JP7066562B2 - Image processing equipment, image processing methods and programs - Google Patents

Description

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

Each pixel constituting the digital image data is generally represented by multi-gradation of 8 bits or 16 bits per color. Therefore, in recent years, when printing such digital image data with a printer, it is known to perform halftone processing that pseudo-expresses the original gradation while reducing the number of gradations that can be printed by the printer. ing.

As one of the halftone processing, there is a so-called dither processing using a threshold matrix. In this dither processing, a threshold matrix in which a plurality of thresholds are arranged with equal probability is periodically arranged, the pixel data and the threshold corresponding to the pixel data are compared, and the image data is converted into a halftone image according to the comparison result. This is a process of reducing the number of gradations by converting.

The threshold matrix used for dither processing includes a dot-dispersed threshold matrix that has many high-frequency components that appear evenly dispersed in the comparison results, and low-frequency components that appear concentrated at certain points in the comparison results. There are many dot-intensive threshold matrices. Further, the halftone processing using the dot-distributed threshold matrix is called an FM (Frequency Modulation) screen, and the halftone processing using the dot-intensive threshold matrix is called an AM (Amplitude Modulation) screen.

In any threshold matrix, when a flat image with uniform gradation is input, the thresholds are arranged so that the printer that executes printing can express the gradation as an ideal processing result. It is decided in advance. However, in the halftone processing using the dot-concentrated threshold matrix (that is, AM screen), although the dot stability is high, when the pixel data and each threshold are compared, the comparison result does not have the desired frequency of appearance. It may be biased. As a result, the distribution pattern of the pixel data of the input image and the spatial frequency of the dot-concentrated threshold matrix interfere with each other to generate a folding component, which causes image quality problems such as moire (interference fringes), graininess, and fine line breaks. There is a problem that appears.

On the other hand, Patent Document 1 discloses a technique of merging a halftone image to which a high frequency halftone screen is applied and a halftone image to which a low frequency halftone screen is applied by weighting according to the feature amount of the image. ing.

Japanese Unexamined Patent Publication No. 11-331562

However, in the technique of Patent Document 1, increasing the specific density of a high frequency halftone image in which moire is difficult to be observed and increasing the specific density of a low frequency halftone image having high dot stability are in a trade-off relationship and are optimal. It is difficult to calculate the specific density. Therefore, it was not possible to obtain a halftone image that simultaneously satisfies the dot arrangement with high dot stability and the visibility of the moire (the moire is not visible).

The present invention has been made in view of the above-mentioned conventional problems, and an object of the present invention is to obtain a suitable halftone image in which moire is not visually recognized while ensuring a dot arrangement with high dot stability. And.

In order to solve the above problems, the image processing apparatus of the present invention is an image processing apparatus that performs dither processing on image data for each predetermined processing area, and is a flatness level detection that detects a flatness level of the processing area. AM placement information generation that generates AM placement information from the means, the difference level detecting means for detecting the difference level by subtracting the flatness level from each pixel value in the processing area, and the flatness level and the first threshold matrix. Based on the means, the FM placement information generating means that generates FM placement information from the difference level and the second threshold matrix, and the AM placement information and the FM placement information, the output gradation of each pixel in the processing area. It is characterized by comprising a gradation value determining means for determining a value.

According to the present invention, it is possible to acquire a suitable halftone image in which moire is not visually recognized while ensuring a dot arrangement with high dot stability.

It is a block diagram which shows the hardware composition of an image processing apparatus. It is a block diagram which shows the functional structure of the image processing part of an image processing apparatus. It is a flowchart which shows the processing procedure in an image processing part. It is a figure for demonstrating an area to be an input image and a processing unit. It is a figure which shows the dot concentration type threshold matrix. It is a figure which shows the dot dispersion type threshold matrix. It is a figure for demonstrating the flatness level detection process. It is a figure for demonstrating the generation method of AM arrangement information. It is a figure for demonstrating the detection process of a difference level. It is a figure for demonstrating the generation method of FM arrangement information. It is a figure for demonstrating the method of determining an output gradation value. It is a block diagram which shows the functional structure of the image processing part of an image processing apparatus. It is a figure for demonstrating the flatness level detection process. It is a figure which shows the AM arrangement information. It is a figure for demonstrating the generation method of FM arrangement information. It is a figure for demonstrating the method of determining an output gradation value. It is a figure which shows the AM arrangement information. It is a figure for demonstrating the method of determining an output gradation value.

Hereinafter, the present invention will be described based on a preferred embodiment with reference to the accompanying drawings. The configuration shown in the following embodiments is only an example, and the present invention is not necessarily limited to the illustrated configuration.

Embodiment 1

In the present embodiment, in the halftone processing for converting a 256-value (that is, 8-bit) input image into a 2-value (that is, 1-bit) output image, the processing is performed for each region of 4 pixels × 4 pixels. explain.

