and under color removal unit 16, a spatial filter processing unit 17, and a tone reproduction processing unit. 19, and the input signal output from the input tone correction unit 13 is output to the subsequent color correction unit 15 as it is.

  The color correction unit 15 performs a process of removing color turbidity based on the spectral characteristics of CMY color materials including unnecessary absorption components in order to faithfully reproduce colors. As a processing method in this case, there are a method of holding a correspondence relationship between an input RGB signal and an output CMY signal as an LUT (look-up table), a color masking method using a conversion matrix as shown in Equation 1 below, and the like.

For example, when the color masking method is used, an L ^* a ^* b ^* value (CIE 1976 L ^* a ^* b ^* signal (CIE: Commission International de l ' ^{) of} a color output when a certain CMY is given to the image output device 30. Eclairage: International Lighting Commission, L ^* : Lightness, a ^* , b ^* : Chromaticity))) The RGB data when the scanner reads a color patch having the same L ^* a ^* b ^* and the image output device 30 A large number of combinations with the given CMY data are prepared, and coefficients of the transformation matrix from a ₁₁ to a _{33 in} the following equation 1 are calculated from the combinations. Color correction processing is performed using these coefficients. When higher accuracy is desired, a higher-order term of second order or higher may be added.

  The black generation and lower color removal unit 16 generates a new CMY signal by subtracting a portion where the original CMY signal overlaps with a black generation process that generates a black (K) signal from the CMY three-color signal after color correction. The CMY three-color signal is converted into a CMYK four-color signal.

  The spatial filter processing unit 17 performs a spatial filter process using a digital filter on the image data of the CMYK signal input from the black generation and under color removal unit 16 based on the region identification signal. As a result, the spatial frequency characteristics are corrected, and blurring and graininess deterioration of the output image are prevented.

  The output tone correction unit 18 performs output tone correction processing according to the output characteristic value of the image output device 30 on the CMYK signal that has been subjected to the spatial filter processing.

  Similar to the spatial filter processing unit 17, the gradation reproduction processing unit (dither processing unit) 19 performs gradation reproduction processing on the image data of the CMYK signal based on the region identification signal, and finally the image is simulated. So that the gradation can be reproduced.

  Processing in the spatial filter processing unit 17 and the gradation reproduction processing unit 19 will be further described. For example, in order to improve the reproducibility of black characters or color characters particularly for the region separated as the character edge region by the region separation processing unit 14, the spatial filter processing unit 17 performs high frequency by sharp enhancement processing in the spatial filter processing. Emphasize ingredients. On the other hand, the gradation reproduction processing unit 19 performs multilevel processing on a high-resolution screen suitable for reproducing high-frequency components.

  In addition, the spatial filter processing unit 17 performs low-pass filter processing for removing the input halftone component on the region separated as the halftone region by the region separation processing unit 14. Further, the gradation reproduction processing unit 19 performs multi-value processing on the halftone dot area with a screen that emphasizes gradation. Further, the gradation reproduction processing unit 19 performs multi-value processing on a region separated as a photographic region by the region separation processing unit 14 using a screen that emphasizes gradation reproducibility.

  In the digital color copying machine 40 of the present embodiment, a multi-tone input image is reproduced with a smaller number of gradations (number of gradations that can be expressed by the image output device 30) than the number of gradations of the input image. For this purpose, the gradation reproduction processing unit 19 performs multi-value dither processing using a dither screen (dither matrix). The present embodiment is characterized by multi-value dither processing performed by the gradation reproduction processing unit 19. This multi-value dither process will be described in detail later.

  The operation panel 41 includes, for example, a touch panel in which a display unit such as a liquid crystal display and an operation unit such as a setting button are integrated. The operations of the image input device 20, the image processing device 10, and the image output device 30 are controlled based on information input from the operation panel 41.

  In the image processing apparatus 10, the image data subjected to the above-described processes is temporarily stored in a storage unit (not shown), read out at a predetermined timing, and output to the color image output apparatus 30. The image output device 30 is an electrophotographic printer, and outputs (prints) an image indicated by image data on a recording medium (for example, paper). The above processes are controlled by a CPU (Central Processing Unit) (not shown).

[Overview of multi-value dither processing]
In the following, first, an outline of the multi-value dither process will be described. In the present embodiment, an image obtained by performing multi-value dither processing on an input image is referred to as a dither image. In principle, an input image having 256 gradations (an image before multi-value dither processing) is converted into a dither image having 16 gradations by multi-value dither processing. Of course, the number of gradations of the input image and the dither image is not limited to these, and the number of gradations M of the input image and the number of gradations N of the dither image are values that satisfy M> N (where N ≧ 3). Any value can be used. Further, input image data (hereinafter referred to as “input image data”) is a gradation value (value indicating density) of each color component of CMYK, and multi-value dither processing is performed on input image data of each color component. Done. Therefore, in the following description, only processing for input image data of one color component will be described.

  FIG. 2A is a diagram showing a configuration of a dither matrix used in the multi-value dither processing of the present embodiment. In the present embodiment, as shown in FIG. 2A, the dither matrix 2 is configured by four sub-matrices 2a to 2d.

  Further, each of the sub-matrices 2a to 2d has a position corresponding to each pixel of the input image and each pixel of the dither image generated by the multi-value dither processing (hereinafter referred to as “pixel corresponding position”). Yes. Specifically, one square which is a constituent element of each of the sub-matrices 2a to 2d is one pixel corresponding position.

  That is, each of the sub-matrices 2a to 2d in FIG. 2A has 10 pixel corresponding positions, and the dither matrix 2 has 40 pixel corresponding positions. In FIG. 2A, the numbers described in the squares are values indicating the order in which high-density output values are assigned in the dither pattern generated corresponding to the dither matrix 2.

  Then, 15 threshold values and 16 output values (dither image gradation values) are associated with each pixel corresponding position in the dither matrix 2.

  FIG. 3A is a diagram illustrating an example of a threshold value and an output value associated with one pixel corresponding position. In FIG. 3A and the following description, the j-th threshold value associated with the pixel corresponding position i (i = 0, 1,..., N) is set to Th [i] [j] (j = 0 , 1,..., 14), and the m-th output value associated with the pixel corresponding position i is referred to as Out [m] (m = 0, 1,..., 15). Note that i is a value indicating the order in which high-density output values are assigned in the dither pattern generated corresponding to the dither matrix 2 (that is, i is a square in FIG. 2A). Is the value shown). Further, it is assumed that Th [i] [j] ≦ Th [i] [j + 1] always holds, and Out [m] ≦ Out [m + 1] always holds.

