JP7191665B2 - Image processing device, image processing method and program - Google Patents

Description

The present invention relates to an image processing apparatus, an image processing method, a program, and a storage medium that generate quantized data for forming an image on a recording medium.

A dither method is known as a quantization method when recording an image using the pseudo-gradation method. The dithering method uses a dither matrix in which thresholds associated with individual pixels are arranged, and compares these thresholds with pixel values indicated by image data to determine dot printing or non-printing for each pixel. It is a way to

Japanese Patent Application Laid-Open No. 2002-200001 discloses a dither method in which one dither matrix is commonly used while being corrected among image data corresponding to a plurality of types of color materials used in a printing apparatus. Specifically, the color image data to be processed in advance is quantized using the threshold values stored in the dither matrix as they are. Then, for image data of colors to be processed subsequently, the thresholds stored in the dither matrix are corrected based on the image data of colors processed in advance, and quantization processing is performed using the corrected thresholds. conduct.

Further, in Japanese Patent Application Laid-Open No. 2002-200010, while adopting the dither method, an upper limit is set for the number and size of dots to be printed in duplicate on the same pixel. A quantization method is disclosed that modifies the pixel value of a pixel.

According to Patent Literatures 1 and 2, it is possible to suppress the frequency of overlapping printing of dots of different colors on the same position on the printing medium, and to alleviate the poor color development due to the overlap of the dots.

U.S. Pat. No. 6,867,884 Japanese Patent Application Laid-Open No. 2014-739

However, in Japanese Patent Laid-Open No. 2004-100001, if the image data of the color to be processed in advance contains a characteristic pattern, the interference between the image data and the threshold matrix affects the dot arrangement of the color to be processed subsequently. In some cases, the graininess of the entire image is deteriorated.

Further, in Japanese Patent Laid-Open No. 2002-200022, although the frequency of overlapping dots of different colors arranged in the same pixel is suppressed, the difference generated in the pixel is distributed to the surrounding pixels, so that the dots are not placed in the surrounding pixels. The probability of arrangement increases, and as a result, the graininess may be impaired.

That is, in Patent Documents 1 and 2, although it is possible to alleviate the color development failure due to overlapping of dots, it is difficult to stably output an image in which texture and graininess are suppressed.

The present invention has been made to solve the above problems. Therefore, the objective is to provide a quantization process capable of outputting an image with reduced graininess while alleviating color development defects caused by dot overlap when printing an image using a plurality of types of color materials. to provide.

To this end, the present invention provides an image processing apparatus for determining the arrangement of dots for each of a plurality of types of colorants based on an image corresponding to each of a plurality of types of colorants, comprising: obtaining means for obtaining dot arrangement information corresponding to a coloring material for which dot arrangement has already been determined among the plurality of types of coloring materials; target value deriving means for deriving a target value for a predetermined undetermined colorant based on the image; and evaluation value deriving means for deriving an evaluation value for each pixel included in the predetermined region based on the arrangement information. and dot arrangement determination means for determining the arrangement of dots of the predetermined color material in the predetermined area based on the target value and the evaluation value , wherein the evaluation value is included in the predetermined area. and the evaluation value deriving means is a pixel having a small number of already arranged dots in the predetermined area. The evaluation value is set such that the higher the priority, the higher the priority .

According to the present invention, when an image is printed using a plurality of types of color materials, it is possible to output an image with reduced graininess while alleviating poor color development due to dot overlap.

1 is a schematic diagram of a recording device and a recording head; FIG. FIG. 3 is a block diagram for explaining a control configuration; FIG. 4 is a flowchart for explaining steps of image processing; 3 is a block diagram for explaining the software configuration in the first embodiment; FIG. 4 is a flowchart for explaining steps of quantization processing according to the first embodiment; FIG. 4 is a conceptual diagram for explaining a method of setting a processing target area; FIG. 4 is a diagram showing how quantization processing is performed in the first embodiment; FIG. 10 is a diagram showing how quantization processing according to the second embodiment is performed; FIG. 12 is a block diagram for explaining the software configuration in the third embodiment; FIG. FIG. 12 is a flowchart for explaining steps of quantization processing according to the third embodiment; FIG. FIG. 10 is a diagram showing a state of quantization processing for the first color; FIG. 10 is a diagram showing how the quantization process for the second color is performed; FIG. 10 is a diagram showing how quantization processing for a third color is performed; FIG. 10 is a diagram showing how a quantization process is performed for a fourth color; FIG. 11 is a comparison diagram for explaining the effects of the third embodiment; FIG. 11 is a comparison diagram for explaining the effects of the third embodiment; FIG. 4 is a comparison diagram for explaining the effects of the present invention;

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. Note that the configurations shown in the following embodiments are merely examples, and the present invention is not necessarily limited to the illustrated configurations.

(First embodiment)
1A and 1B are schematic diagrams of a printing apparatus 100 and a printing head 102 used in this embodiment. The printing apparatus 100 used in this embodiment is a serial type inkjet printing apparatus. In the drawing, the x direction indicates the main scanning direction of the print head, the y direction indicates the transport direction of the print medium S, and the z direction indicates the ink ejection direction.

As shown in FIG. 1B, in the print head 102, nozzle arrays for ejecting cyan (C), magenta (M), yellow (Y), and black (K) inks are arranged in parallel in the x direction. It is In each nozzle row, a plurality of nozzles 101 for ejecting ink in the z-direction according to print data are arranged in the y-direction.

As shown in FIG. 1A, the recording head 102 is mounted on a carriage 103, and the carriage 103 is movable along a carriage shaft 107 in ±x directions. While the carriage 103 moves in the ±x directions, the print head 102 ejects ink from the ejection openings 101 according to print data, thereby printing one band of an image on the print medium S. FIG. A flat platen platen 105 is arranged at a position facing the ejection port surface of the recording head 102 . The platen 105 supports the print medium S in the area to be printed by the print head 102 from behind, and keeps the distance between the ejection port surface of the print head 102 and the print medium S constant.

When the print head 102 completes the print scan for one band, the two sets of conveying rollers 104 that sandwich the print medium S rotate to convey the print medium in the +y direction by a distance corresponding to the one band. By alternately repeating the printing scan for one band by the printing head 102 and the conveying operation of the printing medium S by the conveying roller 104 as described above, an image is formed on the printing medium S step by step.