(Hardware configuration of image processing device)
FIG. 1 is a block diagram showing a hardware configuration of an image processing device according to the present embodiment. As shown in FIG. 1, the image processing apparatus includes a CPU 100, a RAM 101, a ROM 102, an operation unit 103, a display unit 104, an image processing unit 105, an I / F (interface) unit 106, and a bus 107.

The CPU (Central Processing Unit) 100 controls the operation of the entire image processing device by using computer programs and data stored in the RAM 101 and the ROM 102. The RAM (Random Access Memory) 101 temporarily stores computer programs and data loaded from an external storage device via the I / F unit 106. The RAM 101 also functions as a storage area used when the CPU 100 executes various processes and a storage area used when the image processing unit 105 executes image processing. As described above, the RAM 101 can appropriately provide various storage areas.

The ROM (Read Only Memory) 102 stores the setting data of the image processing device, the boot program, and the like. The operation unit 103 is composed of an input device such as a keyboard and a mouse, and an operator of the image processing device can input various instructions to the CPU 100 by using the operation unit 103.

The display unit 104 is composed of a CRT (Cathode Ray Tube), a liquid crystal screen, or the like, and can display the processing result by the CPU 100 as an image, characters, or the like. The image processing unit 105 is composed of a processor capable of executing a computer program and a dedicated image processing circuit. When the image processing unit 105 receives an instruction to execute image processing from, for example, the CPU 100, the image processing unit 105 executes halftone processing on the input image stored in the RAM 101 and outputs the execution result (image processing result). As described above, the image processing unit 105 uses the storage area of the RAM 101 in the process of image processing.

The I / F unit 106 functions as an interface for connecting an image processing device to an external storage device or the Internet. The image processing device appropriately loads the program, image data, image processing parameters, etc. to be processed by the CPU 100 stored in the external storage device into the RAM 101 via the I / F unit 106. The bus 107 connects each of the above-mentioned parts and transfers various data and the like.

The hardware configuration of the image processing device is not necessarily limited to the configuration shown in FIG. Hereinafter, the functional configuration of the image processing unit 105 of the image processing apparatus will be described with reference to the block diagram shown in FIG. Further, the processing procedure in the image processing unit 105 of the image processing apparatus will be described with reference to the flowchart shown in FIG. As a supplement, when the image processing unit 105 is realized as a dedicated image processing circuit, each component shown in FIG. 2 may be mounted on the image processing circuit. Further, when the function of the image processing unit 105 is realized by the program operation of the processor, the processing may be executed according to the flowchart shown in FIG.

(Functional configuration of image processing unit)
FIG. 2 is a block diagram showing a functional configuration of an image processing unit 105 of the image processing apparatus according to the present embodiment. As shown in FIG. 2, the image processing unit 105 has a pixel value acquisition unit 200, an AM threshold value acquisition unit 201, a flat level detection unit 202, an AM arrangement information generation unit 203, an FM threshold value acquisition unit 204, and a difference level detection unit 205. , FM arrangement information generation unit 206, and output gradation value determination unit 207 are provided.

The pixel value acquisition unit 200 acquires a pixel group constituting an area for executing halftone processing from an input image stored in the RAM 101. In the present embodiment, a pixel group 401 having 4 vertical pixels and 4 horizontal pixels is acquired as a processing unit (block) from the 8-bit input image 400 as shown in FIG.

The AM threshold value acquisition unit 201 acquires a threshold value group of an AM screen having a size suitable for an area for executing halftone processing from the threshold value group stored in the RAM 101. Here, in the present embodiment, it is assumed that a threshold group (periodic threshold pattern) as shown in FIG. 5 is used. The threshold group shown in FIG. 5 is a dot-concentrated threshold matrix (first threshold matrix) having 13 thresholds surrounded by a broken line as a set. This first threshold matrix is repeatedly tiled across the image. Then, based on such a threshold group, a block-based AM screen threshold group (16 thresholds surrounded by a solid line) corresponding to the pixel group acquired by the pixel value acquisition unit 200 is acquired.

In the threshold group shown in FIG. 5, when the threshold value of the AM screen is smaller, the probability of dots being arranged increases. Further, the area to be the processing unit (hereinafter referred to as the processing area) and the shape of the threshold matrix are not necessarily limited to this. Further, the threshold group does not necessarily have to be acquired from the RAM 101, and can be generated by the Holladay halftone method, or the relative positions of the processing area and the threshold matrix can be prepared in advance.

The flatness level detection unit 202 detects the flatness level from the processing area. In the present embodiment, the average value of the pixel values of the pixels in the pixel group of 4 pixels × 4 pixels is detected as the flat level. The AM placement information generation unit 203 generates AM placement information using the flat level detected by the flat level detection unit 202 and the threshold group of the AM screen acquired by the AM threshold value acquisition unit 201. The AM placement information indicates centralized dot placement.

