JP2020102767A - Image processing device, image processing method, and program - Google Patents

Abstract

To more appropriately reproduce color tone of a detailed line when performing quantization processing on a color image including a detailed line.SOLUTION: An image processing device includes: deriving means for deriving the number of dots to be arranged in a predetermined area based on a pixel value of each pixel included in the predetermined area for each predetermined area regarding the input image data of color to be processed; determination means for determining an arrangement priority order indicating a priority order of pixels for arranging dots in the predetermined area using an evaluation value based on each pixel value in the predetermined area and a threshold value group corresponding to the predetermined area in a threshold matrix; correction means for correcting the number of dots derived based on the accumulated representative value obtained by accumulating a representative value of pixel values in the predetermined area between colors when the derived dot number is less than a reference dot number; and generation means for generating the dot data corresponding to the predetermined area based on the corrected dot number and the arrangement priority determined by the determining means.SELECTED DRAWING: Figure 2

Description

The present invention relates to a technique for converting image data into higher quality halftone image data.

2. Description of the Related Art Image forming apparatuses such as printers that print digital image data (hereinafter, simply referred to as image data) of an image captured by a camera or an image created by a method such as CG are widely used. In image data, each pixel is generally expressed in multi-gradation of 8 bits or 16 bits per color. On the other hand, since the image forming apparatus expresses gradation by turning dots on and off by the recording material, the number of gradations that can be output is small. Therefore, a quantization method for converting multi-tone image data into the number of tones that can be output by the image forming apparatus is used. Generally, there is halftone processing as a quantization method for reducing the number of gradations. The halftone image data obtained after the halftone processing represents a dot pattern output on the recording medium by the image forming apparatus, and pseudo-expresses the gradation of the image. Dither processing is often used as a type of halftone processing. In the dither processing, a threshold value matrix in which a threshold value is arranged corresponding to each pixel in the image data is used, and for each pixel, the pixel value of the image data is compared with the threshold value of the threshold value matrix, and the dot is turned on for each pixel. Or decide to turn off.

However, when inputting and processing image data containing a high frequency component, the high frequency component contained in the image data interferes with the threshold period of the threshold matrix, and moiré (texture), deterioration of sharpness, breakage of fine lines, etc. may occur. It was likely to occur. Therefore, in Patent Document 1, the image data is divided into a plurality of processing regions, the number of dots to be arranged in the region is determined according to the pixel value of the image for each processing region, and the dots are arranged in each pixel in the region. A halftone process for deciding a priority order for arranging the positions is disclosed.

JP, 2016-21735, A

However, the method described in the related art may not be able to successfully reproduce the detail line by the halftone process. The detail line is a line having high brightness (small pixel value) and narrow line width. Therefore, the average pixel value becomes low in the processing region including only the detail line. Therefore, in the method of determining the number of dots based on the pixel value, the number of dots arranged in the processing area is small, and the detail line may not be reproduced well in the output image. In particular, when printing a color image, the density value of each color tends to be small in the process of separating the RGB image into the CMYK image, and a low density area is likely to occur in the image data of each color of CMYK. As a result, the number of dots sufficient to reproduce the color tone of the detail line may not be obtained in the image data after the halftone process.

Therefore, it is an object of the present invention to provide an image processing device that can more appropriately reproduce the color tone of a detail line when performing a quantization process on a color image.

The image processing apparatus according to the present invention generates dot data for determining whether or not to arrange dots in each pixel used for forming an image on a recording medium for each color of a color material used for forming the image. A device for deriving input image data of a color to be processed, for each predetermined region, deriving means for deriving the number of dots to be arranged in the predetermined region based on the pixel value of each pixel included in the predetermined region. , An arrangement priority order indicating a priority order of pixels in which dots are arranged in the predetermined area is determined by using an evaluation value based on each pixel value of the predetermined area and a threshold value group corresponding to the predetermined area in a threshold matrix. Determining means, and when the number of dots derived by the deriving means is less than a reference number of dots, the derivation is performed based on a cumulative representative value obtained by accumulating representative values of pixel values in the predetermined area between colors. Correction means for correcting the number of dots derived by the means, the number of dots corrected by the correction means, and the layout priority order determined by the determining means, based on the dot data corresponding to the predetermined area And generating means for generating.

According to the present invention, the color tone of the detail line can be more appropriately reproduced when the quantization processing is performed on the color image.

FIG. 3 is a block diagram showing a configuration of an image forming system including an image processing unit according to the first embodiment. FIG. 3 is a block diagram showing a configuration of an image processing unit in the first embodiment. The figure which shows the processing area in input image data. The figure which shows an example of the processing area containing a detail line and the threshold value group corresponding to this processing area. 6 is a flowchart showing the operation of the image processing unit in the first embodiment. FIG. 6 is a diagram for explaining a processing result obtained by the processing shown in FIG. 5. 6 is a flowchart showing the processing of the correction value derivation unit in the first embodiment. It is a flow chart which shows processing of a correction value derivation part in a 2nd embodiment. The figure for demonstrating the arrangement priority order determination process in 2nd Embodiment. The block diagram which shows the structure of the image processing part in 3rd Embodiment. 9 is a flowchart showing the operation of the image processing unit in the third embodiment. The figure for demonstrating the process of the correction|amendment output value determination part in 3rd Embodiment. FIG. 8 is a diagram for explaining the operation of the image processing unit in the third embodiment.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. The following embodiments do not limit the present invention, and all combinations of the features described in the present embodiment are not necessarily essential to the solving means of the present invention. The same configurations will be described with the same reference numerals.

<First Embodiment>
(Structure of image processing device)
FIG. 1 is a block diagram showing a configuration of an image forming system including an image processing unit (hereinafter, also referred to as an image processing apparatus) of the first embodiment. In the present embodiment, as an example of an image processing apparatus, an image processing controller built in a printer that forms an image on a recording medium using a recording material will be described as an example.

The image forming system according to the first embodiment includes a CPU 100, a RAM 101, a ROM 102, an operation unit 103, a display unit 104, an external storage device 105, an image processing unit 106, an image forming unit 107, an I/F (interface) unit 108, and The bus 109 is provided.

The CPU 100 controls the operation of the entire image forming system using the input data and a computer program stored in a RAM or a ROM described later. Although the case where the CPU 100 controls the entire image forming system is described here as an example, the entire image forming system may be controlled by a plurality of pieces of hardware sharing the processing.