FIG. 2 is a block diagram for explaining the configuration of control in this embodiment. The image processing apparatus 200 includes a host PC or the like, and a CPU 201 executes various processes according to a program held in an HDD 203, which is a nonvolatile storage, while using a RAM 202, which is a volatile storage, as a work area. For example, the CPU 201 generates print data that can be printed by the printing apparatus 100 according to commands received from the user via the keyboard/mouse I/F 205 and programs held in the HDD 303 , and outputs the print data to the printing apparatus 100 . Information received from the recording apparatus 100 via the data transfer I/F 204 is displayed on a display (not shown) via the display I/F 206 .

On the other hand, in the recording apparatus 100, the CPU 211 executes various processes while using the RAM 212, which is a volatile storage unit, as a work area according to a program held in a ROM 213, which is a nonvolatile storage unit. Further, the printing apparatus 100 includes an image processing accelerator 216 for performing high-speed image processing and a head controller 215 for controlling the printing head 102 .

The image processing accelerator 216 is hardware capable of executing image processing faster than the CPU 211 . The image processing accelerator 216 is activated when the CPU 211 writes parameters and data necessary for image processing to a predetermined address in the RAM 212. After reading the parameters and data, the image processing accelerator 216 executes predetermined image processing on the data. However, the image processing accelerator 216 is not an essential element, and equivalent processing may be executed by the CPU 211 .

A head controller 215 supplies print data to the print head 102 and controls the print operation of the print head 102 . The head controller 215 is activated when the CPU 211 writes print data that can be printed by the print head 102 and control parameters to a predetermined address in the RAM 212, and executes an ejection operation according to the print data.

A carriage controller 217 controls movement of the carriage 103 in the ±x directions according to instructions from the CPU 211 . Also, the transport controller 218 controls the rotation of the transport roller 104 , that is, the transport of the recording medium S according to instructions from the CPU 211 .

USB, IEEE1394, LAN, or the like can be used as a connection method for the data transfer I/F 204 of the image processing apparatus 200 and the data transfer I/F 214 of the recording apparatus 100 .

FIG. 3 is a flowchart for explaining image processing steps executed by the CPU 201 of the image processing apparatus 200 when a print command is issued. This processing is executed by the CPU 201 using the RAM 202 as a work area according to the program stored in the HDD 203 .

When this process is started, the CPU 201 first acquires image data created by an application program or the like and develops it in the RAM 202 . In this embodiment, it is assumed that image data having 8-bit (256 gradation levels) pixel values indicating the brightness of each of red (R), green (G), and blue (B) is acquired for each pixel. Image data composed of pixels having pixel values of a plurality of components such as R, G, and B is hereinafter referred to as RGB data using the components.

In S<b>301 , the CPU 201 executes color correction processing on the image data developed in the RAM 202 . Color correction processing is processing for converting image data representing a standardized color space such as sRGB into image data associated with a color space printable by the printing apparatus 100 . Specifically, the CPU 201 refers to a pre-stored RGB three-dimensional lookup table, converts RGB data having 8-bit (256 gradation) pixel values to R data having 8-bit (256 gradation) pixel values. Convert to 'G'B' data.

In S302, the CPU 201 executes color conversion processing on the R'G'B' data generated in S301. The color conversion process converts R'G'B' data indicating luminance information into data indicating respective density information of K, C, M, and Y corresponding to ink colors used by the printing apparatus 100. This process is for Specifically, the CPU 201 refers to a prerecorded three-dimensional lookup table, and converts 8-bit (256 gradation) R′G′B′ data of each pixel into 8-bit (256 gradation) KCMY Convert to data. Data corresponding to each ink color that has undergone color conversion processing is collectively referred to as KCMY data.

In S303, the CPU 201 performs quantization processing on the KCMY data generated in S302. The quantization process will be described in detail later, but this quantization process converts 8-bit (256 gradation) KCMY data into 1-bit K'C'M'Y' data. In this embodiment, the K'C'M'Y' data is binary data that designates dot printing (1) or non-printing (0) for each pixel for each ink color of KCMY.

In S304, the CPU 201 outputs the K'C'M'Y' data generated in S303 to the printing apparatus 100 as print data. This completes the processing.

The CPU 211 of the printing apparatus 100 that has received the print data develops the print data in the RAM 212 . Then, while controlling the head controller 215 , the carriage controller 217 and the transport controller 218 , an image is printed on the printing medium according to the printing data developed in the RAM 212 .

FIG. 4 is a block diagram for explaining the functional configuration in the quantization processing of this embodiment. The role of each block will be briefly described below. The KCMY data generated by the ink color separation processing in S302 is input to the area selection unit 401 for each ink color. Data for each ink color that constitutes the KCMY data is hereinafter referred to as gradation data in this specification. Each gradation data has the same number of pixels, and the number of pixels in the vertical direction and the number of pixels in the horizontal direction are also the same. In FIG. 4, the first color is black (K), the second color is cyan (C), the third color is magenta (M), and the fourth color is yellow (Y).

The region selection unit 401 divides the gradation data composed of a plurality of pixels into a plurality of unit regions, and sets one unit region among them as a processing target region. At this time, the area selection unit 401 divides the gradation data of each color into a plurality of unit areas at the same position. Then, the ink color selection unit 402 is provided with gradation data of each color corresponding to the set processing target area.

The ink color selection unit 402 refers to the quantization order information 408 stored in the memory, selects a color to be processed from among the first to fourth colors, and selects a color to be processed corresponding to the selected ink color. The gradation data is provided to the target dot number setting unit 403 . The quantization order information 408 is information indicating the color order for executing the quantization process.

The target dot number setting unit 403 refers to the threshold matrix 411 stored in the memory, and based on the gradation data provided from the ink color selection unit 402, sets the target number of dots D to be arranged in the processing target area. set for each The target number of dots is the number of dots to be arranged in the processing target area. Here, each pixel is converted by quantization processing into a binary value that designates dot printing (1) or non-printing (0) for each pixel. That is, the target number of dots can be rephrased as the number of pixels to be set to a value (here, 1) indicating printing by quantization among the pixels included in the processing target area. Then, the set target dot number D is provided to the reference value acquiring unit 404 together with the gradation data corresponding to the processing target area.

The reference value acquisition unit 404 refers to the reference value information 409 stored in the memory and acquires the reference value R corresponding to the processing target area of the processing target color. In the present embodiment, the reference value information 409 is information that provisionally indicates, as a reference value, the order of priority in arranging dots for each pixel in the processing target area 701 .