  As shown in FIG. 3A, each threshold corresponding to the pixel corresponding position i is within a range of values (0 to 255) that the gradation value (input gradation value X) for each pixel of the input image can take. Set to As shown in FIG. 3 (a), the range of values that the input gradation value X can take (0 to 255) is divided into 16 sections, which is the same as the number of gradations of the dither image, by 15 threshold values. Each of these 16 sections is associated with output values (dither image gradation values) from 0 to 15 in order from the smallest threshold.

  In the multi-value dither processing, the dither matrix 2 is superimposed on the input image, and the gradation value (0 to 255) of the pixel of the input image is associated with the pixel corresponding position in the dither matrix 2 that overlaps the pixel. Based on the threshold value group, the output value (integer) of 0 or more and 15 or less is quantized. When the dither matrix 2 shown in FIG. 2A is used, as shown in FIG. 4, the quantization process is performed along the direction indicated by the broken line (screen angle direction) while superimposing the dither matrix 2 on the input image. repeat.

  The details of the quantization process are as follows. A size comparison between a gradation value (input image data) of a pixel of an input image (hereinafter referred to as “input pixel”) and a threshold value group associated with a pixel corresponding position in the dither matrix 2 overlapping the input pixel is performed. Then, a section including the gradation value of the input pixel is specified from the 16 sections described above. Then, an output value (dither image gradation value) associated with the identified section is output.

  In other words, the dither matrix 2 defines the correspondence between the gradation value (gradation value range) of each pixel of the input image and the gradation value of each pixel of the dither image for each pixel corresponding position. is there. In the present embodiment, the gradation value associated with the pixel corresponding position indicates a lower density as the value is smaller, and indicates a higher density as the value is larger.

[About the gradation reproduction processing section]
In addition, as shown in FIG. 2B, the gradation reproduction processing unit 19 according to the present embodiment converts the pixel corresponding position indicated by the circle among all the pixel corresponding positions in the dither matrix 2 to the density adjustment position ( Limiter processing target position). Then, the gradation reproduction processing unit 19 has a pixel (specific pixel) of the dither image corresponding to the density adjustment position (specific position) when shifting to the toner save mode than in the normal mode other than the toner save mode. It is characterized in that it suppresses the maximum concentration of. Note that the numbers written in the cells in FIG. 2B are assigned high-density gradation values in the dither pattern generated corresponding to the dither matrix 2 as in FIG. This is a value indicating the order, and is a value indicating i.

  In the following, first, a mode switching unit for switching between the toner save mode and the normal mode will be described, and then the configuration and processing contents of the gradation reproduction processing unit 19 will be described in detail.

  Note that the toner save mode (save mode) is a mode that places importance on saving toner, and the normal mode is a mode that places importance on the image quality of the output image. Specifically, the toner save mode is a consumption of color material (toner) for printing than in the normal mode when the dither image output from the gradation reproduction processing unit 19 is printed by the image output device 30. In this mode, image processing (including multi-value dither processing) is performed so that the amount is suppressed. In the normal mode, when the dither image output from the gradation reproduction processing unit 19 is printed by the image output device 30, image processing is performed so that the image quality of the print image is higher than that in the toner save mode. Mode.

  As shown in FIG. 1, the image processing apparatus 10 according to the present embodiment includes a mode switching unit (switching unit) 31.

  When the user has selected the normal mode, the mode switching unit 31 transmits a normal signal indicating the normal mode to the output tone correction unit 18 and the tone reproduction processing unit 19, and the user selects the toner save mode. If it is, the toner save signal indicating the toner save mode is transmitted to the output tone correction unit 18 and the tone reproduction processing unit 19. Note that the user can select either the normal mode or the toner save mode by operating the operation panel 41.

  Thereby, when the user selects the normal mode, the output tone correction unit 18 and the tone reproduction processing unit 19 can be shifted to the normal mode, and the user selects the toner save mode. In this case, the output tone correction unit 18 and the tone reproduction processing unit 19 can be shifted to the toner save mode.

  Next, the gradation reproduction processing unit 19 will be described. As shown in FIG. 5, the gradation reproduction processing unit 19 includes a threshold processing unit 51, a threshold selection unit (adjustment unit) 52, and a storage unit 53.

  The storage unit 53 stores the threshold value and the output value for each pixel corresponding position of the dither matrix 2. More specifically, the storage unit 53 is a first combination of 15 threshold values and 16 output values for pixel corresponding positions (hereinafter referred to as “normal positions”) other than the density adjustment position. A combination of is stored. The storage unit 53 also has a first combination that is a combination of 15 threshold values and 16 output values (0 to 15), 4 threshold values, and 5 output values (0) for the density adjustment position. ˜4) are stored as the second combination.

  Here, the first combination and the second combination will be described. In the multi-value dither processing of the present embodiment, the gradation values of the input image from 0 to 255 are set to 0 to 15 at each normal position of the dither matrix 2 in any of the normal mode and the toner save mode. The process of quantizing the tone value of the dither image is performed. Therefore, the storage unit 53 stores the first combination for each normal position for each normal position. The combination of the threshold value and the output value shown in FIG. 3A corresponds to the first combination.

  Also, in the normal mode, at each density adjustment position of the dither matrix 2, a process of quantizing the gradation value of the input image from 0 to 255 into the gradation value of the dither image from 0 to 15 is performed as in the normal position. . However, when the toner save mode is selected, at each density adjustment position of the dither matrix 2, a process of quantizing the gradation value of the input image from 0 to 255 to the gradation value from 0 to 4 is performed. . Therefore, the storage unit 53 stores not only the first combination for each density adjustment position but also the second combination for each density adjustment position for each density adjustment position. Here, the combination of the threshold value and the output value shown in FIG. 3B corresponds to the second combination.

  That is, at the density adjustment position in the toner save mode, the range of possible values (0 to 255) of the gradation value X of the input image is divided into five sections by the four threshold values in the second combination. Each of the five sections is associated with output values from 0 to 4 in order from the smallest threshold. At the density adjustment position, the gradation value of the input pixel is compared with the four threshold values associated with the density adjustment position, and the gradation value of the input pixel among the five sections described above is compared. Identify the included sections. Then, the output value associated with the identified section is set as the gradation value in the output dither image.