The RAM 101 has a storage area for temporarily storing a computer program or data read from the external storage device 105 and data received from the outside via the I/F unit 108. Further, the RAM 101 is used as a storage area used when the CPU 100 executes various processes and a storage area used when the image processing unit 106 executes image processing. That is, the RAM 101 can appropriately provide various storage areas. The ROM 102 stores setting parameters for setting each unit in the image forming system, a boot program, and the like.

The operation unit 103 is composed of a keyboard, a mouse, and the like, and receives an instruction from the operator through an operation performed by the operator. This allows the operator to input various instructions to the CPU 100.

The display unit 104 is composed of a CRT, a liquid crystal screen, or the like, and can display the processing result by the CPU 100 as an image or characters. When the display unit 104 is a touch panel that can detect a touch operation, the display unit 104 may function as a part of the operation unit 103.

The external storage device 105 is a large capacity information storage device represented by a hard disk drive. The external storage device 105 stores an OS (operating system), a computer program for causing the CPU 100 to execute processing, data, and the like. Further, the external storage device 105 stores temporary data (image data to be input/output, a threshold matrix used in the image processing unit 106, etc.) generated by the processing of each unit. The computer programs and data stored in the external storage device 105 are appropriately read according to the control of the CPU 100, stored in the RAM 101, and subjected to processing by the CPU 100.

The image processing unit 106 is realized as a processor capable of executing a computer program or a dedicated image processing circuit. The image processing unit 106 executes various types of image processing in order to convert the image data input as a print target into image data that can be output by the image forming unit 107. For example, when an instruction to execute image processing is received from the CPU 100, a process of converting N gradation input image data stored in the external storage device 105 into M gradation output image data is executed.

The image forming unit 107 forms an image using a recording material on a recording medium such as paper based on the output image data output from the image processing unit 106. The image forming unit 107 according to the first embodiment employs an inkjet method that forms an image by ejecting ink from a nozzle onto a recording medium. Note that the image forming unit 107 may employ an electrophotographic method in which an image is formed by exposing a charged image carrier to development with toner and transferring the toner image onto a recording medium.

The I/F unit 108 functions as an interface for connecting the image forming system according to this embodiment and an external device. Furthermore, the I/F unit 108 also functions as an interface for exchanging data with a communication device using infrared communication, wireless LAN, or the like. All of the above components are connected to the bus 109, and exchange data with each other via the bus 109.

(Structure of Image Processing Unit 106)
The image processing unit 106 in this embodiment will be described. The image processing unit 106 executes halftone processing for converting the input image data into halftone image data having a smaller number of gradations than the input image data. FIG. 2 is a block diagram showing the configuration of the image processing unit 106 in the first embodiment. The image processing unit 106 in this embodiment is realized as a dedicated image processing circuit having the configuration shown in FIG. The image processing unit 106 includes a pixel value acquisition unit 201, an arrangement priority order determination unit 202, a threshold value acquisition unit 203, a target value derivation unit 204, a correction value derivation unit 205, a total output value derivation unit 206, an output value determination unit 207, and It has an inter-color cumulative value derivation unit 208.

The image processing unit 106 according to the present embodiment receives, as input image data, monochromatic image data after color separation for each color material used by the printer. The color separation is a process of converting image data for each RGB into image data for each CMYK corresponding to the color material used in the printer. In the present embodiment, each image data after color separation is input to the image processing unit 106 in the order of KCMY, and the image processing unit 106 individually performs image processing on the image data of each color. The image data of each color is 8-bit data indicating any value of 0 to 255 for each pixel. Then, the image processing unit 106 converts 8-bit input image data per pixel into 1-bit binary halftone image data (output image data) having a value of 0 or 1 for each pixel. Thereby, halftone image data for four colors of KCMY are generated. In the halftone image data, a pixel having a pixel value (output value) of 0 represents a dot off, and a pixel having a pixel value of 1 represents a dot on. In such halftone image data, the input image data is reproduced in a pseudo manner with a gradation number smaller than the gradation number represented by the input image data. In the following, the halftone image data may be referred to as dot data.

The pixel value acquisition unit 201 acquires each pixel value of a plurality of pixels included in a unit area (hereinafter, referred to as a processing area or a predetermined area) to be processed in the input image data. FIG. 3 is a diagram showing a processing area in the input image data 300. In the present embodiment, the region of 4 pixels×4 pixels indicated by the thick frame in FIG. 3 is set as the processing region. For example, when the processing region is the region 301, the pixel value acquisition unit 201 acquires each pixel value of 16 pixels included in the region 301.

The threshold acquisition unit 203 acquires a threshold group used for dither processing from the RAM 101 or the external storage device 105. FIG. 4 is a diagram showing an example of a processing area including a detail line and a threshold value group corresponding to the processing area. In the present embodiment, it is assumed that the threshold value group 401 shown in FIG. 4 is used for dither processing. The threshold group 401 is a part of a threshold matrix stored in the RAM 101 or the external storage device 105. The threshold value acquiring unit 203 acquires the threshold value group 401 shown in FIG. 4 every time the pixel value acquiring unit 201 reads each pixel value of the processing region. In the present embodiment, a common threshold value matrix is used for each color of KCMY for simplicity of description, but different threshold value matrices may be used for each color.

The target value derivation unit 204 derives a target value for determining the total output value (total output value) in the processing area based on the pixel values of the plurality of pixels included in the processing area and the threshold values of the threshold value group. As described above, the output value is represented by 1s and 0s indicating that dots are turned on and off. Therefore, the target value in the processing area corresponds to the number of dots that should be turned on in the processing area. In the present embodiment, the average value of the pixel values of each pixel in the processing area is compared with each threshold value of the threshold value group, and the number of threshold values less than the average value is derived as the target value.

The inter-color cumulative value deriving unit 208 outputs the inter-color cumulative value used when the correction value deriving unit 205 derives the correction value. The inter-color cumulative value will be described later.

The correction value derivation unit 205 corrects the total output value based on the pixel value of each pixel in the processing region, each threshold value of the threshold value group, and the inter-color cumulative value derived by the inter-color cumulative value deriving unit 208. The correction amount (correction value) of is derived. In the present embodiment, the correction value derivation unit 205 compares the sum of the average value of pixel values in the processing area and the inter-color cumulative value with the minimum threshold value of the threshold value group. As a result of the comparison, if the sum is greater than or equal to the minimum threshold, 1 is set as the correction value, and if less than the minimum threshold, 0 is set as the correction value.