The FM threshold value acquisition unit 204 acquires a threshold value group of the FM screen having a size suitable for the area where the halftone processing is executed from the threshold value group stored in the RAM 101. Here, in this embodiment, it is assumed that the threshold group as shown in FIG. 6 is used. The threshold group shown in FIG. 6 is a dot-dispersed threshold matrix (second threshold matrix) having 64 vertical and horizontal pixels as a set. Then, from such an image group, the threshold group 600 (16 thresholds surrounded by a solid line) of the block-shaped FM screen corresponding to the pixel group acquired by the pixel value acquisition unit 200 is acquired. In the threshold group shown in FIG. 6, when the threshold value of the FM screen is smaller, the probability of dots being arranged increases.

The difference level detection unit 205 detects the difference level of each pixel in the processing area. The difference level detection unit 205 calculates the difference value of each pixel by subtracting the flatness level acquired from the flatness level detection unit 202 from the pixel value of each pixel in the processing area. Here, the difference values of all the pixels in the processing area are collectively called the difference level.

The FM placement information generation unit 206 generates FM placement information using the difference level detected by the difference level detection unit 205 and the threshold group 600 of the FM screen acquired by the FM threshold value acquisition unit 204. The FM placement information is information obtained by converting the difference level into a distributed dot placement, and is used to correct the AM placement information. The output gradation value determination unit 207 determines the output gradation value of each pixel in the processing area from the AM arrangement information acquired from the AM arrangement information generation unit 203 and the FM arrangement information acquired from the FM arrangement information generation unit 206. ..

(Processing procedure in the image processing unit)
Next, the operation (processing) in the image processing unit 105 will be specifically described with reference to the flowchart shown in FIG. In the following, the symbol "S" in the description of the flowchart represents a step.

First, in S300, the pixel value acquisition unit 200 acquires pixel data from the RAM 101. The pixel value acquisition unit 200 acquires pixel data. In S301, the pixel value acquisition unit 200 determines whether or not all the pixel data of the image group to be the processing area are complete.

In the present embodiment, as described above, halftone processing is executed for the 8-bit input image 400 as shown in FIG. 4 in block units indicated by the pixel group 401 of 4 pixels × 4 pixels. Therefore, in S301, it is determined whether or not the pixel groups in the rectangular region of 4 pixels × 4 pixels are aligned.

As a result of the determination, when all the image data of the pixel group is not available (S301 No), the pixel value acquisition unit 200 returns the process to S300 and repeatedly executes the pixel data acquisition process. On the other hand, when all the image data of the pixel group is prepared (S301 Yes), the pixel value acquisition unit 200 shifts the processing to S302.

In S302, the flatness level detection unit 202 detects the flatness level in the processing area. In the present embodiment, the flatness level detection unit 202 calculates the average value of the pixel values of the pixel groups in the processing area, and the calculated value is set as the flatness level.

Here, the flat level detection process will be described with reference to FIG. 7. FIG. 7A shows the pixel positions of the pixel group in alphabets (ap), and FIG. 7B shows the pixel values of the pixel group. Further, FIG. 7C is a graph showing the pixel positions of the pixel group on the horizontal axis and the pixel values on the vertical axis. The flatness level detection unit 202 calculates the average value of the pixel values in FIG. 7B, and sets “110”, which is the average value of this pixel group, as the flatness level. Since the average value of the pixel group is a value that does not fluctuate within the processing region, it is shown as a flat value without unevenness in FIG. 7 (c).

In S303, the AM threshold value acquisition unit 201 acquires the threshold value data of the AM screen. When the AM threshold value acquisition unit 201 acquires the threshold value data, it determines whether or not all the threshold value data of the threshold value group used as the processing unit in S304 are prepared. In this embodiment, as described above, from the dot-concentrated threshold matrix shown in FIG. 5, the threshold group 500 of the AM screen in the block unit corresponding to the pixel group in the rectangular region of 4 pixels × 4 pixels, which is the processing unit. To get.

As a result of the determination, when all the threshold data of the threshold group are not prepared (S304 No), the AM threshold acquisition unit 201 returns the process to S303 and repeatedly executes the process of acquiring the threshold data. On the other hand, when all the threshold data of the threshold group are prepared (S304 Yes), the AM threshold acquisition unit 201 shifts the processing to S305.

In S305, the AM placement information generation unit 203 generates AM placement information in the processing area. In the present embodiment, the AM arrangement information generation unit 203 performs dither processing on the flatness level detected by the flatness level detection unit 202 using the threshold group 500 of the AM screen in block units corresponding to the pixel group of the processing area. By applying, AM placement information is generated.

Here, a method of generating AM placement information in the AM placement information generation unit 203 will be described with reference to FIG. In FIG. 8, the flatness level (that is, the value “110”) corresponding to the pixel group shown in FIG. 7B is subjected to dither processing using the threshold group 500 of the AM screen, and the AM arrangement information 800 is obtained. The process (method) to generate is shown. The AM arrangement information generation unit 203 compares the flatness level with each threshold value as dither processing. That is, in the processing area consisting of 16 pixels, it corresponds to performing dither processing by regarding the pixel values of all the pixels as flat level values. When the flatness level is equal to or higher than the threshold value, "1" which means that dots are arranged is output, and when the flatness level is less than the threshold value, "0" which means that dots are not arranged is output as arrangement information. For example, for the upper left pixel in the processing area, the flatness level 110 is larger than the corresponding threshold value 29, so that “1” is determined as the arrangement information.