The evaluation value setting unit 405 refers to the exclusion value information 410 stored in the memory, and based on the exclusion value information and the reference value R acquired by the reference value acquisition unit 404, the color included in the processing target region of the processing target color. An evaluation value Ev is set for each pixel in the Here, the exclusion value is information regarding the arrangement of dots used to control the dots so that they do not overlap. A detailed calculation method will be described later, but here, it means that it is better not to place dots on a pixel with a larger exclusive value. In the present embodiment, cumulative dot number information 410 is used as the exclusive value information including the exclusive value of each pixel included in the processing target area. The evaluation value Ev is a value indicating the final priority of dot placement.

The dot arrangement unit 406 arranges dots for each pixel included in the processing target area based on the evaluation value Ev set by the evaluation value setting unit 405 and the target dot number D determined by the target dot number determination unit. Also, the dot placement unit 406 updates the accumulated dot number information 410 based on the placement result.

A quantized data generation unit 407 generates quantized data printable by the printing apparatus 100 based on the dot arrangement data generated by the dot arrangement unit 406, and outputs the quantized data to the printing apparatus 100 as print data. The quantized data of each color referred to here corresponds to the K'C'M'Y' data described in the flow chart of FIG.

Note that in FIG. 4, the memory is shown by integrating the nonvolatile HDD 203 and the volatile RAM 202 in the image processing apparatus 200 described in FIG. Each block described above is substantially a functional unit on software executed by the CPU 201 shown in FIG.

FIG. 5 is a flowchart for explaining processing steps executed by the CPU 201 using each block shown in FIG. 4 in the quantization processing of S303. FIGS. 6 and 7(a) to (k) are diagrams for specifically explaining how the quantization process of this embodiment is performed and how pixel values of individual pixels are converted. . The quantization process performed in this embodiment will be described in detail below according to the flowchart shown in FIG. 5 with reference to FIGS. 6 and 7(a) to (k).

When this process is started, first in S500, the area selection unit 401 sets one unit area from among the image areas to be printed as a process target area.

FIG. 6 is a conceptual diagram for explaining a method of setting a processing target area. The area selection unit 401 divides an image area 600 to be printed into a plurality of unit areas 601 of the same type, and sets one unit area among them as a processing target area. In this embodiment, the image area to be printed is divided into units of 4 pixels×4 pixels, and each 4 pixels×4 pixel area is treated as a unit area 601 . Then, these plurality of unit regions 601 are sequentially set as a processing target region 701 to perform a predetermined quantization process. FIG. 6 shows a state in which the hatched unit area is set as the processing target area 701 . The order in which the processing target areas are set is not particularly limited, but in this embodiment, they are set in order from the upper left of the figure in the +x direction, and when the processing of the farthest end is completed, the order is shifted to the adjacent stage in the +y direction. and

Returning to the flow chart of FIG. When the processing target area is set in S500, the ink color selection unit 402 selects one of black (first color), cyan (second color), magenta (third color), and yellow (fourth color) in S501. Set the processing target color from . The color order to be set for the colors to be processed is stored in advance in the memory of the HDD 203 as quantization order information 408, and the ink color selection unit 402 selects a new color each time this process is performed. Set according to the color sequence shown.

In S502, the ink color selection unit 402 develops the gradation data of the processing target color of the processing target region in the memory. FIG. 7A shows an example of gradation data developed in S502. A gradation value I of 0 to 255 is associated with each of the 4×4 pixels included in the processing target area 701 .

In S503, the target dot number setting unit 403 sets the target dot number of the processing target color to be arranged in the processing target area. In this embodiment, the target dot number setting unit 403 sets the target dot number D by referring to the threshold matrix 411 stored in advance in the memory.

FIG. 7B shows an example of the threshold matrix 411 referenced in S503. The threshold matrix 411 stores thresholds Th from 0 to 254 in association with pixel positions. Although FIG. 7B shows only a pixel region of 8 pixels×8 pixels, in reality, pixel positions and threshold values Th are stored in association with a wider pixel region.

The target dot number setting unit 403 first calculates the average value Ave of the gradation values I in the process target area based on the gradation data of the process target color of the process target area. Next, the target dot number setting unit 403 reads a 4×4 (16) threshold value Th matrix (area surrounded by a thick line) corresponding to the processing target area from the threshold matrix 411, and The threshold value Th is compared with the average value Ave of the gradation value I shown in FIG. 7(a). Then, among the 16 pixels, the number of pixels whose average value Ave is larger than the threshold value is counted, and the count value is set as the target dot number D of the processing target color of the processing target region 701 .

That is, when the gradation data of the processing target area is shown in FIG. 7A, the average value of the gradation values I is calculated as Ave=1756/16=109.75. Among the 16 pixels included in the thick line in FIG. 7B, the number of pixels where the threshold value Th satisfies Ave>Th is seven. Therefore, in this case, the target dot number D of the processing target area 701 is D=7.

It should be noted that the threshold matrix 411 of the present embodiment has blue noise characteristics that emphasize the dispersibility of dots. That is, when the dots are arranged in ascending order of the thresholds 0 to 254 included in the threshold matrix 411, a dot arrangement with excellent dispersibility can be stably obtained regardless of the number of dots. In addition, it is preferable that the threshold matrix 411 has the same number of threshold values Th arranged in a predetermined area such as 16 pixels×16 pixels or 256 pixels×256 pixels. That is, for example, in an area of 256 pixels×256 pixels, it is preferable that each of 257 or 256 values from 0 to 254 is included.

Returning to the flow chart of FIG. In S504, the reference value acquisition unit 404 acquires reference value information stored in advance in the memory.

7C to 7E are diagrams showing examples of the reference value information 409. FIG. Here, since the processing target area 701 is 16 pixels, which is 4 pixels multiplied by 4 pixels, the reference value information is information in which any value from 1 to 16 is associated as a provisional priority order for each of the 16 pixels. .

Here, FIG. 7E shows reference value information in this embodiment. The reference value acquisition unit 404 reads a 4-pixel×4-pixel region corresponding to the processing target region 701 from the threshold matrix 411 shown in FIG. R is arranged to create reference value information. In this case, since the threshold matrix 411 can be used together as the reference value information, the memory capacity prepared for the reference value information can be saved. Although the reference value information is created based on the threshold matrix in this embodiment, reference value information prepared in advance may be held. For example, FIG. 7D shows the reference value information held in advance. Similar to the threshold matrix used for quantization processing, the reference value information is also of dot dispersion type. In the case of the dot distribution type, the reference value information is arranged with values indicating priority distributed. By using the threshold matrix and the dot dispersion type reference value information in this manner, dots can be arranged with high dispersion, and graininess can be suppressed. In any reference value information shown in FIGS. 7(c) to (e), the reference values R indicated by 1 to 16 indicate the provisional values of the priorities in which dots are arranged. In other words, the provisional order of dot allocation is highest for pixels with a reference value of “1” and lowest for pixels with a reference value of “16”.