The total output value deriving unit 206 derives the total output value in the processing area (total output value) based on the target value derived by the target value deriving unit 204 and the correction value derived by the correction value deriving unit 205. .. In the present embodiment, the total output value deriving unit 206 derives the result of adding the target value and the correction value as the total output value.

The arrangement priority order determination unit 202 determines the arrangement priority order indicating from which pixel in the processing area the dots are to be arranged preferentially, based on the pixel value of each pixel in the processing area and each threshold value of the threshold value group. In the present embodiment, the arrangement priority order determination unit 202 determines the difference value between the pixel value corresponding to the position and the threshold value (here, the difference value obtained by subtracting the pixel value by the threshold value) as the dot arrangement priority order. Is derived as an evaluation value for determining. Then, the arrangement priority order determination unit 202 determines the dot arrangement priority order by sorting the derived evaluation values of each pixel. When the input image includes the detail line, the number of effective pixels (pixels that are not white pixels) forming the line is small as shown in the processing area 402, and most of the white pixels having a pixel value of 0 occupy the majority. Further, since the difference in pixel value between the pixels forming the detail line and the white pixels is small, in a region such as the processing region 402, the thresholds of the threshold group have a great influence on the dot arrangement priority order, and the arrangement of the white pixels is large. Priority may be higher. If dots are arranged in white pixels, an image with deteriorated sharpness is generated. Therefore, the dot arrangement priority order is preferably the effective pixels excluding the white pixels in the higher priority order. Note that in the present embodiment, a pixel having a pixel value of 0 is a white pixel, but a pixel having a pixel value of a predetermined value or less may be a white pixel, as in known background processing for an image read by a document scanner. .. In that case, the effective pixel is a pixel having a pixel value exceeding the predetermined value.

The output value determination unit 207 allocates 1 as an output value to each pixel until the total output value derived by the total output value derivation unit 206 is reached based on the dot arrangement priority order determined by the arrangement priority order determination unit 202. Go Pixels to which 1 is not assigned as the output value are treated as those to which 0 is assigned as the output value. As a result, the output value of each pixel in the processing area is determined.

Here, the inter-color cumulative value derived by the inter-color cumulative value deriving unit 208 will be described with reference to FIG. The inter-color cumulative value is a value that is accumulated every time the correction value derivation unit 205 performs the correction value derivation process for each color of KCMY. In the present embodiment, the sum of the average value of the pixel values of the input image data of the channel (color plane) input immediately before and the inter-color cumulative value is set as the inter-color cumulative value of the color plane being processed. Since this sum can be said to be an accumulation of the average values of the pixel values of the input image data of each channel input up to this time, it can also be called a cumulative average value (or a cumulative representative value). Here, consider a case where the processing area 402 shown in FIG. 4 is input as image data common to KCMY. In this case, when K input image data is input to the image processing unit 106, K is the color plane that is input first, so the inter-color cumulative value is 0. Next, when the input image data of C is input, the inter-color cumulative value is the sum of the average value of the pixel values of the color plane K input immediately before and the inter-color cumulative value. Since the average value of the processing area 402 is 3 (=(18+14+10+6)÷16), the inter-color cumulative value is 3 (=3+0). Similarly, the inter-color cumulative value when M input image data is input is 6 (=3+3), and the inter-color cumulative value when Y is input is 9 (=3+6). The most suitable inter-color cumulative value of the color plane being processed is the sum of the average value of the pixel values input immediately before and the inter-color cumulative value. However, the sum of other representative values (statistics) such as the total value, maximum pixel value, median value, and minimum value in the processing area and the inter-color cumulative value may be used as the inter-color cumulative value of the color plane being processed. .. Hereinafter, the inter-color cumulative value may be referred to as a cumulative representative value. Further, the sum of the value obtained by multiplying these values by the coefficient α and the inter-color cumulative value may be used as the inter-color cumulative value of the color plane being processed.

(Operation in the image processing unit 106)
FIG. 5 is a flowchart showing the operation of the image processing unit 106 in the first embodiment. The series of processes shown in the flowchart of FIG. 5 is performed by the CPU 100 expanding the program code stored in the ROM 102 into the RAM 101 and executing the program code. Alternatively, some or all of the functions of the steps in FIG. 5 may be realized by hardware such as an ASIC or an electronic circuit. The symbol "S" in the description of each process means a step in the flowchart. FIG. 6 is a diagram for explaining the processing result obtained by the processing shown in FIG. The output results 608 to 610 represent the output results after halftone, and the colors at which the dots are turned on at each pixel position are indicated by "K", "C", "M", and "Y". The image processing unit 106 sequentially performs the processes of S501 to S510 described below on the input image data of each color of KCMY after color separation acquired from the RAM 101 or the external storage device 105. Here, for the sake of simplicity of description, it is assumed that the image data 601, 602, 603 and 604 of 16 pixels×16 pixels shown in FIG. 6 are respectively acquired as K, C, M and Y input image data. .. The image data 601 to 604 correspond to a part (areas 301 to 304) of the image data 300 shown in FIG. Further, it is assumed that the size of the processing area is 4 pixels×4 pixels, and the processing is sequentially executed from the upper left area to the upper right area, the lower left area, and the lower right area for each input image data. The size of the processing area is not limited to the above size. Further, the execution order of the processing on the processing area is not limited to the above order.

First, the pixel value acquisition unit 201 acquires the pixel value of each pixel in the processing region from the input image data (S501). Next, the threshold acquisition unit 203 acquires a threshold group corresponding to the processing area from the RAM 101 or the external storage device 105 (S502). The threshold matrix 605 shown in FIG. 6 is a threshold matrix commonly used for the input image data 601 to 604. Therefore, here, the threshold value group at the position corresponding to the processing region is acquired from the threshold value matrix 605. A different threshold matrix may be used for each color plane. Further, since the processes of S501 and S502 do not depend on each other, whichever may be executed first, or the respective processes may be executed in parallel.