In S306, the difference level detection unit 205 detects the difference level corresponding to the processing area by subtracting the flatness level from the pixel value of the pixel group in the processing area. Here, the difference level will be described with reference to FIG. 9 (a) shows the difference level obtained by subtracting the flat level “110” from the pixel group in the processing area shown in FIG. 7 (b). Further, FIG. 9B is a graph showing the pixel position of this image group on the horizontal axis and the difference level on the vertical axis. In FIG. 9B, the axis of the difference value “0” corresponds to the flatness level.

In S307, the FM threshold value acquisition unit 204 acquires the threshold value data of the FM screen. When the FM threshold value acquisition unit 204 acquires the threshold value data, the FM threshold value acquisition unit 204 determines whether or not all the threshold value data of the threshold value group used as the processing unit in S308 are prepared. In this embodiment, as described above, from the dot-distributed threshold matrix shown in FIG. 6, the threshold group 600 of the FM screen of the block unit corresponding to the pixel group of the rectangular region of 4 pixels × 4 pixels, which is the processing unit. To get.

As a result of the determination, when all the threshold data of the threshold group are not prepared (S308 No), the FM threshold acquisition unit 204 returns the process to S307 and repeatedly executes the process of acquiring the threshold data. On the other hand, when all the threshold data of the threshold group are prepared (S308 Yes), the FM threshold acquisition unit 204 shifts the processing to S309.

In S309, the FM placement information generation unit 206 generates FM placement information corresponding to the processing area. Here, a method of generating FM placement information in the FM placement information generation unit 206 will be described with reference to FIG. 10. The FM placement information generation unit 206 first generates the FM placement information 1000 by performing dither processing on the difference level detected by the difference level detection unit 205 using the threshold group 600 of the FM screen. The FM arrangement information generation unit 206 compares the difference value of each pixel at the difference level with the threshold value corresponding to each pixel as dither processing. If the difference value is equal to or greater than the threshold value, "1" indicating pixels to which dots should be relatively arranged is output as arrangement information. If the difference value is less than the threshold value, "0" indicating a pixel that is not a pixel to which dots should be relatively arranged is output as arrangement information. The pixel to which the dot should be arranged is a pixel having a pixel value relatively larger than that of the surrounding pixel, and is likely to be a pixel that characterizes the pixel value distribution in the processing area. Further, by using the threshold group 600 of the FM screen, the positions of the pixels to which the dots should be arranged are not concentrated in the processing area and are evenly dispersed.

Further, the FM arrangement information generation unit 206 performs dither processing on the difference level detected by the difference level detection unit 205 using the threshold value group adjusted by subtracting the offset value “255” from the threshold value group 600 of the FM screen. As a result, FM arrangement information 1001 is generated. The offset value "255" is the maximum value of the pixel value range in the input image. The FM arrangement information generation unit 206 compares the difference value of each pixel at the difference level with the offset-adjusted threshold value corresponding to each pixel as dither processing. If the difference value is less than the adjusted threshold value, "0" indicating a pixel to which dots should not be arranged relative to the difference level is output as arrangement information. Further, if the difference value is equal to or higher than the adjusted threshold value, "1" indicating a pixel that is not a pixel to which dots should not be relatively arranged is output as arrangement information. It should be noted that the pixels that should not be dot-arranged are pixels whose pixel values are relatively smaller than those of the surrounding pixels, and are likely to be pixels that characterize the pixel value distribution in the processing area. Further, due to the threshold group 600 of the FM screen, the positions of the pixels in which the dots should not be arranged are not concentrated and are evenly distributed in the processing area.

In S310, the output gradation value determination unit 207 determines the output gradation value of each pixel in the processing area based on the AM arrangement information generated in S305 and the FM arrangement information generated in S309. Here, a method of determining the output gradation value in the output gradation value determination unit 207 will be described with reference to FIG. 11. First, the output gradation value determination unit 207 determines all the candidates 1100 to be the dot arrangement target by calculating the logical sum of the AM arrangement information 800 and the FM arrangement information 1000. That is, dots are further added to the AM placement information based on the FM placement information 1000. In the candidate 1100, the pixel whose pixel value is "1" is a candidate for arranging dots, and the pixel whose pixel value is "0" is a candidate for a pixel where dots are not arranged. Next, the output gradation value determination unit 207 calculates the logical product of all the candidates 1100 to be arranged for the dots and the FM arrangement information 1001, for the pixels of the candidates 1100 where the dots should not be arranged. Pull out the dots. That is, the dots are reduced from the candidate 1100 based on the FM arrangement information 1001. As a result, the output gradation value of each pixel is obtained as the processing result 1101 in the processing area. In the processing result 1101, the output gradation value of the pixel in which the dots are arranged is "1", and the output gradation value of the pixel in which the dots are not arranged is "0".