  Note that the number of output values (number of gradations) in the second combination is not limited to five, but is set to be smaller than the number of output values (number of gradations) in the first combination and three or more. There is a need. In addition, the maximum value of the output values of the second combination needs to be set to be 2 or more than the maximum value of the output values of the first combination. The reason for setting in this way is to enable multi-value dither processing while suppressing the maximum density level at the density adjustment position in the toner save mode as compared with that in the normal mode. Further, by setting the maximum value of the output values of the second combination to 2 or more, the input gradation value indicating black (maximum density level) is quantized to the output gradation value indicating white (minimum density). Can be suppressed.

  The threshold selection unit 52 is a block that reads the first combination threshold and the second combination threshold from the storage unit 53.

  More specifically, as shown in FIG. 5, the threshold selection unit 52 shifts to the normal mode when receiving a normal signal from the mode switching unit 31, and shifts to the toner save mode when receiving a toner save signal. Then, during the transition to the normal mode, the threshold selection unit 52 sequentially reads 15 thresholds included in the first combination associated with each pixel corresponding position for each pixel corresponding position.

  Further, even when the threshold selection unit 52 is in the toner save mode instead of the normal mode, the normal positions of the all-pixel corresponding positions are sequentially associated with the normal positions. Fifteen threshold values included in the first combination are read out. On the other hand, when the threshold selection unit 52 has shifted to the toner save mode, each of the density adjustment positions in the all-pixel corresponding positions is sequentially associated with the second density adjustment position. Four threshold values included in the combination are read out.

  The threshold processing unit 51 performs a quantization process on each pixel corresponding position in the dither matrix 2 in order using the threshold read by the threshold selection unit 52. More specifically, the threshold processing unit 51 sequentially reads out (1) the gradation value (input image data) of the input pixel and the threshold selection unit 52 for each pixel corresponding position in the dither matrix 2. Comparison with a threshold value group is performed. (2) Based on the result of the comparison, an output value corresponding to the gradation value of the input pixel is specified from the output values stored in the storage unit 53. (3) The specified output value is handled as the tone value of the output dither image.

  Next, a processing procedure of the gradation reproduction processing unit 19 when a certain input pixel is a processing target will be described. FIG. 6 is a flowchart showing the flow of processing executed by the gradation reproduction processing unit 19.

  First, the threshold selection unit 52 determines whether or not the current processing mode is the toner save mode (S1). If the threshold selection unit 52 determines that the toner save mode is not selected (No in S1), the 15 threshold values corresponding to the pixel corresponding to the input pixel to be processed (threshold values included in the first combination). ) Is read from the storage unit 53 (S2). Thereafter, the threshold processing unit 51 performs a quantization process on the gradation value of the input pixel to be processed using the 15 threshold values read by the threshold selection unit 52 (S3). Specifically, referring to the combination (first combination) of 15 threshold values and 16 output values shown in FIG. 3A, the 16 output values corresponding to the input gradation values are set. Are selected from the output values (0 to 15), and the selected output value is set as the gradation value in the output dither image.

  If it is determined in S1 that the toner save mode is selected (Yes in S1), the threshold selection unit 52 determines whether the pixel corresponding position corresponding to the input pixel to be processed is the density adjustment position ( S5). When the threshold selection unit 52 determines that the position is not the density adjustment position (No in S5), the 15 threshold values corresponding to the pixel corresponding to the input pixel to be processed (thresholds included in the first combination) Is read from the storage unit 53 (S6). Thereafter, the threshold processing unit 51 performs a quantization process on the gradation value of the input pixel to be processed using the 15 threshold values read by the threshold selection unit 52 (S3). Specifically, referring to the combination (first combination) of 15 threshold values and 16 output values shown in FIG. 3A, the 16 output values corresponding to the input gradation values are set. Are selected from the output values (0 to 15), and the selected output value is set as the gradation value in the output dither image.

  On the other hand, when it is determined in S5 that the position is the density adjustment position (Yes in S5), the threshold selection unit 52 uses the four thresholds (second combination) at the pixel corresponding position corresponding to the input pixel to be processed. Is read from the storage unit 53 (S7). Thereafter, the threshold processing unit 51 performs a quantization process on the gradation value of the input pixel to be processed using the four threshold values read by the threshold selection unit 52 (S3). Specifically, referring to the combination (second combination) of the four threshold values and the five output values shown in FIG. 3B, the five output values corresponding to the input gradation values are set. Are selected from the output values (0 to 4), and the selected output value is set as the gradation value in the output dither image. And the process of S1-S7 is repeated with respect to all the pixels of an input image (it is No at S4).

  According to the processing of the present embodiment described above, either the normal mode or the toner save mode is selected according to the user's intention. In the toner save mode, density reduction is not performed by partial white mask processing as in Patent Document 1, but a density adjustment position is set in the dither matrix 2, and multi-tone expression is expressed at the density adjustment position. By suppressing the maximum output value (maximum density) while making it possible, toner consumption can be suppressed by toner saving while maintaining good gradation. Further, as shown in FIG. 2B, since the density adjustment positions are uniformly distributed in the entire dither matrix 2, the pixels corresponding to the density adjustment positions are uniformly distributed in the entire output image, and partially. Not unevenly distributed. Therefore, even if the density adjustment position is set, it can be suppressed that the gradation is impaired.

  Further, the multi-value dither processing of the present embodiment is a form using threshold processing, but is not limited to the form using threshold processing. For example, the correspondence between the input value (tone value of the input pixel) and the output value (tone value of the dithered image) for each pixel corresponding position of the dither matrix 2 shown in FIGS. 2 (a) and 2 (b). The dither output conversion table indicating the above may be stored, and the multi-value dither processing may be performed using the dither output conversion table.

  Here, FIG. 10 shows a configuration example of the gradation reproduction processing unit 19a in the case of adopting a form in which multi-value dither processing is performed using a dither output conversion table. As shown in FIG. 10, the gradation reproduction processing unit 19 a includes a table conversion unit 61, a table selection unit 62, and a table storage unit 63. The table storage unit 63 is a block that stores a dither output conversion table. The table selection unit 62 selects and reads the dither output conversion table for the normal mode from the table storage unit 63 in the normal mode, and selects the dither output conversion table for the toner save mode from the table storage unit 63 in the toner save mode. This is the block to read. The table conversion unit 61 quantizes the gradation value of the input pixel using the dither output conversion table read by the table selection unit 62 and converts it into an output value, and converts the output value into the gradation value in the output dither image. Treat as.