Returning to the flow chart of FIG. In S505, the evaluation value setting unit 405 acquires cumulative dot number information stored in the memory. In the present embodiment, the cumulative dot number information is information in which the cumulative number of dots already arranged in the processing target area by the quantization process is stored for each pixel included in the processing target area. That is, when there is a color that has undergone quantization processing before the color to be processed, the cumulative number of dots corresponding to the other colors is held as cumulative dot number information. Therefore, when the processing target color is the first color (black), the accumulated dot number information is set to "0" for all pixels within the processing target area.

FIG. 7F shows an example of accumulated dot number information. Here, the current color to be processed is the second color (cyan), and the accumulated dot number information reflects the quantization result of the first color (black). That is, among the 16 pixels shown in FIG. It shows that no one is placed. In the present embodiment, such accumulated dot number information is used as exclusive value information to determine the dot arrangement of the color to be processed next. Hereinafter, the cumulative dot number information is referred to as exclusive value information, and the cumulative dot number of each pixel is referred to as exclusive value H.

In S506, the evaluation value setting unit 405 sets the evaluation value Ev for each pixel included in the processing target area based on the exclusion value information acquired in S505 and the reference value information acquired in S504. The evaluation value setting unit 405 sets the evaluation value Ev for the pixels included in the processing target area such that the evaluation value Ev increases for pixels with a smaller exclusion value and a smaller reference value. Note that the evaluation value Ev is a value indicating priority, so the smaller the value, the higher the priority. .

7(g) and (h) are diagrams showing procedures for setting the evaluation value Ev based on the reference value information shown in FIG. 7(e) and the exclusion value information shown in FIG. 7(f). In this embodiment, dots of different colors are arranged in the same pixel as little as possible. Therefore, when the exclusion value information is shown in FIG. 7F, the evaluation value setting unit 405 first extracts only five pixels whose exclusion value H is "0", and prints dots in these five pixels. The priority (evaluation value) is set based on the reference value information shown in FIG. 7(e). That is, the evaluation values Ev=1 to 5 are set in order of the pixel having the smallest reference value R for the five pixels having the exclusion value H of "0". FIG. 7G shows a state in which evaluation values Ev=1 to 5 are set for five pixels whose exclusion value H is "0".

Next, the CPU 201 sets evaluation values Ev=6 to 16 for the remaining pixels whose exclusion value H is “1” in ascending order of the reference value R. FIG. FIG. 7(h) shows a state in which evaluation values Ev=1 to 16 are set for all pixels.

Returning to the flow chart of FIG. In S507, the dot arrangement unit 406 arranges dots of the color to be processed in the area to be processed based on the target dot number D set in S503 and the evaluation value Ev set in S506. Specifically, the number of dots indicated by the target number of dots D is arranged in each pixel in the process target area in ascending order of the evaluation value Ev set in S506 to generate dot arrangement data. . The dot arrangement unit 406 generates dot arrangement data of the color to be processed in the area to be processed by storing 1 in pixels where dots are arranged and 0 in pixels where no dots are arranged.

FIG. 7(i) schematically shows the dot arrangement determined for the processing target area in which the evaluation value Ev is set as shown in FIG. 7(h) and the target dot number D is D=7. FIG. 4 is a diagram showing; In FIG. 7, black areas indicate pixels in which dots of the color to be processed are arranged, and white areas indicate pixels in which dots of the color to be processed are not arranged. In FIG. 7(i), it can be seen that dots are arranged in all of the pixels whose exclusive value H is "0" and some of the pixels whose exclusive value H is "1" in FIG. 7(f). FIG. 7(k) shows quantized data generated according to the dot arrangement data shown in FIG. 7(i). For each pixel in the processing target area, printing (1) is associated with pixels in which dots of the processing target color are arranged, and non-printing (0) is associated with other pixels. The quantized data indicating recording (1) or non-recording (0) of the processing target area is output to the quantized data generation unit 407 .

In S508, the dot arrangement unit 406 updates exclusion value information (accumulated dot information) based on the dot arrangement data generated in S507. That is, in S507, 1 is added to the exclusion value H corresponding to the pixel in which the dot of the processing target color is arranged, and stored in the memory as new exclusion value information (accumulated dot information).

FIG. 7(j) shows the result of updating the exclusion value information shown in FIG. 7(f) according to the dot arrangement data shown in FIG. 7(i). In FIG. 7(j), a pixel with an exclusive value H of "2" means that dots of the first color and dots of a second color are arranged together, and a pixel with an exclusive value H of "1". means that either the dots of the first color or the dots of the second color are arranged.

Returning to the flow chart of FIG. In S509, the quantized data creation unit 407 creates quantized data of the color to be processed according to the dot arrangement data created in S507. In this embodiment, the quantized data creation unit 407 accumulates quantized data for each region and creates quantized data for the entire image of the color to be processed.

In S510, the area selection unit 401 determines whether or not the quantization process for all ink colors has been completed for the area to be processed. If there are still ink colors that have not been quantized, the process returns to S501, and after setting the next color to be processed, the processes of S502 to S509 are performed for the newly set color to be processed. On the other hand, if it is determined in S510 that the quantization process for all ink colors for the processing target area has been completed, the process proceeds to S511.

In S511, the quantized data creation unit 407 determines whether or not the quantization processing of S502 to S509 has been completed for all unit areas. If there remains a unit area that has not been quantized yet, the process returns to S500, the area selection unit 401 sets the next unit area as a new processing target area, and the new processing target area undergoes steps S501 to S510. process. On the other hand, if it is determined in S511 that the quantization process has been completed for all unit areas, this process ends.

According to the series of processes described above, the smaller the value of the exclusion value H in the processing target area, that is, the smaller the number of dots arranged in the previously processed ink color, the smaller the evaluation value for that pixel. Ev is set. Then, the smaller the evaluation value Ev of the pixel, the more preferentially the dots of the color to be processed are arranged in the region to be processed. As a result, the number of dots arranged in one pixel in the processing target area can be reduced, and graininess can be suppressed while alleviating poor color development caused by overlapping dots on the recording medium.

Further, since the dots are rearranged in a unit area composed of a plurality of pixels, which is the processing target area, interference between the image data and the threshold matrix can be suppressed. Furthermore, in each unit area, the number of dots indicated by the gradation data (the target number of dots) for each ink color is allocated. is stored in the unit area and can be reproduced on the recording medium.