In S311, the image processing unit 105 determines whether or not the processing has been completed for each processing area for all the pixels of the input image 400, and if not (S311 No), returns the processing to S300. Repeat the process.

In this embodiment, as described above, the average value of the pixel values of the pixel groups in the processing area is used as the flat level. Therefore, the AM arrangement information represents the gradation value (corresponding to the total number of dots) of the entire pixel group in the processing area. On the other hand, the FM arrangement information calculated from the difference level expresses the shape of the pixel value distribution. Here, particularly using an FM screen, there are FM arrangement information 1000 indicating a pixel position where dots should be arranged and FM arrangement information 1001 indicating a pixel position where dots should not be arranged. Here, in the method of determining the output gradation value described with reference to FIG. 11, it is determined that the dots are always arranged at the pixel positions where it is determined that the dots should be arranged using the FM screen, and the dots should not be arranged. Dots are not always arranged at the pixel positions that have been set. As described above, according to the method described with reference to FIG. 11, since the emphasis is on expressing the FM shape, the gradation value of the entire pixel group may fluctuate. When maintaining the gradation in the processing area is important, the gradation value in the processing area can be maintained by keeping the number of dots added by the FM arrangement information 1000 and the number of dots deleted by the FM arrangement information 1001 constant. ..

By the above-mentioned processing, the pixel value of each pixel is divided into a flat level and a difference level corresponding to the processing area, and the arrangement information is generated by the threshold matrix suitable for each, so that the dot stability is high and the moire is increased. A dot arrangement that does not occur is realized. In this regard, the explanation is briefly supplemented.

Originally, the dot-concentrated threshold matrix with high dot stability has many frequency components in the low frequency band that easily interfere with the input image, so in order to avoid the occurrence of moire, the area to be processed by the AM screen. It is common to limit. On the other hand, in order to improve the dot stability, it is common to adopt the processing result by the AM screen in the adjacent plurality of pixels.

Therefore, in the present embodiment, the flatness level is detected for a predetermined processing area. In the present embodiment, since dither processing using the dot-concentrated threshold matrix is performed assuming that each pixel in the processing region has a uniform flat level, the dot-concentrated threshold has a high proportion of frequency components in the low frequency band. Moire does not occur even in the matrix. In addition, by generating AM placement information with a constant value in the processing area, it is possible to secure dot placement with high dot stability for any image. Further, in the present embodiment, the shape of the output image can be corrected by the difference level expressing the shape of the pixel value distribution and the dot dispersion type threshold matrix. Therefore, deterioration of image quality (image blur) due to averaging of pixel value distribution can be suppressed. In the FM screen, since the ratio of the frequency component in the high frequency band is high, it is originally difficult to visually recognize the moire.

As described above, a flat level is used by using a dot-concentrated threshold matrix (first threshold matrix) that originally has many low-frequency components that easily generate moire and can obtain output results with high dot stability. Generates AM placement information. Further, the FM arrangement information is generated according to the difference level by using the dot dispersion type threshold matrix (second threshold matrix) having many high frequency components. Then, by correcting the AM arrangement information with the FM arrangement information, it is possible to secure a dot arrangement with high dot stability and to make a dot arrangement in which moire is not visually recognized, and a suitable halftone image can be obtained.

Embodiment 2

In the present embodiment, in determining the output gradation value from the AM arrangement information and the FM arrangement information, the output gradation value is determined based on the target gradation value in the processing area, so that the output floor of the entire processing area is determined. An example of holding a key value will be described. Further, here, an example in which the minimum value in the processing area is used as the flat level is shown. Since the hardware configuration of the image processing apparatus according to the present embodiment can be realized by the same hardware configuration as the hardware configuration of the image processing apparatus according to the first embodiment shown in FIG. The explanation is omitted.

FIG. 12 is a block diagram showing a functional configuration of the image processing unit 105 of the image processing apparatus according to the present embodiment. Similar to the first embodiment, the image processing unit 105 includes a pixel value acquisition unit 200, an AM threshold value acquisition unit 201, a flat level detection unit 202, an AM arrangement information generation unit 203, an FM threshold value acquisition unit 204, and a difference level detection unit 205. It includes an FM arrangement information generation unit 206 and an output gradation value determination unit 207. Further, the image processing unit 105 includes a target gradation value calculation unit (target gradation value derivation unit) 208 in addition to these functional blocks. In the following description, the functional blocks with the same reference numerals will be omitted, and the functional blocks having different reference numerals will be focused on.

As described above, the flatness level detection unit 202 calculates the minimum value in the processing area as the flatness level. For example, in the pixel group of FIG. 7B, the flatness level detection unit 202 detects the minimum value “87” as the flatness level.