  As described above, the image processing apparatus 10 according to the present embodiment saves printing toner (color material for printing) more than in the normal mode and when the dither image is printed. And a mode switching unit 31 for switching the save mode in which the multi-value dither processing is performed. Furthermore, the image processing apparatus 10 according to the present embodiment includes a threshold selection unit 52, and this threshold selection unit 52 corresponds to a density adjustment position (specific position) among all the pixel corresponding positions of the dither matrix 2. When the pixel of the dither image is a specific pixel, the specific pixel has multiple gradations regardless of whether the specific mode is the normal mode or the toner save mode, and when the save mode is the normal mode Therefore, the number of gradations of the specific pixel and the gradation value indicating the maximum density are adjusted so that the maximum density of the specific pixel is suppressed (specifically, the toner save mode). In such a case, the number of gradations of the specific pixel is reduced in the toner save mode than in the normal mode so that the maximum density of the specific pixel is suppressed more than in the normal mode. To change the tone value indicating the highest density in the case of the as in the toner saving mode Normal mode). According to such a configuration, in the toner save mode, some pixels (the specific pixels) included in the dithered image to be printed are maintained to have multiple gradations (the number of gradations is 3 or more). However, since the maximum density level is suppressed, it is possible to suppress the consumption of toner during printing while suppressing the unnatural gradation reproducibility of the entire printed dither image.

  Also, the density adjustment position (specific position) in the dither matrix 2 when performing multi-value dither processing for an input image of a certain color component and when performing multi-value dither processing for an input image of another color component. If the same, the situation where the density of the area corresponding to the density adjustment position in the printed color image is extremely lowered may occur. When this situation occurs, the gradation reproducibility becomes unnatural. there is a possibility. Therefore, it is preferable that the threshold selection unit 52 changes the location of the density adjustment position in the dither matrix 2 for each color component. In this way, the density adjustment position is made different between when multi-value dither processing is performed on an input image of a certain color component and when multi-value dither processing is performed on an input image of another color component. Therefore, it is possible to suppress the occurrence of a situation where the density of the region corresponding to the specific position in the printed color image is extremely reduced, and it is possible to suppress the gradation reproducibility from being deteriorated.

  Furthermore, if the specific pixels corresponding to the density adjustment position are concentrated in a certain area in the dither image, there is a possibility that the density of the area will be extremely lower than the density of other areas. There is a possibility that gradation reproducibility becomes unnatural when this situation occurs. In this regard, according to the present embodiment, as shown in FIG. 2B, the density adjustment positions are not continuous along the row direction of the dither matrix 2, and the column direction (the direction orthogonal to the row direction). ), A plurality of density adjustment positions are arranged in the dither matrix 2 so that the density adjustment positions are not continuous. Therefore, it is possible to suppress the specific pixels corresponding to the density adjustment position from being concentrated on a certain area in the dither image, and it is possible to further suppress the gradation reproducibility from becoming unnatural. When the input image handled by the image processing apparatus 10 is read by the CCD line sensor, the row direction of the dither matrix 2 is the main scanning direction (pixel arrangement direction in the CCD line sensor) in the CCD line sensor or It coincides with the sub-scanning direction (direction orthogonal to the main scanning direction).

[About dither pattern]
Next, a dither pattern obtained by multilevel dither processing in the normal mode will be described. Assuming an input image having the same size and shape as the dither matrix 2 in FIG. 2A and having a uniform density, when the density of the input image is sequentially increased in the normal mode, a dither pattern (dither processing) The later dither image) changes as shown in FIG. In FIG. 7, the numbers assigned to the pixels indicate the gradation values of the pixels in the dither pattern.

More specifically, in the dither matrix 2, the dither pattern as shown in FIG. 7 can be obtained by setting the threshold value relationship between the pixel corresponding positions so that the following formulas 2 to 4 are established. it can.
Th [i] [j] ≦ Th [i + 1] [j] Equation 2
Th [4k + 3] [j] ≦ Th [4k] [j + 1] Equation 3
Th [4k + 3] [14] ≦ Th [4k + 4] [0] Equation 4
(However, k = 0,1, ..., 8)
That is, the output value at each pixel corresponding position i is sequentially increased to 15 based on the order of the numbers shown in FIG. For example, in this embodiment, the output value of the pixel corresponding position (the pixel corresponding position serving as the nucleus in each of the sub-matrices 2a to 2d) whose values shown in FIG. After that, the output value of the pixel corresponding position whose values shown in FIG. 2A are 4, 5, 6, and 7 is increased.

  In this embodiment, the output value Out [m] at each pixel-corresponding position in the normal mode is a 4-bit integer value from 0 to 15. However, an output of 8-bit integer values from 0 to 255 is output. It may be 16 values selected in consideration of the characteristics of the apparatus.

  Next, a dither pattern obtained by multi-value dither processing in the toner save mode will be described. In the multi-value dither processing in the toner save mode, the density adjustment position is set in the dither matrix 2 as shown in FIG. Then, the maximum value of the output value (dither data) of the dither image corresponding to the density adjustment position is suppressed more than in the normal mode. As shown in FIGS. 2A and 2B, the order given to each pixel corresponding position in the dither matrix 2 (the order in which high-density output values are assigned) is in the toner save mode. Is the same as in the normal mode.

  Accordingly, assuming an input image having the same size and shape as the dither matrix 2 in FIG. 2B and a uniform density, if the density of the input image is sequentially increased in the toner save mode, The pattern (dithered image after dithering) changes as shown in FIG. In FIG. 8, the numbers given to the pixels indicate the tone values of the pixels in the dither pattern, as in FIG.

  Hereinafter, the dither pattern will be described more specifically. In this embodiment, it is a 4-bit output, and 16 output values are associated with each of all pixel corresponding positions in the dither matrix 2 in the normal mode. Therefore, as shown in FIG. 7, when the dot growth level is maximized in the normal mode, the gradation value of all the pixels of the dither image is 15.

  On the other hand, in the toner save mode, as shown in FIG. 2B, some of the pixel corresponding positions included in the dither matrix 2 are set as the density adjustment positions. Suppresses the maximum output value (limiter processing). Specifically, in the toner save mode, the maximum value (maximum density level) of the output value at the position (normal position) other than the density adjustment position among the positions corresponding to all pixels of the dither matrix 2 is set to 15. The maximum value (maximum density level) of the output value at the density adjustment position is set to 4. That is, in the dither matrix 2 in the toner save mode, a pixel corresponding position having a maximum density level of 4 and a pixel corresponding position having a maximum density level of 15 are mixed.