That is, according to the present embodiment, when an image is printed using a plurality of types of color materials, it is possible to output an image in which graininess is suppressed while alleviating poor color development due to dot overlap. Become.

In this embodiment, the quantization process is performed using a threshold matrix having blue noise characteristics and dot dispersion type reference value information. On the other hand, there is also a dot concentration type in which dots are concentrated as much as possible in quantization processing, but the present embodiment can also be applied to the dot concentration type. FIG. 7(c) shows reference value information of the dot concentration type. In the case of the dot concentration type, since multiple dots are printed in clusters on the recording medium, the output density is less likely to be affected by misalignment between dots, and the gradation value of the pixel and the output density are kept linear. be able to.

(Second embodiment)
Also in this embodiment, the image processing apparatus described with reference to FIGS. 1 to 4 is used, and quantization processing is performed according to the flowchart shown in FIG. In this embodiment, the contents of the exclusion value information (accumulated dot information) are made different from those in the first embodiment. In the first embodiment, "1" is added to the pre-update exclusion value H regardless of the ink color of the color to be processed in the pixels for which dots of the color to be processed have been determined to be placed. On the other hand, in the present embodiment, a different weighting factor wi is prepared for each ink color, and the weighting factor wi of the color to be processed is added to the exclusion value H before updating.

In this embodiment, the weighting factor for each ink color is set based on the ratio of the optical densities of the dots printed on the printing medium. Here, it is assumed that an optical density ratio of 4:2:2:1 is obtained in a combination of black, cyan, magenta, and yellow. The weighting factor for yellow is "2", and the weighting factor for yellow is "1".

8A to 8H are diagrams showing the relationship between dot arrangement data and exclusive value information in this embodiment. The quantization processing in this embodiment will be described below according to the flowchart of FIG. 5 with reference to FIGS. 8(a) to 8(h). Also in this embodiment, in S501, the colors to be processed are set in the order of black, cyan, magenta, and yellow.

FIG. 8A shows an example of dot arrangement data set in S507 when the color to be processed is the first color (black). Also, FIG. 8B shows the exclusion value information updated in S508 in this case. In FIG. 8(b), the black weighting factor "4" is added to the initial value "0" only for pixels where dots are arranged in FIG. 8(a).

FIG. 8C shows an example of dot arrangement data set in S507 when the color to be processed is the second color (cyan). Since the cyan dots are arranged based on the exclusive value information shown in FIG. 8B, the cyan dot arrangement data and the black dot arrangement data shown in FIG. 8A are generally in an exclusive relationship. .

FIG. 8D shows the exclusive value information updated in S508 when the color to be processed is cyan. In FIG. 8(d), a cyan weighting factor of "2" is added to the exclusion value H shown in FIG. 8(b) only for the pixels for which cyan dots are arranged in FIG. 8(c).

A case where the processing target color is set to the third color (magenta) in such a state will be described below. Here, it is assumed that the target dot number is set to D=7 in S503, and the reference value information shown in FIG. 8E is obtained in S504.

In this case, in S506, the CPU 201 refers to the exclusion value information shown in FIG. 8(d) and rearranges the reference values R shown in FIG. 8(e). Specifically, first, evaluation values Ev=1 to 3 are set for the three pixels whose exclusive value H is “0” in order of the pixel whose reference value R is small. Next, evaluation values Ev=4 to 10 are set for the seven pixels with the exclusion value of "2" in order of the pixel with the smallest reference value R. FIG. Furthermore, evaluation values Ev=11 to 15 are set in ascending order of the reference value R for the four pixels with the cumulative dot number of "4". Finally, an evaluation value Ev=16 is set for one pixel with an accumulated dot number of "6". FIG. 8(f) shows evaluation value information obtained by setting the evaluation value Ev of each pixel in this manner.

In S507, the CPU 201 arranges the seven dots indicated by the target dot number D in the processing target area one by one in ascending order of the evaluation value Ev set in S506, thereby generating dot arrangement data. FIG. 8(g) shows dot arrangement data generated in this way. In the exclusion value information shown in FIG. 8D, it can be seen that magenta dots are preferentially arranged for pixels with a small exclusion value H. FIG.

I will explain in more detail. When comparing FIGS. 8A, 8C, and 8G showing black, cyan, and magenta dot arrangement data, magenta dots are preferentially arranged in pixels in which neither black dots nor cyan dots are arranged, Next, it is arranged in pixels where only cyan dots are arranged. This is because the weighting factor "4" for black is greater than the weighting factor "2" for cyan, so the evaluation value Ev of a pixel on which a black dot is arranged is greater than the evaluation value Ev of a pixel on which a cyan dot is arranged. This is because the priority of arranging magenta dots is lowered. The order of priority for arranging magenta dots is as follows: pixels with no dots, pixels with only cyan dots, pixels with only black dots, and pixels with both black and cyan dots. , lower in the order of

In S508, the CPU 201 updates the exclusion value information based on the dot arrangement data generated in S507. That is, in S507, the weighting coefficient w2=2 for magenta is added to the exclusion value H of the pixel in which the dot of the processing target color is arranged, and the result is stored in the memory as new exclusion value information. FIG. 8(h) shows the exclusion value information updated in S508. A magenta weighting factor w2=2 is added to the exclusive value shown in FIG. 8(d) only for pixels where dots are arranged in FIG. 8(g). The exclusive value information shown in FIG. 8(g) is subsequently used when the CPU 201 generates dot arrangement data for the fourth color (yellow).

On a recording medium, overlapping dots formed by overlapping a plurality of ink colors are more conspicuous than single dots formed by one ink color, and their coloring properties are lowered. However, even for the same overlapping dots, the conspicuousness and coloring of the dots differ depending on the combination of ink colors forming the overlapping dots. For example, overlapping dots formed with cyan and yellow are less conspicuous than overlapping dots formed with black and cyan, and have less effect on graininess. If the exclusion value H is managed using a different weighting factor for each ink color as in the present embodiment, and dots of the color to be processed are preferentially arranged in pixels with a small exclusion value H, overlapping dots will have to be printed. Even under such circumstances, it is possible to suppress graininess while suppressing deterioration of color developability as much as possible.

In the above description, the weighting factor for each color is set based on the ratio of the optical densities of the dots printed on the printing medium, but the present embodiment is not limited to this. For example, saturation reduction when dots are overlapped with special color inks such as green, orange, and purple has a greater effect on the image than saturation reduction when dots are overlapped with the four-color inks described above. Therefore, in such a case, it is preferable to set the weighting factor for each ink color based not only on optical density but also on other factors such as hue and saturation.