Here, the flat level detection process will be described with reference to FIG. 13 (a) is a graph in which the pixel positions of the pixel groups shown in FIG. 7 (b) are shown on the horizontal axis and the pixel values are shown on the vertical axis. In addition, in FIG. 13A, the flatness level “87” is also shown. Further, the difference level detection unit 205 detects the difference level based on the flatness level “87” detected by the flatness level detection unit 202, as shown by the arrow in FIG. 13A. The magnitude of the detected difference level is shown in FIG. 13 (b). As shown in FIG. 13B, since the minimum value in the processing area is set as the flat level, all the difference values at the difference level are positive values.

The AM arrangement information generation unit 203 performs dither processing on the flatness level “87” detected by the flatness level detection unit 202 using the threshold group 500 of the AM screen in block units corresponding to the pixel group of the processing area. Then, the AM arrangement information shown in FIG. 14 is generated.

The FM placement information generation unit 206 generates FM placement information using the difference level detected by the difference level detection unit 205 and the threshold group 600 of the FM screen acquired by the FM threshold value acquisition unit 204. The FM placement information generation unit 206 creates an evaluation value from the difference level and the threshold group 600 of the FM screen, and further determines the priority (priority order) for arranging the dots based on the created evaluation value. Generate FM placement information.

Here, a method of generating FM placement information in the FM placement information generation unit 206 will be described with reference to FIG. The FM arrangement information generation unit 206 first subtracts the threshold value group 600 of the FM screen from the difference level detected by the difference level detection unit 205 (that is, the difference level shown in FIG. 13B), thereby evaluating the evaluation value group. Get 1500. The evaluation value of the evaluation value group 1500 is information indicating the priority order in which the dots are arranged. The evaluation value is configured so that the larger the difference value is, the easier it is for dots to be arranged, and the smaller the corresponding threshold value is, the easier it is for dots to be arranged.

When the FM placement information generation unit 206 acquires the evaluation value group 1500, the FM placement information generation unit 206 subsequently calculates the priority for arranging the dots from the size of the evaluation value. For pixels having the same evaluation value, in the threshold group 600 of the FM screen, the priority of arranging dots is set higher than that of pixels having a smaller threshold (that is, pixels in which dots are more likely to be arranged). .. The FM placement information generation unit 206 acquires the FM placement information 1501 by performing the above processing.

The target gradation value calculation unit 208 calculates a target gradation value which is the total output value of each pixel in the processing area. Regarding the target gradation value, for example, using the techniques disclosed in Patent Document "Japanese Patent Laid-Open No. 2016-21735", Patent Document "Japanese Patent Laid-Open No. 2016-11483", etc., from the pixel group and the threshold value group corresponding to the processing area. It should be calculated. In the present embodiment, the average value of the pixel group in the processing area is subjected to dither processing using the threshold group corresponding to the processing area, and the target gradation value is obtained by summing the results of the dither processing. calculate.

Further, when the threshold value group used for this dither processing matches the threshold value group used when generating the AM arrangement information, the dot arrangement in the pixel value where the processing area is flat becomes the same as the processing result by the normal AM screen. preferable. In the case of the present embodiment, the dot arrangement as shown in FIG. 8 is acquired by performing dither processing on the average value of the pixel group using the AM threshold value group. Further, the target gradation value is calculated as "8" by taking the sum of the results of this dither processing.

The output gradation value determination unit 207 determines the output gradation value of each pixel in the processing area from the above-mentioned AM arrangement information, FM arrangement information, and target gradation number. Here, a method of determining the output gradation value will be described with reference to FIG. FIG. 16A is a flowchart showing the procedure of the process of determining the output gradation value.

First, in S1600, the output gradation value determining unit 207 temporarily sets the AM arrangement information as shown in FIG. 14 as the output gradation value of each pixel. In S1601, the output gradation value determination unit 207 aggregates the number of dots of each pixel and acquires the total of the output gradation values.

Then, in S1602, it is determined whether or not the sum of the output gradation values aggregated in S1601 described above matches the target gradation value. When it is determined that the output gradation value determination unit 207 matches the target gradation value (S1602 Yes), the output gradation value determination unit 207 outputs the current dot arrangement as the output gradation value. Further, when the output gradation value determination unit 207 determines that the target gradation value does not match (S1602 No), the process returns to S1603.

In S1603, the output gradation value determination unit 207 arranges the dots according to the priority (priority order) for arranging the dots shown in the FM arrangement information 1501 (FIG. 16 (b)). Specifically, the output gradation value determination unit 207 arranges dots for one target pixel in order from the first priority order in which the dots shown in the FM arrangement information 1501 are arranged. However, if the dots are already arranged by the AM arrangement information in the pixel to be the dot arrangement target, the priority is raised to the next pixel so as not to overlap with the dot arrangement by the AM arrangement information. To place. Then, when dots are arranged for one pixel, the output gradation value determination unit 207 returns the processing to S1601.