Next, the target value derivation unit 204 sets the target value in the processing area (the target of the total number of pixels for which dots should be turned on in the processing area) based on the pixel value acquired in S501 and the threshold value acquired in S502. A value) is derived (S503). Here, the target value derivation unit 204 calculates the average value of all the pixels in the processing area, compares the calculated average value with each threshold value of the threshold value group, and targets the number of threshold values that are less than the average value. Derived as a value. When the upper left area of the input image data 601 is used as the processing area, the average value of pixels in the processing area is 12.5. In addition, the target value is 1 because only one threshold value less than 12.5 is 11 in the threshold value group corresponding to the processing area in the threshold value matrix 605. When the upper right, lower left, and lower right areas are used as the processing areas, the same calculation results in 0 as the target value.

Next, the image processing unit 106 determines whether or not the correction value derivation unit 205 should execute the correction value derivation (S504). Specifically, when the target value derived in step S503 is less than the reference dot number (YES in S504), the image processing unit 106 proceeds to the process of step S505. In this embodiment, the reference number of dots is 1. The reference number of dots is not limited to one, and may be changed according to the size of the processing area. Therefore, if the target value is 0, the process proceeds to S505. On the other hand, when the target value is not 0 (NO in S504), the image processing unit 106 sets 0 as the initial value of the correction value, and the process proceeds to S506. The purpose of this embodiment is to improve the visibility of detail lines. Therefore, the correction value is not derived in the region where the target value is 1 or more, which is assumed to be the high-density region where the detail line does not exist, and the correction value is derived only in the low-density region where the target value is 0.

Next, the correction value derivation unit 205, based on the pixel value acquired in S501, the threshold value acquired in S502, and the inter-color cumulative value derived by the inter-color cumulative value deriving unit 208, the correction value ( A correction amount of the target value) is derived.

Here, the correction value derivation process executed by the correction value derivation unit 205 in S505 will be described. FIG. 7 is a flowchart showing the process of the correction value derivation unit 205 in the first embodiment. First, the correction value derivation unit 205 sets 0 to the correction value and initializes the correction value (S701). Next, the correction value derivation unit 205 determines whether dots have not been arranged in the same processing area, including other color planes (S702). When the dots are arranged (NO in S702), the correction value derivation unit 205 ends the process. The purpose of this embodiment is to prevent the loss of detail lines and improve reproducibility. Therefore, regarding the processing area where dots are already arranged, it is judged that the lines are continuous to some extent (the detail line is not missing) or not in other colors, and excessive correction processing is performed. To prevent this, the correction value derivation process ends.

Next, the correction value derivation unit 205 acquires the average value and the minimum threshold value in the processing area (S703, S704). Further, the correction value derivation unit 205 acquires the inter-color cumulative value (S705). Next, the correction value derivation unit 205 compares the sum of the average value acquired in S703 and the inter-color cumulative value acquired in S705 with the minimum threshold value acquired in S704. When the sum of the average value and the inter-color cumulative value is greater than or equal to the minimum threshold value (YES in S706), the correction value derivation unit 205 sets the correction value to 1. On the other hand, if it is less than the minimum threshold value (NO in S706), the correction value derivation unit 205 ends the process.

Returning to the explanation of FIG. After S505, the total output value derivation unit 206 derives the number of pixels for which dots should be turned on in the processing area (total output value) from the target value derived in S503 and the correction value derived in S505 ( S506). Here, the sum of the target value and the correction value is derived as the total output value. When the target value is 0 as in the upper right, lower left, and lower right processing areas of the input image data 601, the correction value is set as it is as the total output value. At this time, the range of the target value is 0 to 16, while the range of the correction value is 0 or 1. Therefore, the branching process of S504 may be omitted, and the larger value of the target value and the correction value may be adopted as the total output value in S506. In that case, since the correction value is reflected in the total output value in S506 when the target value is 0, the same processing as the processing shown in FIG. 5 is performed.

Next, the arrangement priority order determination unit 202 determines the priority order of the pixels whose dots are to be turned on for the pixels whose dots should be turned on in the processing area (S507). That is, the arrangement priority order determination unit 202 determines the arrangement priority order of dots to be arranged in the processing area. Here, the placement priority order determination unit 202 first derives the evaluation value of each pixel position by subtracting the pixel value of each pixel position of the processing region by the threshold value of the corresponding pixel position. Then, the placement priority order determination unit 202 sorts the derived evaluation values in descending order, and sets the sorted result as the placement priority order. For example, the evaluation value group is derived based on the input image data 601 to 604 and the threshold matrix 605. At this time, the evaluation value is derived only for a pixel having an effective pixel. Further, in all of the input image data 601 to 604 of each color plane, since the pixel values of the effective pixels in the image data are all the same, the evaluation value group 606 common to KCMY is derived, and the dot arrangement priority order is determined. To be done. The arrangement priority data 607 is data representing the dot arrangement priority order determined at this time. Here, for simplicity of explanation, the case where the evaluation value group 606 derived for K is commonly used for KCMY is taken as an example, but the pixel value of the effective pixel in the image data is different for each input image data of each color plane. In this case, the evaluation value group may be derived for each color plane.

Next, the output value determination unit 207 sets 1 as an output value to the corresponding pixel according to the arrangement priority order determined in S507 so that the dots are turned on by the number of the total output values obtained in S506 (S508). ). For the other pixels, the output value determination unit 207 sets 0 as the output value so that the dot is turned off.

After S508, the image processing unit 106 determines whether the processing has been completed for all the processing areas (S509). If not completed (NO in S509), the image processing unit 106 returns to the processing in S501 and repeats the processing in S501 to S508. When the processing is completed (YES in S509), the image processing unit 106 determines whether the processing is completed for all KCMY colors (S510). If not completed (NO in S510), the image processing unit 106 returns to the process of S501 and repeats the processes of S501 to S509. When it is completed (YES in S510), the image processing unit 106 ends the process.