On the other hand, for certain ink color combinations, such as black and gray or cyan and light cyan, overlapping dots do not significantly affect hue and saturation. Therefore, in such a case, a plurality of pieces of exclusive value information may be prepared in association with ink colors, and the weighting factor used when updating the exclusive value H may be changed for each piece of exclusive value information. Then, when each ink color becomes a color to be processed, in S505, one corresponding exclusive value information may be acquired from a plurality of pieces of exclusive value information.

For example, the weighting factor for black in the exclusive value information for gray is preferably set smaller than the weighting factor for black in the exclusive value information for cyan. Also, the cyan weighting factor in the exclusive value information for light cyan is preferably set smaller than the cyan weighting factor in the exclusive value information for magenta. In this way, overlap between ink colors with similar hues is prioritized over overlap between ink colors with different hues, and the color development of the entire image can be improved.

Furthermore, the weighting factor as described above may be changed for each processing target area. For example, when the hue of the processing target area indicated by the R'G'B' data obtained in S301 of FIG. 3 is close to G (green), it may be preferable to prioritize the overlap of cyan and yellow dots. In such a case, in the exclusive value information for yellow, the weighting factor for cyan should be set smaller than the weighting factor for black or magenta.

According to the present embodiment described above, when an image is printed using a plurality of color materials, it is possible to output an image in which graininess is suppressed while alleviating poor color development due to overlapping of dots. Become.

(Third embodiment)
Also in this embodiment, a series of image processing is performed according to the flowchart shown in FIG. 3 using the image processing apparatus explained in FIGS.

FIG. 9 is a block diagram for explaining the software configuration in the quantization processing of this embodiment. The difference from FIG. 4 is that the evaluation value setting unit 901 uses the threshold matrix 411 as reference value information, and dot arrangement history information 903 is prepared instead of the cumulative dot number information 410 . Here, the dot arrangement history information 903 is information in which dot arrangement information for ink colors for which dot arrangement data has already been generated is stored in association with ink colors. The evaluation value setting unit 901 of this embodiment uses the threshold matrix 411 and the dot arrangement history information 903 to set the evaluation value Ev for each pixel. Also, the dot placement unit 902 of this embodiment updates the dot placement history information 903 based on the set dot placement data.

FIG. 10 is a flowchart for explaining the processing steps executed by the CPU 201 of this embodiment using the blocks shown in FIG. 9 in the quantization processing of S303. FIGS. 11A to 11J are diagrams for specifically explaining the quantization process when the color to be processed is the first color (black). The quantization process for the first color (black) will be described below according to the flowchart shown in FIG. 10 with reference to FIGS. 11(a) to (j).

In S1002, the CPU 201 develops the black gradation data of the processing target area in the memory. Here, it is assumed that the gradation data shown in FIG. 11(a) is expanded. In S1003, the CPU 201 sets the target number D of black dots to be arranged in the processing target area. Specifically, first, 4×4 thresholds corresponding to the processing target area are read out from the threshold matrix 411, and each threshold is set to the average value Ave of the gradation values of the gradation data shown in FIG. Compare with Then, among the 16 pixels, the number of pixels whose average value Ave is larger than the threshold value is counted, and the count value is set as the target dot number D. FIG. When the gradation data is shown in FIG. 11A, the average value Ave is Ave=128. Also, assuming that the 4×4 threshold value read in association with the processing target area is shown in FIG. 11C, the target dot number is set to D=8.

In S1004, the CPU 201 acquires and develops reference value information stored in the memory in advance. Here, the 4×4 threshold shown in FIG. 11C, which was also used in S1003, is used as the reference value information.

In S1005, the CPU 201 acquires the dot placement history information 903 (see FIG. 9) saved in the memory. In this embodiment, the dot arrangement history information is dot arrangement data for each ink color for which quantization processing has been completed in the processing target area. When the color to be processed is the first color (black), the CPU 201 acquires null data. When the color to be processed is the second color (cyan), the CPU 201 acquires black dot arrangement history information. When the color to be processed is the third color (magenta), the CPU 201 acquires black dot placement history information and cyan dot placement history information.

In S1006, the CPU 201 derives and sets an evaluation value Ev for each pixel included in the processing target area based on the dot arrangement history information acquired in S1005 and the reference value information (FIG. 11C) acquired in S1004. do. The evaluation value derivation method in this embodiment will be described in detail below.

To obtain the evaluation value Ev, the CPU 201 of this embodiment first obtains a first evaluation value Ev1 for each pixel included in the processing target area. The first evaluation value Ev1 is obtained according to (Equation 1).
E1 = (I/Imax) - (R/Rmax) - (H/Hmax) (Equation 1)

Here, I is the gradation value of the pixel of interest indicated by the gradation data, and Imax indicates its maximum value. In this embodiment, since the gradation data is 8 bits (256 gradations), Imax=255. For example, when the gradation data is 16 bits (65536 gradations), Imax=65535. FIG. 11(b) shows normalized values (I/Imax) for 4×4 pixels corresponding to the region to be processed. (I/Imax) is a real number with a value between 0 and 1, meaning that pixels with larger values are more likely to have dots placed thereon.

R is the reference value of each pixel indicated by the reference value information, and Rmax indicates its maximum value. In the case of this embodiment, since the reference value information is obtained from the threshold matrix 411, Rmax=254. FIG. 11(d) shows the normalized value (R/Rmax) of the reference value information. (R/Rmax) is a real number with a value between 0 and 1, meaning that a pixel with a larger value has a lower provisional priority for arranging dots.

H is an exclusive value obtained from dot placement history information. In this embodiment, the exclusion value Hn can be obtained using (Equation 2).
H=Σ(wi×Di) (Formula 2)

Here, i is a number indicating the processing order of the color to be processed. In this embodiment, quantization processing is performed in the order of black, cyan, magenta, and yellow. Therefore, i=1 when the ink to be treated is black, i=2 when it is cyan, and i=3 when it is magenta. , i=4 for yellow. Also, wi indicates a weighting factor for each ink color. In this embodiment, the weighting coefficients for each ink color are set based on the ratio of the optical densities of the dots printed on the printing medium, w1=4, w2=2, w3=2, and w4=1.

Also, in (Equation 2), Di indicates the number of dots of the ink color i arranged in each pixel. For example, when one black dot is arranged in a certain pixel, D1=1 in that pixel. Furthermore, Σ indicates the sum of the 1st to (i−1)ths.