  In this way, the maximum output value of all the pixel corresponding positions in the normal mode is 15, and the maximum output value of the normal position (position other than the density adjustment position) is 15 in the toner save mode. On the other hand, the maximum output value at the density adjustment position is 4 in the toner save mode. Therefore, when the dither pattern is grown by sequentially increasing the density of the input image having a uniform density in the toner save mode, the density at the pixel position corresponding to the density adjustment position as shown in FIG. The level can only go up to level 4. That is, even if the density of the entire image increases, the density level of 4 is continuously maintained at the pixel position corresponding to the density adjustment position.

  In the toner save mode, first, output values of pixel corresponding positions (pixel corresponding positions serving as nuclei in each of the sub-matrices 2a to 2d) whose values shown in FIG. Increase to 15. Thereafter, the output value is increased to 15 for the pixel corresponding position (normal position) other than the density adjustment position, but the number shown in FIG. 2B is set so that the output value is increased only to 4 at the density adjustment position. The output values are assigned according to the order.

  Further, the arrangement of the density adjustment positions in the dither matrix 2 is not limited to the example of FIG. However, it is preferable to arrange a plurality of density adjustment positions in the dither matrix 2 so that the density adjustment positions are not adjacent to each other as much as possible.

  Further, the maximum value (upper limit value) of the output value in the toner save mode at the density adjustment position is not limited to 4. There is no particular limitation as long as the maximum value of the output value at the density adjustment position is suppressed in the toner save mode compared to that in the normal mode, and the output value at the density adjustment position has multiple gradations. It can be set according to the level of toner consumption reduction to be performed and the image quality of the output image.

  In the present embodiment, also in the toner save mode, as in the normal mode, the output value Out [m] at each pixel corresponding position is expressed by a 4-bit integer value. Specifically, the output value of 0 to 4 is expressed by 4-bit data for the density adjustment position, and the output value of 0 to 15 is expressed by 4-bit data for the normal position other than the density adjustment position. However, the output value is not limited to such a value, and may be a value selected in consideration of the characteristics of the output device among 8-bit integer values from 0 to 255.

[About output tone correction section]
The output tone correction unit 18 performs tone correction based on the output characteristics of the image output device 30. The input / output relationship of this tone correction is not only the characteristics of the image output device 30 but also the tone reproduction processing unit. 19 is determined by the processing method. Therefore, if the contents of the multi-value dither processing in the tone reproduction processing unit 19 are different from each other, an input / output relationship for gradation correction corresponding to each multi-value dither processing is required. That is, if this tone correction is performed by table conversion, a plurality of different tables are required.

  That is, when the contents of the multi-value dither processing are changed between the normal mode and the toner save mode as in the present embodiment, the normal mode gradation correction table (tone correction curve) and the toner save are originally set. A tone correction table for the mode is required. The tone correction table is used for the output tone correction process in the output tone correction unit 18, and includes a pre-correction tone value (input value) and a post-correction tone value (output value) in the correction process. It is the table which showed the corresponding relationship.

  However, only the gradation correction table showing the input / output relationship according to the multi-value dither processing in the normal mode is stored in the output gradation correction unit 18, and the gradation correction table is used in the toner save mode. It is also possible to perform output gradation correction. This point will be described below.

  FIG. 9A shows a gradation value (input value) for each pixel of the input image in the normal mode and a total output value in one dither matrix 2 (output values of all pixel corresponding positions in one dither matrix 2). 9B is a graph showing the relationship between the gradation value for each pixel of the input image and the total output value in one dither matrix 2 in the toner save mode. It is.

  As shown in FIG. 2A, in the normal mode, one dither matrix 2 includes 40 pixel corresponding positions, and the maximum output value at each pixel corresponding position is 15. Therefore, as shown in FIG. 9A, in the normal mode, the maximum total output value in one dither matrix 2 is 600 (15 × 40 = 600). As shown in FIG. 2B, in the toner save mode, one dither matrix 2 includes 16 density adjustment positions and 24 normal positions, and the maximum output value at each density adjustment position. The value is 4, and the maximum output value at each normal position is 15. Therefore, as shown in FIG. 9B, in the toner save mode, the maximum total output value in one dither matrix 2 is 424 (4 × 16 + 15 × 24 = 424).

  As shown in FIGS. 9A and 9B, although the total output value corresponding to the input value is lower in each input value in the toner save mode than in the normal mode, The order given to the pixel corresponding positions (the order in which high density output values are assigned) is the same as in the normal mode even in the toner save mode.

  The reduction rate of each total output value associated with each input value is uniformly the highest value of the total output value of the dither matrix 2 in the toner save mode and the total output value of the dither matrix 2 in the normal mode. It can be considered that it substantially coincides with or coincides with the ratio with the highest value of.

  Therefore, if the gradation correction table is changed so that the output value level (gradation level) decreases in accordance with the ratio in the gradation correction table for the normal mode, the gradation correction table after the change is displayed. It can also be used in toner save mode. Therefore, if only the gradation correction table for the normal mode is stored in the output gradation correction unit 18, the gradation correction table for the toner save mode is stored in addition to the gradation correction table for the normal mode. There is no need to keep it.

  For example, for each output value in the gradation correction table for the normal mode, a constant coefficient (the highest total dither matrix output value in the toner save mode and the highest total dither matrix output value in the normal mode) If the gradation correction table is changed, the output value level (gradation level) can be lowered by multiplying the ratio by the value), and the changed gradation correction table is displayed in the toner save mode. Can be used for the output gradation correction.

The constant coefficient can be obtained by the following equation 5.
Constant coefficient = (maximum value of total output value of dither matrix in toner save mode) / (maximum value of total output value of dither matrix in normal mode) Equation 5
As shown in FIG. 9, in the case of the dither matrix 2 of the present embodiment, the maximum total output value in the toner save mode is 424, and the maximum total output value in the normal mode is 600. ,
Constant coefficient = 424 ÷ 600 = 0.71
It becomes.

  Then, by multiplying each output value of the gradation correction table for the normal mode by this constant coefficient, the gradation correction table after multiplication can be used also in the toner save mode. As a result, the gradation correction table for the normal mode can also be used in the toner save mode. Therefore, if the gradation correction table for the normal mode is stored in the image processing apparatus 10, the gradation set for the toner save mode is stored. The correction table need not be stored, and the storage capacity of the image processing apparatus 10 can be saved.