The output result obtained when the process shown in FIG. 5 is sequentially executed on each input image data of KCMY (image data 601 to 604) will be described. Since the number of effective pixels is 0 in the lower left processing area, the arrangement priority order is not determined in S507 and the dots are not arranged, and thus the description thereof is omitted. First, when input image data of K is input, the average value (50×4÷16=12.5)>minimum threshold value (11) in the upper left processing area, so the target value of K is 1. .. In the upper right and lower right processing regions, the target value of K derived by the target value derivation unit 204 is 0, so the correction value derived by the correction value derivation unit 205 becomes the total output value as it is. Here, in the upper right processing region, the average value+the inter-color cumulative value (3.125+0=3.125)<the minimum threshold value (7), so the correction value of K is 0. In the lower right processing area, the average value+the inter-color cumulative value (9.375+0=9.375)<the minimum threshold value (15), so the correction value for K is 0. Therefore, when the processing for the K input image data is completed, one K dot is placed in the upper left processing area as shown in the output result 608. Next, when the input image data of C is input, the target value of C derived by the target value deriving unit 204 becomes 0 in the processing regions at the upper left, upper right, and lower right, and therefore the correction value deriving unit 205 derives it. The correction value becomes the total output value as it is. Here, since K dots have already been arranged in the upper left processing area, the correction value of C is set to 0 by the branch processing of S702. In the upper right processing area, the average value+the inter-color cumulative value (2.5+3.125=5.625)<the minimum threshold value (7), and thus the correction value of C is 0. Further, in the lower right processing area, the average value+the inter-color cumulative value (7.5+9.375=16.875)≧minimum threshold value (15), and thus the correction value is set to 1. Therefore, when the processing on the C input image data is completed, as shown in the output result 609, one K dot is arranged in the upper left processing area and one C dot is arranged in the lower right processing area. .. Similarly, when processing is performed on the M and Y input image data, one K dot is finally located in the upper left processing area, one M dot is located in the upper right processing area, and one is located in the lower right processing area. An output result 610 in which one C dot is arranged is obtained.

The effects of the present embodiment will be described below. First, when the correction value derivation process in the correction value derivation unit 205 is not performed, the number of dots arranged in each of the upper right, lower left, and lower right processing regions is 0. Therefore, the visibility of the detail line of the input image data 601 is significantly reduced after the halftone process. That is, the shape of the object in the input image data is not retained. Further, when the average value and the minimum threshold value are independently compared for each color plane without considering the inter-color cumulative value when deriving the correction value, K, C, M, and No dot is arranged because the average value of Y exceeds the minimum threshold value (7). Similarly, no dots are arranged in the lower right processing area. Therefore, in the image after the halftone process shown in the output result 610 shown in FIG. 6, M dots and C dots are not recorded, and the detail line is reproduced only with K dots. On the other hand, in the present embodiment in which the inter-color cumulative value is taken into consideration when deriving the correction value, M dots and C dots are recorded in the upper right and lower right processing regions, respectively, as in the output result 610, so that the detail line The color tone is adjusted.

The process of S507 (placement priority order determination process) can be executed any time after the pixel values and the threshold matrix included in the processing region are acquired by the processes of S501 and S502. Further, the process of S507 and the processes of S503 to S506 are processes that do not depend on each other. Therefore, whichever of the processing in step S507 and the processing in steps S503 to S506 may be executed first, or each of them may be executed in parallel.

Further, the main point of this embodiment is that a correction process considering the inter-color cumulative value is added when determining the total output value. Therefore, the present embodiment is also applicable to an image forming system configured to derive the total output value within a predetermined area and determine the dot arrangement separately from the total output value derivation.

<Second Embodiment>
In the first embodiment, when the correction value deriving unit 205 derives the correction value, if the dots of another color plane are already arranged in the same processing area, the correction value is not derived. I explained. In the first embodiment, such a configuration prevents the detail line from being overemphasized and the brightness from largely changing. In the present embodiment, a configuration will be described in which the adjustment of the color tone of the detail line is facilitated by allowing the change in the brightness to some extent.

The configuration and operation of the image processing unit 106 of the second embodiment are similar to those of the first embodiment. However, as shown in FIG. 8, the operations of the correction value derivation unit 205 and the placement priority order determination unit 202 are different from those of the first embodiment. The same components as those in the first embodiment are designated by the same reference numerals, and detailed description thereof will be omitted.

FIG. 8 is a flowchart showing the process of the correction value derivation unit 205 in the second embodiment. In the correction value derivation process of the first embodiment, if it is determined in S702 that dots of another color plane have already been arranged in the same processing area so that the brightness does not change significantly, the process is terminated. The correction value range is set to 0 or 1. On the other hand, in this embodiment, the range of the correction value is expanded to 0 to 16 by not performing the process corresponding to the branching process of S702 of the first embodiment. As a result, the number of dots that can be arranged in the processing area can be increased, and the adjustment of the color tone is facilitated. Note that the processing of S801, S802 to S804, S805 to S807 is the same as the processing of S701, S703 to S705, S708 to S710 of the first embodiment, and a description thereof will be omitted.

After the processing of step S804, the correction value derivation unit 205 determines whether the number of dots arranged in the processing area is smaller than the number of effective pixels (S805). If the number of pixels is equal to or more than the number of effective pixels (NO in S805), the correction value derivation unit 205 ends the process. As a result, the maximum number of dots arranged in the processing area can be limited to the effective pixel number, and the total output value (sum of the target value and the correction value) can be less than the effective pixel number. Therefore, when the output value determination unit 207 arranges the dots in S508 according to the arrangement priority order determined in S507, it is possible to prevent the dot from being arranged in the pixel having the pixel value of 0, and the image sharpness is improved. It is possible to suppress deterioration of the property.

When the number of dots arranged in the processing area is less than the number of effective pixels (YES in S805), the correction value deriving unit 205 sets a correction value (S806). In the first embodiment, the correction value is set to 1 when the sum of the average value of the pixel values of each pixel in the processing area and the inter-color cumulative value of the processing area is equal to or greater than the minimum threshold value in the processing area. On the other hand, in the present embodiment, the sum of the maximum pixel value and the inter-color cumulative value of the processing area is compared with each threshold value of the threshold value group, and the number of threshold values that is less than the sum is already arranged in the processing area. The difference from the number of dots is set as the correction value. Finally, the correction value derivation unit 205 corrects the evaluation value derived by the placement priority order determination unit 202 (S807). Here, the correction value derivation unit 205 sets the evaluation value of these pixels to the minimum value (=0-the maximum value that the threshold value can take) so that the dots are not arranged in the pixels where the dots are already arranged in the processing area. Set to.