Thus, the exclusive value H is obtained by multiplying the number of dots Di allocated to each pixel by a weighting factor wi unique to the ink color i, and multiplying this by the weighting factor wi specific to the color to be processed. It is calculated by adding about When the color to be processed is the first color (black), that is, when i=1, the exclusive value H is H=0 for all pixels. Therefore, the exclusive value information of 4 pixels×4 pixels obtained using (Formula 2) is as shown in FIG. 11(e).

On the other hand, in (Formula 1), Hmax indicates the maximum value of the exclusion value H in the processing target area. In this embodiment, a value obtained by normalizing the exclusion value H by the maximum value Hmax (H/Hmax) is referred to as an exclusion control value Hn. The exclusive control value Hn is a real number having a value between 0 and 1, and indicates that dots are more difficult to be arranged in pixels with a larger value. When the color to be processed is the first color (black), the maximum value is Hmax=0, but the exclusive control value Hn is fixed at zero. FIG. 11F shows exclusive control value information in the processing target area when the processing target color is the first color (black).

FIG. 11(g) shows normalized gradation data (I/Imax) in FIG. 11(b), normalized reference value information (R/Rmax) in FIG. 11(d), and FIG. It is a figure which shows the 1st evaluation value information calculated according to (Formula 1) based on the exclusive control value information (Hn) of f). In this embodiment, the first evaluation value Ev1 is a value that indicates the ease with which dots are arranged for each pixel. A pixel having a larger positive value indicates that dots are more likely to be placed, and a pixel having a larger negative value indicates that dots are less likely to be placed.

Next, the CPU 201 sets a value of 1 to 16 as the second evaluation value Ev2 in the order of pixels having the highest first evaluation value Ev1 in the processing target area. At this time, when there are a plurality of pixels having the same first evaluation value Ev1, priority is given to a pixel having a larger gradation value I, a pixel located in the upper left of the processing target area is given priority, or a random You can set it to .

FIG. 11(h) shows evaluation value information in which second evaluation values Ev2=1 to 16 are set for individual pixels included in the region to be processed. Hereinafter, the second evaluation value Ev2 is simply referred to as the evaluation value Ev in this embodiment.

Returning to the flow chart of FIG. In S1007, the CPU 201 arranges dots of the processing target color in each pixel included in the processing target region based on the target dot number D set in S1003 and the evaluation value Ev set in S1006. Specifically, the number of dots indicated by the target dot number D is arranged in the processing target area one by one in ascending order of the evaluation value Ev set in S1006 to generate dot arrangement data.

FIG. 11I shows the dot arrangement data generated in the dot arrangement determination process of S1007 based on the evaluation value information of FIG. FIG. 10 shows. In the figure, black areas indicate pixels in which black dots are arranged, and white areas indicate pixels in which black dots are not arranged.

In S1008, the CPU 201 updates the dot placement history information based on the dot placement data generated in S1007. Specifically, the dot arrangement data generated in S1007 is associated with the ink color i and stored in the memory. In this example, the dot arrangement data shown in FIG. 11(i) is associated with the first color (black) and stored in the memory. The saved dot arrangement history information is read from the memory as the dot arrangement history information for the first color (black) in S1005 after the post-processing target color is changed.

In S1009, the CPU 201 generates quantized data according to the dot arrangement data generated in S1007, and stores it in the memory. FIG. 11(j) shows quantized data generated according to the dot arrangement data shown in FIG. 11(i).

When the processing for black described above is completed, the CPU 201 next returns to S1001, sets the second color (cyan) as the color to be processed, and performs the processing of S1002 to S1009.

FIGS. 12A to 12J are diagrams similar to FIGS. 11A to 11J showing the state of processing when the color to be processed is the second color (cyan). FIG. 12A is the cyan tone data developed in S1002. In this case, the normalized value (I/Imax) of the gradation data is as shown in FIG. 12(b).

FIG. 12C shows 4×4 threshold values corresponding to the processing target area read out from the cyan threshold matrix. is set to 12(c) is also used as the reference value information R, and the normalized value (R/Rmax) of the reference value information R is as shown in FIG. 12(d).

FIG. 12E shows exclusive value information derived when the color to be processed is cyan. The exclusive value information is derived by referring to the dot arrangement history information of the first color (black) (the same information as the quantized data in FIG. 11(i)) and obtaining the exclusive value H of each pixel according to (Equation 2). be. In the case of the exclusion value information shown in FIG. 12(e), the maximum value of the exclusion value H is Hmax=4, and the exclusion control value information obtained by normalizing the exclusion value H of each pixel is as shown in FIG. 12(f). become.

FIG. 12(g) shows normalized gradation data (I/Imax) shown in FIG. 12(b) and normalized reference value information (R/Rmax) shown in FIG. 12(d). FIG. 10 is a diagram showing first evaluation value information derived according to (Equation 1) based on the exclusive value control value information shown in FIG. (f); FIG. 12(h) is evaluation value information for cyan derived based on the exclusive value control value information of FIG. 12(g). Further, FIG. 12(i) shows dot arrangement data set for a processing target area in which the evaluation value Ev is set as shown in FIG. 12(h) and the target dot number D is D=4. , and FIG. 12(j) are the quantized data based on FIG. 12(i). It can be seen that the dot arrangement data shown in FIG. 12(i) has dots arranged in exclusive positions with respect to the dot arrangement data for the first color (black) shown in FIG. 11(i).

After that, according to the flow described above, the third color (magenta) processing and the fourth color (yellow) processing are performed on the processing target area in this order. Since the specific processing is the same as that for the first color (black) and the second color (cyan), the description is omitted here. FIGS. 13A to 13J are diagrams similar to FIGS. 11A to 11J showing the processing when the color to be processed is the third color (magenta). FIGS. 14A to 14J are diagrams similar to FIGS. 11A to 11J showing the processing when the color to be processed is the fourth color (yellow).

According to the present embodiment described above, for each pixel, the priority for arranging dots is determined based on (Equation 1) composed of three parameters: the tone value I, the reference value R, and the exclusion value H. The evaluation value Ev indicated is obtained. Therefore, compared to the above-described embodiment in which the evaluation value Ev is set using two parameters, the reference value R and the exclusion value H, the gradation value I of each pixel can be more positively reflected as a result of the quantization of that pixel. can be done.