In the above description, the gradation correction table set for the normal mode is stored as a default, and in the toner save mode, the output value of the gradation correction table for the normal mode is multiplied by a constant coefficient. The embodiment in which the gradation correction table is changed and the changed gradation correction table is used has been described. However, the present invention is not limited to this mode, and the tone correction table set for the toner save mode is held as a default, and in the normal mode, each output value of the tone correction table for the toner save mode is stored. The gradation correction table may be changed by applying a constant coefficient to, and the changed gradation correction table may be used. However, in this embodiment, the constant coefficient is obtained by the following equation (6).
Constant coefficient = (maximum total dither matrix output value in normal mode) / (maximum total dither matrix output value in toner save mode) Equation 6
Further, the output tone correction can be performed simultaneously with the threshold processing in the multi-value dither processing of the tone reproduction processing unit 19. This can be achieved by setting the relationship between the input value (input image data) and the total output value in the dither matrix 2 according to the output characteristics of the image output device 30.

  For example, the relationship between the gradation value (input value) of the input pixel in the normal mode and the total output value of the dither matrix 2 is set as shown in FIG. 9A according to the output characteristics of the image output device 30. That is, a relationship is set in which an output value of 0 to 600 is returned for an input value of 0 to 255. Then, 15 threshold values or dither output conversion tables corresponding to the pixel positions are created based on the relationship between the input values and the total output values shown in FIG. If multi-value dither processing in the normal mode is performed using the threshold value or dither output conversion table created in this way, output gradation correction and multi-value dither processing can be executed simultaneously in the normal mode.

  On the other hand, the relationship between the input value and the total output value in the toner save mode can be derived from the relationship between the input value and the total output value of the multi-value dither processing in the normal processing. That is, the relationship between the input value and the total output value in the toner save mode as shown in FIG. 9B is derived by multiplying each total output value shown in FIG. 9A by the constant coefficient described above. it can. Then, using the relationship between the input value derived in this way and the total output value, 15 or 4 threshold values or dither output conversion tables corresponding to the pixel positions are created. If multi-value dither processing in the toner save mode is performed using the threshold value or dither output conversion table created in this way, output gradation correction and multi-value dither processing can be executed simultaneously in the toner save mode.

[About the maximum output value]
In the present embodiment, the dither matrix is composed of a plurality of sub-matrices. In the example of FIG. 2B, the dither matrix 2 is composed of four sub-matrices 2a to 2d. As shown in FIG. 2B, each of the sub-matrices 2a to 2d includes a density adjustment position. In the toner save mode, the threshold selection unit 52 detects the pixel corresponding to the density adjustment position so that the maximum density of the pixel (specific pixel) corresponding to the density adjustment position (specific position) is suppressed compared to that in the normal mode. The maximum output value is adjusted. In the toner save mode, the output value changes in 16 steps from 0 to 15 at all normal positions (positions other than the density adjustment position), and the output value ranges from 0 to 4 at all density adjustment positions. It changes in 5 stages. That is, in the toner save mode, the maximum output value at the normal position is 16, but the maximum value at the density adjustment position is suppressed to 4. Further, the output values at the respective density adjustment positions are set so that the maximum output values in the toner save mode are equal to each other at all the density adjustment positions in the dither matrix 2.

  FIG. 8 shows changes in density levels in such a form that the maximum output values in the toner save mode are equal to each other at all density adjustment positions in the dither matrix 2. According to the example of FIG. 8, all the pixels corresponding to the normal position change in density in 16 levels, and all the pixels corresponding to the density adjustment position change in density in 4 levels. All pixels corresponding to the normal position are indicated by black (output value is 15), and the highest gradation of all pixels corresponding to the density adjustment position is indicated by light black (output value is 4). Will be.

  As described above, in the present embodiment, the threshold selection unit 52 determines the pixel level corresponding to each density adjustment position so that the maximum output values are the same at all density adjustment positions in the same dither matrix 2. This is a mode for adjusting the logarithm and the maximum output value. However, the present invention is not limited to this form.

  For example, the threshold selection unit 52 sets the maximum output value to the same value at the density adjustment positions belonging to the same sub-matrix, and the maximum output value from the density adjustment positions belonging to the different sub-matrices to different values. Alternatively, the number of gradation levels and the maximum output value of the pixel corresponding to the density adjustment position may be adjusted. Below, the Example of the said form is described based on FIG.2 (b).

  The threshold selection unit 52 sets selectable output values from 0 to 1 (that is, sets the maximum output value to 1) for all density adjustment positions belonging to the sub-matrix 2a in FIG. To do). The threshold selection unit 52 sets selectable output values from 0 to 3 (that is, sets the maximum output value to 3) for all density adjustment positions belonging to the submatrix 2b in FIG. Further, the threshold selection unit 52 sets selectable output values from 0 to 4 (that is, sets the maximum output value to 4) for all density adjustment positions belonging to the sub-matrix 2c in FIG. Further, the threshold selection unit 52 sets selectable output values from 0 to 2 (that is, sets the maximum output value to 2) for all density adjustment positions belonging to the sub-matrix 2d in FIG. FIG. 12 shows changes in the density level when the maximum output value is set as described above. As is apparent from FIG. 12, the maximum densities of the corresponding pixels are equal to each other at the density adjustment positions belonging to the same sub-matrix. However, at the density adjustment positions belonging to different sub-matrices, the maximum density of the corresponding pixels is It will be different from each other.

  FIG. 13A is an example of an output image in which the maximum output values of all density adjustment positions in the dither matrix 2 are equal to each other, and FIG. 13B is a density adjustment position belonging to different sub-matrices. It is an example of the output image of the form which makes a maximum output value differ between. As is apparent from these drawings, in the case where the maximum output value is made different between the density adjustment positions belonging to different sub-matrices, an image maintaining more gradation levels can be obtained, and toner saving was performed. Even in this case, the image quality can be improved to some extent.

  Note that when the maximum output values are made the same for all density adjustment positions in the dither matrix 2 as in the pattern of FIG. 8, the maximum output value needs to be 2 or more as described above. This is because, unless the maximum output value is set to 2 or more, the binary dither processing is performed only with respect to the density adjustment position. On the other hand, when there are a plurality of density adjustment positions that make the maximum output values different from each other among all density adjustment positions in the dither matrix 2, as shown in the pattern of FIG. There may be an adjustment position. In this case, since the density adjustment position where the maximum output value is larger than 1 always exists in the matrix 2, even if attention is paid only to the density adjustment position, it does not correspond to the binary dither process, and the multi-value dither process is performed. It is because it corresponds.