In this way, by repeating the dot arrangement until the total sum of the output gradation values matches the target gradation value (here, until the total sum of the output gradation values becomes the target gradation value "8"), The output gradation value shown in FIG. 16C is acquired. In FIG. 16C, two dots added by the FM placement information to the dot placement shown in the AM placement information (FIG. 14) (priority order shown in FIG. 16B is “1” and “2”. A total of eight dots (pixels circled)) are arranged.

As described above, an example is shown in which a target gradation value is calculated and dots derived from FM arrangement information are added (arranged) to the AM arrangement information until the output gradation value matches the calculated target gradation value. .. Further, by processing in units of pixel groups in this way, the target number of gradations in the processing area is maintained, and good gradation expression can be obtained. In addition, by using the minimum value as the flatness level of the processing area, the minimum dot stability is ensured in the AM arrangement information, and moire caused by the AM screen is not generated. Further, the FM arrangement information is generated based on the difference level that expresses the unevenness on the pixel value distribution by the difference level, and the presence or absence of the dot arrangement of the pixel having the difference level with a large deviation from the flat level is determined, so that the image quality is deteriorated. Does not cause (image blur).

Embodiment 3

In this embodiment, an example in which dots are arranged at an arbitrary ratio in pixels having a high priority (priority order) of each arrangement information in determining an output gradation value from AM arrangement information and FM arrangement information will be described. do.

Since the hardware configuration of the image processing apparatus according to the present embodiment can be realized by the same hardware configuration as the hardware configuration of the image processing apparatus according to the first embodiment shown in FIG. The explanation is omitted. Further, in the following description, the description of the functional blocks having the same reference numerals will be omitted, and the functional blocks having differences will be described.

The AM placement information generation unit 203 uses the threshold group 500 of the AM screen to generate a priority for arranging dots. The priority order in which the dots are arranged is determined by the magnitude relationship of the threshold values of the pixels constituting the threshold value group 500 of the AM screen. That is, a high priority is set for a pixel having a small threshold value in which a dot is likely to be arranged. Here, the AM arrangement information generated based on the threshold group 500 of the AM screen is shown in FIG. In the AM arrangement information shown in FIG. 17, the priority of the pixel located on the left side is set higher with respect to the pixel having the same threshold value.

The output gradation value determination unit 207 determines the output gradation value so as to be the target gradation number from the AM arrangement information generated by the AM arrangement information generation unit 203 and the FM arrangement information 1501 shown in FIG. 15 above. .. Here, a method of determining the output gradation value in the output gradation value determination unit 207 will be described with reference to FIG.

First, the ratio of dots to be arranged is determined in advance based on each arrangement information. In the present embodiment, the ratio of arranging the dots is 1: 1 and the dots are arranged in the pixels having a high priority by the AM arrangement information, and then the dots are arranged in the pixels having a high priority by the FM arrangement information. That is, as shown in FIG. 18, the pixels having the highest priority of the AM arrangement information and the FM arrangement information are selected in order, and the dots are arranged.

As described above, when arranging the dots, it is assumed that the dots are already arranged in the pixel to which the dots are arranged. Specifically, since the pixel having the fourth priority of the FM placement information (the lower right pixel in the processing area) corresponds to the pixel having the second priority of the AM placement information, the priority of the FM placement information. At the stage of arranging the dots based on, the dots are already arranged. In this case, the dots cannot be newly arranged, and the dots are arranged based on the pixel having the next priority of the FM arrangement information (that is, the pixel having the fifth priority). Then, the output gradation value is determined by carrying out this dot arrangement until it matches the target gradation number.

As described above, an example of arranging dots according to the priority of each of AM arrangement information and FM arrangement information is shown. Also in this embodiment, the minimum value in the processing area is used as the flat level of the processing area. Therefore, for example, even in the case where only one pixel has the minimum value and the other pixels have a pixel value having a certain large pixel value, the dot arrangement by the AM screen is formed to some extent, and the dot stability is improved. A high placement pattern can be secured. Further, as described above, when the dots are scheduled to be arranged in the pixel in which the dots are already arranged according to the priority of the other arrangement information, new dots cannot be arranged, so that the dots are next to the priority. Dots are placed for pixels with high priority. This is possible because the output value is determined for each region, whereby the gradation in the region is maintained.

(Other embodiments)
In the first embodiment, the average value of the pixel values in the processing region was used as the flat level. As a result, the AM arrangement information can determine the arrangement information of the number of dots according to the gradation group (density) corresponding to the processing area. Further, in the second embodiment, the minimum value of the pixel value in the processing region was used as the flat level. This makes it easier to process the difference level, as the difference level is configured only with positive difference values. However, for the flatness level, other values such as the maximum value and the median value in the processing area can be used as long as they are fixed values corresponding to the processing area. Further, in the second embodiment, a relatively small value (for example, a pixel value next to the minimum value) can be used even if the value is not necessarily the minimum value. Further, regarding the flatness level, it is possible to configure a plurality of pixel values having similar values in the processing area.