Next, the process of the placement priority order determination unit 202 (the placement priority order determination process of S507) in this embodiment will be described. In the first embodiment, the sum of the correction values derived for each color plane in the same processing area by the branching process of step S702 is 1 at the maximum for all color planes. On the other hand, in the present embodiment, the range of the correction value is expanded to 0 to 16, so that it is possible to arrange a plurality of dots in the same processing area. However, when the arrangement priority order of the dots determined in the arrangement priority order determination processing of S507 is commonly used in all the color planes, the dots overlap between the plurality of color planes, and the color tone changes due to the overlapping dots. , It may be difficult to reproduce the color tone. Therefore, when determining the dot arrangement priority order in S507, the arrangement priority order determination unit 202 of the present embodiment controls the dot arrangement position so that the dots are not arranged at the same position among the plurality of color planes. Specifically, the placement priority order determination unit 202 treats dots placed by a correction process in a certain color plane in the same manner as white pixels when determining the placement priority order of dots in another color plane, and Control so that they are not overlapped.

The arrangement priority order determination process of this embodiment will be specifically described with reference to FIG. 9. Here, the case where the threshold value group 901 is acquired in S501 and the image data 902 and 903 are acquired in S502 is taken as an example. The image data 902 is KMY input image data after color separation. The image data 903 is C input image data after color separation. At this time, in S507, the arrangement priority order as represented by the arrangement priority order data 904 is determined for all the input image data of KCMY. Further, the average values of the input image data 902 and 903 are 2 and 4, respectively, and since there are 0 threshold values less than the average value in the threshold value group 901, the target value is 0. Therefore, the correction value is directly reflected in the total output value. Further, in this embodiment, since the cumulative value of the maximum pixel values in the processing area is used as the cumulative value between colors, the cumulative values between colors of K, C, M, and Y are 0, 8, 24, and 32, respectively.

In the arrangement priority order determination process for the input image data of K, it is determined in S805 that the number of dots (0) arranged in the processing area is smaller than the number of effective pixels (4), and therefore the process of S806 (correction value setting process) Is executed. At this time, the added value of the maximum pixel value (8) and the inter-color cumulative value (0) is 8, and only one threshold value which is less than the added value is 7 in the threshold value group. Therefore, based on the threshold number (1) that is less than the added value and the number (0) of dots that are already arranged, the correction value of K is set to 1 (=1-0), and the number of K dots is 1 dot. Become. Next, in the arrangement priority order determination process for the input image data of C, the added value of the maximum pixel value (16) and the inter-color cumulative value (8) is 24, and the threshold values less than the added value in the threshold value group are 7, 15 , 23. Since the number of dots already arranged is 1, the correction value for C is set to 2 (=3-1), and the number of dots for C is 2. Similarly, in the arrangement priority order determination process for the M input image data, the added value of the maximum pixel value (8) and the inter-color cumulative value (24) is 32, and the threshold values less than the added value in the threshold value group are 7, 15 ,23,31. Since the number of dots already arranged is 1 dot of K and 2 dots of M, a total of 3 dots, the correction value of M is set to 1 (=4-3), and the number of dots of M is 1 dot. Become. On the other hand, in the arrangement priority order determination process for the Y input image data, the number of dots (4) that have already been arranged becomes equal to the number of effective pixels (4), and therefore the processes of S806 and S807 are not executed. Therefore, the correction value of Y is set to 0 which is an initial value. As a result, the numbers of dots arranged for K, C, M, and Y are 1, 2, 1, 0, respectively, and the dots are arranged in the order of K, C, M, and Y according to this dot number and the arrangement priority order. As a result, the output result 905 is finally obtained. As shown in FIG. 9, according to the correction value derivation process according to the present embodiment, the total output value in the low gradation processing region where the target value is 0 is a value larger than 1 (4 in the example shown in FIG. 9). Is set to, the process of greatly changing the brightness is performed. On the other hand, in the output result 905, the number of C dots having a high gradation number in the input image data is larger than that of the other color planes, and the C tone is emphasized. That is, according to the correction value derivation process according to the present embodiment, the color tone of the input image can be easily reflected in the output result even when the low gradation region is set as the processing target region.

<Third Embodiment>
In the second embodiment, the configuration in which the adjustment of the color tone is facilitated by expanding the range of the correction value derived for each processing region has been described. In the method of deriving the correction value for each processing area as in the second embodiment, since the maximum pixel value and the minimum threshold value in the processing area are compared, the correction value can be determined regardless of the phase of the dither matrix. It is possible to increase the probability that dots are arranged. On the other hand, the above method of determining the correction value based on the low-frequency component in the processing area, such as the average value or the maximum pixel value in the processing area, may reduce the reproducibility of high-frequency color tone for each pixel. There is. Therefore, in the present embodiment, in low-density input image data having a target value of 0, dot on/off is determined by taking into consideration the inter-color cumulative value for each pixel, thereby improving the reproducibility of the color tone of the detail line. A method of increasing the value will be described.

(Structure of Image Processing Unit 106)
The configuration of the image processing unit 106 in this embodiment will be described. FIG. 10 is a block diagram showing the arrangement of the image processing unit 106 according to the third embodiment. As shown in FIG. 10, the configuration of the image processing unit 106 according to the present embodiment does not include the correction value derivation unit 205 and the total output value derivation unit 206, and the correction output value determination unit 209 is newly added. The added point is different from the configurations of the first and second embodiments.

The corrected output value determination unit 209 includes a pixel value of the processing area acquired by the pixel value acquisition unit 201, a threshold value acquired by the threshold value acquisition unit 203, and an inter-color cumulative value derived by the inter-color cumulative value deriving unit 208. The corrected output value is determined based on At that time, the corrected output value determination unit 209 determines the corrected output value so that the output image data to be output by the image processing unit 106 can be obtained. Details of the operation of the correction output value determination unit 209 will be described later.

(Operation in the image processing unit 106)
In the image processing unit 106 of the first embodiment, the total output value derivation unit 206 derives the total output value using the correction value derived by the correction value derivation unit 205, and the output value determination unit 207 determines the placement priority order. The output value is determined according to the arrangement priority order determined by the unit 202. On the other hand, in the present embodiment, when the target value is 0, the corrected output value determination unit 209 uses the arrangement information indicating the arrangement state of the dots of the other color planes already arranged, and outputs the corrected output. Determine the value. On the other hand, when the target value is not 0, the output value determination unit 207 determines the output value based on the target value and the arrangement priority order. The operation of the image processing unit 106 of this embodiment will be described below with reference to FIG.