[Modification]
Note that the dither matrix used in the present embodiment is composed of a plurality of sub-matrices, but the dither matrix does not have to be composed of a plurality of sub-matrices, and is multi-valued using one sub-matrix. Dither processing may be performed (that is, a dither matrix including a single sub-matrix is used for multi-value dither processing). In such a dither matrix, the maximum output values in the save mode at all density adjustment positions (specific positions) in the dither matrix 2 may be the same, or all the density adjustment positions in the dither matrix 2 The maximum output value of at least the first density adjustment position may be different from the maximum output value of the second density adjustment position.

  Further, according to the above embodiment, the mode is switched to the normal mode when the user selects the normal mode, and is switched to the toner save mode when the user selects the toner save mode. Switching between the normal mode and the toner save mode is not limited to this mode.

  For example, in the image output device 30, a toner cartridge that stores toner used for printing processing, a remaining amount sensor that outputs remaining amount information indicating the remaining amount of toner in the toner cartridge, and the remaining amount sensor are output. A transmission unit that transmits the remaining amount information to the image processing apparatus 10. Further, in the image processing apparatus 10, when the determination unit determines whether or not the remaining amount of toner indicated by the remaining amount information is equal to or less than a predetermined amount, A notification unit for notifying a user or an administrator of information for prompting replacement. In the image output device 30 and the image processing device 10 as described above, when the determination unit determines that it is not less than the predetermined amount, the mode switching unit 31 switches to the normal mode, and when the determination unit determines that it is less than the predetermined amount, The mode switching unit 31 may be configured to switch to the toner save mode. In such a form, the toner consumption can be saved in the image output device 30 after the toner cartridge needs to be replaced. Therefore, when the user is notified of information prompting the user to replace the toner cartridge, the image forming process must be performed in a situation where the toner cartridge cannot be replaced immediately (for example, when the cartridge is out of stock). Even in this case, it is possible to suppress the occurrence of toner out in the image output apparatus 30. The notification unit may realize the notification by displaying information prompting the replacement of the toner cartridge on the display unit of the operation panel 41, or information prompting the administrator's terminal device or the like to replace the toner cartridge. The notification may be realized by transmitting.

  In this embodiment, the image processing apparatus 10 is provided in the digital color copying machine 40. However, the image processing apparatus 10 may be provided in a digital color multifunction peripheral. That is, the gradation reproduction processing unit 19 of the present embodiment can be applied to an image processing apparatus provided in a digital color multifunction peripheral.

  Further, the image output device 30 of the present embodiment is an electrophotographic printer, but the image output device 30 is not limited to an electrophotographic printer, and for example, an ink jet printer is used as the image output device 30. Also good. When an ink jet printer is used as the image output device 30, the dither processing of the present embodiment can achieve the effect of reducing ink consumption.

  Further, the function performed by the image processing apparatus 10 including the gradation reproduction processing unit 19 that performs the multi-value dither processing described above may be realized by software (application program). In this case, the computer can be provided with a printer driver in which software for realizing multi-value dither processing is incorporated.

  As shown in FIG. 11, a printer driver 201, a communication port driver 202, and a communication port (for example, RS232C / LAN) 203 are incorporated in the computer 200. The printer driver 201 includes a color correction unit 15, a black generation and under color removal unit 16, a gradation reproduction processing unit 19, and a printer language translation unit 91.

  The computer 200 is connected to a printer (image output device) 204. The printer 204 performs printing based on the image data output from the computer 200. The printer 204 may be a digital multifunction machine having a copy function and a fax function in addition to the printer function.

  For example, image data generated by executing various application programs in the computer 200 is processed in the color correction unit 15, the black generation and under color removal unit 16, and the gradation reproduction processing unit 19 as described above. The image data that has undergone the processing of these units is converted into a printer language by the printer language translation unit 91 and input to the printer 204 via the communication port driver 202 and the communication port 203, and the image indicated by the image data is printed on the paper. Printed on.

  Each block of the image processing apparatus 10 and the printer driver 201 shown in the embodiment of the present invention may be configured by hardware logic, or may be realized by software using a CPU as follows.

  That is, each device includes a CPU (central processing unit) that executes instructions of a control program that realizes each function, a ROM (read only memory) that stores the program, a RAM (random access memory) that expands the program, A storage device (recording medium) such as a memory for storing the program and various data is provided. An object of the present invention is to provide a recording medium in which a program code (execution format program, intermediate code program, source program) of a control program for each of the above devices, which is software that realizes the above-described functions, is recorded in a computer-readable manner This can also be achieved by supplying each of the above devices and reading and executing the program code recorded on the recording medium by the computer (or CPU or MPU).

  Examples of the recording medium include tapes such as magnetic tapes and cassette tapes, magnetic disks such as floppy (registered trademark) disks / hard disks, and disks including optical disks such as CD-ROM / MO / MD / DVD / CD-R. Card system such as IC card, IC card (including memory card) / optical card, or semiconductor memory system such as mask ROM / EPROM / EEPROM / flash ROM.

  Further, each of the devices may be configured to be connectable to a communication network, and the program code may be supplied via the communication network. The communication network is not particularly limited. For example, the Internet, intranet, extranet, LAN, ISDN, VAN, CATV communication network, virtual private network, telephone line network, mobile communication network, satellite communication. A net or the like is available. Also, the transmission medium constituting the communication network is not particularly limited. For example, even in the case of wired such as IEEE 1394, USB, power line carrier, cable TV line, telephone line, ADSL line, etc., infrared rays such as IrDA and remote control, Bluetooth ( (Registered trademark), 802.11 wireless, HDR, mobile phone network, satellite line, terrestrial digital network, and the like can also be used. The present invention can also be realized in the form of a computer data signal embedded in a carrier wave in which the program code is embodied by electronic transmission.

  In this case, since the system configuration is such that a communication network including the Internet can be connected, the embodiment of the present invention is a medium that fluidly carries the program code so as to download the program code from the communication network. There may be. When the program code is downloaded from the communication network in this way, the program code for downloading may be stored in the main apparatus in advance or installed from another recording medium. . The embodiment of the present invention can also be realized in the form of a computer data signal embedded in a carrier wave in which the program code is embodied by electronic transmission.