The present invention supplies a program that realizes one or more functions of the above-described embodiment to a system or device via a network or storage medium, and one or more processors in the computer of the system or device reads and executes the program. It can also be realized by the processing to be performed. It can also be realized by a circuit (for example, ASIC) that realizes one or more functions.

202 Flat level detection unit 203 AM layout information generation unit 205 Difference level detection unit 206 FM layout information generation unit 207 Output gradation value determination unit

Claims (17)

An image processing device that performs dither processing on image data for each predetermined processing area.
A flatness level detecting means for detecting the flatness level of the processing area,
A difference level detecting means for detecting the difference level by subtracting the flatness level from each pixel value in the processing area.
An AM placement information generating means that generates AM placement information from the flatness level and the first threshold matrix,
An FM placement information generation means that generates FM placement information from the difference level and the second threshold matrix.
An image processing apparatus comprising: a gradation value determining means for determining an output gradation value of each pixel in the processing region based on the AM arrangement information and the FM arrangement information. The first threshold matrix has a higher proportion of low frequency components than the second threshold matrix, and the second threshold matrix has a higher proportion of high frequency components than the first threshold matrix. Item 1. The image processing apparatus according to item 1. The image processing apparatus according to claim 1 or 2, wherein the first threshold matrix is a dot-intensive threshold matrix, and the second threshold matrix is a dot-dispersed threshold matrix. Further provided with a gradation value deriving means for deriving a target gradation value in the processing area,
The gradation value determining means is characterized in that the output gradation value of each pixel in the processing area is determined so that the sum of the output gradation values of each pixel in the processing area matches the target gradation value. The image processing apparatus according to any one of claims 1 to 3. The image processing apparatus according to claim 4, wherein the gradation value deriving means derives the target gradation value from the average value of each pixel value in the processing region and the first threshold matrix. 4. The FM placement information generating means is characterized in that, as the FM placement information, a priority for arranging dots is generated based on the evaluation value created from the difference level and the second threshold matrix. Or the image processing apparatus according to 5. The claim is characterized in that the gradation value determining means determines the output gradation value of each pixel in the processing area by giving priority to the dot arrangement based on the AM arrangement information over the dot arrangement based on the FM arrangement information. The image processing apparatus according to 6. The gradation value determining means temporarily sets the AM arrangement information as an output gradation value of each pixel in the processing area, and further sets the difference between the target gradation value and the temporarily set output gradation value. The image processing apparatus according to claim 6 or 7, wherein the output gradation value of each pixel in the processing area is determined based on the priority of the FM arrangement information so as to match. The image processing apparatus according to claim 6, wherein the AM arrangement information generating means generates a priority for arranging dots from the first threshold matrix as the AM arrangement information. The gradation value determining means is based on the priority of the AM arrangement information and the priority of the FM arrangement information so that the target gradation value and the output gradation value of each pixel in the processing area match. The image processing apparatus according to claim 9, wherein the output gradation value of each pixel in the processing area is determined. When the priority of the AM arrangement information and the priority of the FM arrangement information are the same, the gradation value determining means gives priority to the dot arrangement in the AM arrangement information over the dot arrangement in the FM arrangement information. The image processing apparatus according to claim 9, wherein the output gradation value of each pixel in the processing area is determined. The claim is characterized in that the gradation value determining means determines the output gradation value of each pixel in the processing area so that the dot arrangement based on the AM arrangement information and the dot arrangement based on the FM arrangement information do not overlap each other. Item 6. The image processing apparatus according to any one of Items 6 to 11. When the dots are already arranged in the pixel to which the dot arrangement is scheduled according to the priority, the gradation value determining means advances the dots to the pixel having the next highest priority after the priority and arranges the dots. The image processing apparatus according to claim 12, wherein the output gradation value of each pixel in the processing region is determined. The image processing apparatus according to any one of claims 4 to 13, wherein the flat level detecting means detects the minimum value of the pixel values in the processing region as the flat level. The FM placement information generating means uses the difference level, the first FM placement information from the second threshold matrix, and the offset-adjusted difference level and the second threshold matrix as the FM placement information. Generate FM placement information and
The gradation value determining means determines the output gradation value of each pixel in the processing area from the logical product of the AM arrangement information, the first FM arrangement information, and the second FM arrangement information. The image processing apparatus according to any one of claims 1 to 3, wherein the image processing apparatus is determined. A program for making a computer function as each means of the image processing apparatus according to any one of claims 1 to 15. It is an image processing method in an image processing apparatus that performs dither processing on image data for each predetermined processing area.
A flatness level detection step for detecting the flatness level of the processing area, and
A difference level detection step for detecting the difference level by subtracting the flatness level from each pixel value in the processing area.
An AM placement information generation step that generates AM placement information from the flatness level and the first threshold matrix,
An FM placement information generation step that generates FM placement information from the difference level and the second threshold matrix,
An image processing method comprising: a gradation value determination step of determining an output gradation value of each pixel of the processing area based on the AM arrangement information and the FM arrangement information.

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