FIG. 11 is a flowchart showing the operation of the image processing unit 106 in the third embodiment. The series of processes shown in the flowchart of FIG. 11 is performed by the CPU 100 expanding the program code stored in the ROM 102 into the RAM 101 and executing the program code. Alternatively, some or all of the functions of the steps in FIG. 11 may be realized by hardware such as an ASIC or an electronic circuit. In this embodiment, the processes corresponding to S505 and S506 in the first and second embodiments are not performed. Further, in this embodiment, the process of S1107 is added as a new process. Note that the processing of S1101 to S1103, S1108, and S1109 is the same as the processing of S501 to S503, S509, and S510 of the first and second embodiments, and a description thereof will be omitted.

If the target value is not 0 in S1104 (NO in S1104), the placement priority order determination unit 202 executes the placement priority order determination process (S1105). Note that this processing is the same as the processing of S507 of the first embodiment. Then, the output value determination unit 207 sets 1 as the output value to the corresponding pixel according to the arrangement priority order determined in S1105 so that the dots are turned on by the number of the target values derived in S1103 (S1106). .. For the other pixels, the output value determination unit 207 sets 0 as the output value so that the dot is turned off. On the other hand, when the target value is 0 (YES in S1104), the corrected output value determination unit 209 determines the output value based on the pixel value acquired in S1101 and the threshold value acquired in S1102 (S1107). .. Here, the process of S1107 will be specifically described with reference to FIG. The processing shown in FIG. 12 is sequentially executed for each pixel in the processing area.

First, the corrected output value determination unit 209 initializes the output value set in the pixel to be processed (target pixel) to 0 indicating that the dot is off (S1201). Next, the corrected output value determination unit 209 determines whether or not dots have already been arranged at the target pixel position in the processing area in the processing of another color plane (S1202). When the dots have already been arranged (YES in S1202), the corrected output value determination unit 209 proceeds to the process of S1206. When the dots are not arranged (NO in S1202), the corrected output value determination unit 209 acquires the inter-color cumulative value of the target pixel (hereinafter, also referred to as cumulative pixel value) (S1203). Next, the correction output value determination unit 209 determines whether the sum of the inter-color cumulative value and the pixel value acquired in S1203 in the target pixel is larger than the threshold value corresponding to the target pixel position (S1204). When it is less than the threshold value (NO in S1204), the corrected output value determination unit 209 proceeds to the process of S1206. If it is greater than or equal to the threshold value (YES in S1204), the corrected output value determination unit 209 sets the output value of the pixel of interest to 1 indicating that the dot is on (S1205). Then, the corrected output value determination unit 209 confirms whether the processing has been completed for all pixels in the processing area (S1206). If the processing has been completed for all pixels (YES in S1206), the corrected output value determination unit 209 ends the processing. When there is an unprocessed pixel (NO in S1206), the correction output value determination unit 209 selects a pixel to be processed from the unprocessed pixel, and performs the processes of S1201 to S1206 with the selected pixel as the pixel of interest. repeat.

The operation of the image processing unit 106 of this embodiment will be specifically described with reference to FIG. Here, the case where the threshold group 1302 is acquired in S1101 and the input image data 1301 common to KCMY is acquired in S1102 is taken as an example. At this time, the average value of the input image data 1301 is 4 (=16×4÷16), which is smaller than the minimum threshold value (11) of the threshold value group 1302, and thus 0 is derived as the target value in S1103. Therefore, in S1107, the corrected output value determination unit 209 determines the corrected output value of each color. It is assumed here that the input image data is processed in the order of KCMY. In the correction output value determination process for the K input image data, the inter-color cumulative value 1303 is obtained in the process of S1203 (inter-color cumulative value acquisition process). Further, since the sum of the pixel value (16) and the inter-color cumulative value (0) is 16, which is larger than the threshold value (11), K dots are arranged at the pixel position where the threshold value (11) is set. .. The output result 1307 represents the output result obtained by the correction output value determination process for the K input image data. In the correction output value determination process for the C input image data, the inter-color cumulative value 1304 is obtained in the process of S1203. It should be noted that, for the pixel in which the K dot has already been arranged, the subsequent processing is not executed by the processing in S1202, so the pixel in which the K dot has already been arranged is treated in the same manner as the white pixel, and the color As shown by the inter-color cumulative value 1304, the inter-color cumulative value is invalidated. Further, since the sum of the pixel value (16) and the inter-color cumulative value (16) is 32, which is larger than the threshold value (23), the C dot is arranged at the pixel position where the threshold value (23) is set. .. The output result 1308 represents the output result obtained when the corrected output value determination processing is performed on the C input image data. Similarly, the correction value determination processing is performed on the M and Y input image data, and the output results 1309 and 1310 are derived based on the inter-color cumulative values 1305 and 1306.

Although the most preferable processing flow in this embodiment is the method shown in FIG. 11, after the output value is determined in S1106, the processing in S1104 is performed to execute the corrected output value determination processing in S1107. It may be determined whether or not. Further, the main point of this embodiment is that the correction processing is performed in consideration of the inter-color cumulative value for each pixel in the low density area. Therefore, when the total of the output values in the predetermined area in the output image data obtained by performing the halftone process on the input image data after color separation is less than or equal to the predetermined value, the correction is performed on the area. This embodiment can be applied to an image forming system.

Further, in the example shown in FIG. 13, the inter-color cumulative value of the pixels in which the dots are arranged is invalidated, but the present invention is not limited to this, and the threshold value may be invalidated. When the threshold value is invalidated, the minimum threshold value in the processing area can be used as shown in the second embodiment. That is, the minimum threshold value of the color plane in which the dots are arranged is invalidated. This makes it possible to suppress interference due to the position of the threshold value.