  Furthermore, the processing of each block of the image processing apparatus 10 and the printer driver 201 can be realized by reading the recording medium by a program reading apparatus provided in a digital color image forming apparatus or a computer system. The computer system includes an image input device such as a flatbed scanner, a film scanner, and a digital camera, a computer that performs processing of each block of the image processing device 10 by loading a predetermined program, and a processing result of the computer Image display devices such as a CRT display and a liquid crystal display, and a printer that outputs the processing results of the computer to paper. Furthermore, a network card, a modem, and the like are provided as communication means for connecting to a server or the like via a network.

  The present invention is not limited to the above-described embodiments, and various modifications are possible within the scope shown in the claims, and the embodiments can be obtained by appropriately combining the respective technical means disclosed in the above-described embodiments. The form is also included in the technical scope of the present invention.

  The present invention is suitable for a printer, a copier, a multifunction machine, a personal computer, etc. having an image processing function.

2 Dither matrixes 2a to 2d Sub-matrix 10 Image processing device 18 Output tone correction unit 19 Tone reproduction processing unit (dither processing unit)
31 Mode switching part (switching part)
40 Digital color copier (image forming device)
51 Threshold Processing Unit 52 Threshold Selection Unit (Adjustment Unit)
53 Storage

Claims (13)

Each of the input images for each pixel corresponding position, which is a position corresponding to each pixel of the multi-tone input image and each pixel of the multi-tone dither image obtained by performing multi-value dither processing on the input image. Based on the dither matrix in which the relationship between the gradation value of the pixel and the gradation value of each pixel of the dither image is determined, the gradation value of each pixel of the input image is converted to the gradation of each pixel of the dither image. In an image processing apparatus having a dither processing unit that performs multi-value dither processing for converting values into values,
There is a switching unit that switches between the normal mode and the save mode in which the multi-value dithering process is performed so that the consumption of the color material for printing is suppressed when the dither image is printed compared to the normal mode. And
When the dither processing unit sets the pixels of the dither image corresponding to some specific positions of all the pixel corresponding positions of the dither matrix as specific pixels, the specific pixels are in the normal mode and the save mode. Regardless of the mode, the number of gradations of the specific pixel and the maximum number of gradations are increased so that the maximum density of the specific pixel is more suppressed in the save mode than in the normal mode. An image processing apparatus comprising: an adjustment unit that adjusts a gradation value indicating density. The image processing device is a color image processing device that handles the input image and the dither image for each color component,
The image processing apparatus according to claim 1, wherein the adjustment unit changes a location of the specific position in the dither matrix for each color component.   A plurality of the specific positions are arranged in the dither matrix so that the specific positions are not continuous along the row direction of the dither matrix and the specific positions are not continuous along the column direction of the dither matrix. The image processing apparatus according to claim 1, wherein the image processing apparatus is an image processing apparatus. The dither matrix has a plurality of specific positions, and the plurality of specific positions include at least first and second specific positions;
The adjustment unit has a different value between a gradation value indicating the highest density of the first specific pixel corresponding to the first specific position and a gradation value indicating the highest density of the second specific pixel corresponding to the second specific position. The image processing apparatus according to claim 1, wherein the adjustment is performed as described above. The dither matrix includes a plurality of sub-matrices, and each sub-matrix has the specific position,
The adjustment unit performs the adjustment so that gradation values indicating the highest density of specific pixels corresponding to a specific position are different from each other at specific positions belonging to different sub-matrices. Item 4. The image processing apparatus according to any one of Items 1 to 3.   The dither processing unit uses the table indicating a correspondence relationship between the gradation value of each pixel of the input image and the gradation value of each pixel of the dither image defined in the dither matrix. 6. The image processing apparatus according to claim 1, wherein a gradation value of each pixel of the image is converted into a gradation value of each pixel of the dither image. For each pixel-corresponding position of the dither matrix, a threshold value for determining the tone value of the dither image corresponding to the tone value of the input image is defined,
The dither processing unit compares a gradation value of the input image with a threshold value, and determines a gradation value of the dither image to be converted from the gradation value of the input image according to a result of the comparison. The image processing apparatus according to claim 1, wherein the image processing apparatus is characterized. An output tone correction unit that performs tone correction processing on the input image before the multi-value dither processing is executed;
The output gradation correction unit
In the normal mode, the gradation correction processing is performed using a gradation correction table which is a table showing the relationship between the gradation value before correction and the gradation value after correction, and is a table set for the normal mode. Done
In the save mode, the gradation correction table is changed by multiplying each corrected gradation value included in the gradation correction table by a constant coefficient, and the gradation correction table is used to change the gradation correction table. The image processing apparatus according to claim 1, wherein tone correction processing is performed. An output tone correction unit that performs tone correction processing on the input image before the multi-value dither processing is executed;
The output gradation correction unit
In the save mode, the gradation correction processing is performed using a gradation correction table that is a table showing a relationship between the gradation value before correction and the gradation value after correction, and is a table set for the save mode. And
In the normal mode, the gradation correction table is changed by multiplying each corrected gradation value included in the gradation correction table by a constant coefficient, and the gradation correction table after the change is referred to The image processing apparatus according to claim 1, wherein gradation correction processing is performed.   An image forming apparatus comprising the image processing apparatus according to claim 1. Each of the input images for each pixel corresponding position, which is a position corresponding to each pixel of the multi-tone input image and each pixel of the multi-tone dither image obtained by performing multi-value dither processing on the input image. Based on the dither matrix in which the relationship between the gradation value of the pixel and the gradation value of each pixel of the dither image is determined, the gradation value of each pixel of the input image is converted to the gradation of each pixel of the dither image. In an image processing method including a dither processing step of performing multi-value dither processing for converting to a value,
A normal mode, and a save mode for performing the multi-value dither processing so that the consumption of the printing color material is suppressed as compared with the normal mode when the dither image is printed,
In the dither processing step, when the pixels of the dither image corresponding to some specific positions among all the pixel corresponding positions of the dither matrix are set as specific pixels, the specific pixels are in the normal mode and the save mode. Regardless of the mode, the number of gradations of the specific pixel and the maximum number of gradations are increased so that the maximum density of the specific pixel is more suppressed in the save mode than in the normal mode. An image processing method comprising adjusting a gradation value indicating density.   An image processing program for causing a computer to function as each unit of the image processing apparatus according to claim 1.   A computer-readable recording medium on which the image processing program according to claim 12 is recorded.

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