(Other embodiments)
The present invention supplies a program that implements one or more functions of the above-described embodiments to a system or apparatus via a network or a storage medium, and one or more processors in a computer of the system or apparatus read and execute the program. It can also be realized by the processing. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

202 Arrangement Priority Order Determination Unit 204 Target Value Derivation Unit 205 Correction Value Derivation Unit 206 Total Output Value Derivation Unit 207 Output Value Determination Unit 208 Inter-Color Accumulated Value Derivation Unit 209 Correction Output Value Determination Unit

Claims (19)

An image processing apparatus for generating dot data for determining whether or not to arrange a dot in each pixel used for forming an image on a recording medium, for each color of a color material used for forming the image,
With respect to the input image data of the color to be processed, for each predetermined area, a deriving unit that derives the number of dots to be arranged in the predetermined area based on the pixel value of each pixel included in the predetermined area,
Using the evaluation value based on each pixel value of the predetermined area and the threshold value group corresponding to the predetermined area in the threshold matrix, the arrangement priority order indicating the priority order of the pixels in which the dots are arranged in the predetermined area is determined. Decision means,
When the number of dots derived by the deriving unit is less than the reference number of dots, the deriving unit derives based on a cumulative representative value obtained by accumulating representative values of pixel values of the predetermined region between colors. Correction means for correcting the number of dots,
Generating means for generating the dot data corresponding to the predetermined area based on the number of dots corrected by the correcting means and the arrangement priority order determined by the determining means;
An image processing apparatus comprising: The correction means derives the representative value from the pixel value of each pixel included in the predetermined area, and corresponds to the derived representative value and the predetermined area already derived for the input image data of another color. The image processing apparatus according to claim 1, wherein a sum of the representative value and the representative value is calculated as the cumulative representative value. When the number of dots derived by the deriving unit is less than the reference number of dots, the correction unit compares the cumulative representative value of the predetermined region with a minimum threshold value of the threshold value group corresponding to the predetermined region. If the cumulative representative value is larger than the minimum threshold value, correction is performed to increase the number of dots derived by the deriving unit to the reference number of dots. .. The image processing apparatus according to claim 3, wherein the correction unit does not correct the number of dots derived by the derivation unit when the cumulative representative value is equal to or less than the minimum threshold value. The determining unit subtracts the pixel value of each pixel included in the predetermined area by each threshold value of the threshold value group corresponding to the predetermined area, and derives and derives the evaluation value for each of the pixels. The image processing apparatus according to any one of claims 1 to 4, wherein the arrangement priority order is set higher for a pixel having a larger evaluation value. The correcting unit corrects the dot number derived by the deriving unit when the number of dots determined to be arranged in the predetermined region is less than the number of non-white pixels included in the predetermined region. The image processing apparatus according to claim 1 or 2, characterized in that. The correction unit subtracts the number of thresholds included in the threshold value group corresponding to the predetermined area, which is less than the cumulative representative value of the predetermined area, by the number of dots determined to be arranged in the predetermined area. The image processing apparatus according to claim 6, wherein the obtained correction value is added to the number of dots derived by the deriving unit to correct the number of dots. The correction unit corrects the evaluation value such that the arrangement priority order of pixels determined to arrange dots of other colors in the predetermined region is lower than that of other pixels. The image processing apparatus according to claim 6 or 7. The representative value is an average value, a total value, a maximum value, a minimum value, a median value of pixel values of pixels included in the predetermined area, or a value obtained by multiplying them by a predetermined coefficient. The image processing device according to any one of claims 1 to 8. The image processing apparatus according to claim 1, wherein the reference number of dots is determined based on the size of the predetermined area. The image processing apparatus according to claim 10, wherein the reference dot number is 1 when the size of the predetermined area is 4 pixels×4 pixels. The correction unit does not correct the number of dots derived by the derivation unit when it is determined that dots of other colors are arranged in the predetermined area. The image processing device according to any one of 11. The deriving unit derives, as the number of dots, a number of threshold values included in the threshold value group corresponding to the predetermined area, which is less than an average value of pixel values of the predetermined area. The image processing device according to claim 12. An image processing apparatus for generating dot data for determining whether or not to arrange a dot in each pixel used for forming an image on a recording medium, for each color of a color material used for forming the image,
With respect to the input image data of the color to be processed, for each predetermined area, a deriving unit that derives the number of dots to be arranged in the predetermined area based on the pixel value of each pixel included in the predetermined area,
When the number of dots derived by the deriving unit is less than the reference number of dots, the cumulative pixel value of each pixel obtained by accumulating the pixel value of each pixel in the predetermined area between colors, and a threshold matrix Based on each threshold value of the threshold value group corresponding to the predetermined area, a generation unit that generates the dot data corresponding to the predetermined area,
An image processing apparatus comprising: The generating means is
When the number of dots derived by the deriving unit satisfies the reference number of dots, dots are calculated in the predetermined area by using an evaluation value based on each pixel value of the predetermined area and a threshold value group corresponding to the predetermined area. Deciding the arrangement priority order indicating the priority order of the pixels to be arranged,
The image processing apparatus according to claim 14, wherein the dot data corresponding to the predetermined area is generated based on the number of dots and the arrangement priority order derived by the deriving unit. The generating means is
The output value of each pixel is determined based on the sum of each pixel value of the predetermined area and the cumulative pixel value of each pixel, and the minimum threshold value of pixels in which dots are not arranged in all other colors in the predetermined area. The image processing apparatus according to claim 15, wherein the image processing apparatus comprises: An image processing method for generating dot data for determining whether or not to arrange dots in each pixel used for forming an image on a recording medium, for each color of a color material used for forming the image,
With respect to the input image data of the color to be processed, for each predetermined region, a deriving step of deriving the number of dots to be arranged in the predetermined region based on the pixel value of each pixel included in the predetermined region,
Using the evaluation value based on each pixel value of the predetermined area and the threshold value group corresponding to the predetermined area in the threshold matrix, the arrangement priority order indicating the priority order of the pixels in which the dots are arranged in the predetermined area is determined. Decision step,
When the number of dots derived in the deriving step is less than a reference number of dots, based on a cumulative representative value obtained by accumulating representative values of pixel values of the predetermined region between colors, the deriving step is performed. A correction step for correcting the derived number of dots,
A generation step of generating the dot data corresponding to the predetermined area based on the number of dots corrected in the correction step and the arrangement priority order determined in the determination step,
An image processing method comprising: An image processing method for generating dot data for determining whether or not to arrange dots in each pixel used for forming an image on a recording medium, for each color of a color material used for forming the image,
With respect to the input image data of the color to be processed, for each predetermined region, a deriving step of deriving the number of dots to be arranged in the predetermined region based on the pixel value of each pixel included in the predetermined region,
When the number of dots derived in the deriving step is less than a reference number of dots, the cumulative pixel value of each pixel obtained by accumulating the pixel value of each pixel in the predetermined region between colors and a threshold matrix A generation step of generating the dot data corresponding to the predetermined area based on each threshold value of the threshold value group corresponding to the predetermined area of
An image processing method comprising: A program for causing a computer to execute the image processing method according to claim 17 or 18.

Images