For example, when printing a photographic image, the uniformity of the entire image is emphasized rather than the gradation value of each pixel, and dots are overlapped within the unit area as in the first and second embodiments. It is preferable to disperse However, when printing a graphic image including characters and line drawings, it is preferable to reflect the gradation value of each pixel in the presence or absence of dots in that pixel as much as possible in order to emphasize the sharpness and contrast of the object. There is also As in the present embodiment, if dots are arranged based on (Equation 1) containing three parameters, the tone value I, the reference value R, and the exclusion value H, the tone value of each pixel is It is possible to suppress overlapping of dots of different colors while reflecting the presence or absence of dots. As a result, it becomes possible to output an image excellent in sharpness and color development.

At this time, the weighting coefficients Wp and Wh having a value of 1 to 0 are set for acting to encourage dot placement (I/Imax) and acting to suppress dot placement (H/Hmax). The balance between sharpness and coloring may be adjusted by multiplying each. In this case, for example, by increasing the weighting factor Wp, it is possible to output an image in which sharpness and contrast are maintained. By increasing the weighting factor Wh, it is possible to increase the exclusion rate of different-color dots and output an image with excellent coloring.

Furthermore, by preparing a weighting factor Wr for the normalized value (R/Rmax) of the reference value R and increasing the value, it is possible to strengthen the influence of the arrangement order defined by the threshold matrix. For example, if a threshold matrix having blue noise characteristics with excellent dispersibility is prepared, a dot arrangement with excellent dispersibility can be obtained. Such weighting factors Wp, Wr, and Wh may be appropriately changed according to the ink color, print mode, type of recording medium, and the like.

According to this embodiment, since the evaluation value Ev is derived based on the gradation value I in addition to the reference value R and the exclusion value H, the frequency at which dots of different colorants are overlapped is the first. , is higher than in the second embodiment. This is a factor that increases graininess compared to the first and second embodiments, but on the other hand, it increases the resistance (robustness) of image quality to various errors and variations of the printing apparatus. effect is also obtained. A brief description will be given below.

For example, even if the dot arrangement data is generated so that the dots of the first ink and the dots of the second ink are mutually exclusive, the print position misalignment between the print head of the first ink and the print head of the second ink may occur. When this happens, some overlapping dots are produced on the recording medium. When the amount of such print position deviation fluctuates due to reciprocating scanning of the print head or cockling of the print medium, the number of overlapping dots to be generated also fluctuates in accordance with the amount of print position shift. It is perceived as density unevenness or color unevenness.

On the other hand, if overlapping dots of different color materials are generated in advance at a predetermined ratio as in the present embodiment, two dots that should be printed at different positions may overlap. There are also places where two dots are separated. Therefore, even if the print position shift amount fluctuates due to reciprocating scanning of the print head or cockling of the print medium, the ratio of overlapping dots is kept within a certain range, and unevenness in density and color on the print medium can be detected. It becomes difficult to be The image robustness against such recording position shift can be adjusted by changing the weighting factor Wh described above.

Hereinafter, the effect of this embodiment will be described with specific examples. For example, if 8 dots are arranged according to the reference value information shown in FIG. 11(c) without obtaining the evaluation value Ev as in this embodiment, the dot arrangement data will be as shown in FIG. 15(a). . Comparing FIG. 15A with FIG. 11I, which is the dot arrangement data of the present embodiment, FIG. It can be seen that the characteristics of the gradation data shown in FIG. 11(a) also appear. That is, according to this embodiment, by including the gradation value I in (Equation 1) when obtaining the evaluation value Ev, the feature of the gradation data can be more strongly reflected in the result of quantization.

Next, the effect of including the exclusion value H in (Equation 1) when deriving the evaluation value Ev will be described. For example, without providing the term of the exclusion value H in (Formula 1), the first evaluation is performed according to the second color (cyan) tone data shown in FIG. 12(a) and the reference value information shown in FIG. When the value information is obtained, it becomes as shown in FIG. 16(a). When four dots are arranged according to this evaluation value information, the dot arrangement data becomes as shown in FIG. 16(b).

On the other hand, when four dots are arranged as they are according to the reference value information shown in FIG. 12(c), the dot arrangement data becomes as shown in FIG. 16(b). Comparing FIG. 16B with FIG. 11I, which is the dot arrangement data of the present embodiment, FIG. is inferior to that in FIG. 11(i).

That is, according to the present embodiment, by including the exclusive value H in (Equation 1) when deriving the evaluation value Ev, the dots of the ink for which the quantization processing has already been completed in the dot arrangement data of the processing target color Exclusivity for placement data can be enhanced.

Note that in (Formula 1), the first evaluation value Ev1 is obtained by performing addition and subtraction on values obtained by normalizing each of the gradation value I, the reference value R, and the exclusive value H in the range of 0 to 1. , such normalization is not essential. The gradation value I, the reference value R, and the exclusive value H may be arranged in the same range. may be

By the way, as in the present embodiment, when the area corresponding to the processing target area in the threshold matrix is used as the reference value information, the contents of the reference value information are different for each unit area. Here, consider a case where the target dot number D is not set as in the present embodiment, but the reference value information is used as it is as a dither matrix to perform dither processing on the gradation data shown in FIG. 11(a). In this case, the dot arrangement data shown in FIG. 11(i) can be obtained from the reference value information shown in FIG. 11(c). Placement data is obtained. That is, even with the dot arrangement data quantized according to the same gradation data shown in FIG. 11A, 8 dots are arranged in one unit area and 11 dots are arranged in the other unit area. will be As a result, there is concern that density unevenness and color unevenness may occur on the recording medium.

On the other hand, if the average value of the gradation values included in the unit area is obtained as in this embodiment and the number of dots to be arranged in the unit area is set based on this average value, the contents of the reference value information will change. Even so, the variation in the number of dots arranged can be stabilized to some extent between unit areas. 15(d) and (e) show the first evaluation when the processing of the present embodiment is executed based on the gradation data of FIG. 11(a) using the reference value information shown in FIG. 15(b). Value information and dot placement data are shown, respectively. In FIG. 15(e), nine dots are arranged, and approximately the same density as in FIG. 11(i), in which eight dots are arranged, can be expressed.

As shown in FIGS. 11 to 14, in this embodiment, different threshold matrices are prepared for the four ink colors to obtain the target dot number D and the evaluation value Ev. good too. In this case, the exclusive value information, which is the sum of the processed ink color dot arrangement data, becomes information according to the threshold matrix common to all colors, and as a result, the total sum of all ink colors has dispersibility according to the threshold matrix. can be obtained. Therefore, when emphasis is placed on increasing the dispersibility, for example, one threshold matrix having blue noise characteristics may be prepared, and this threshold matrix may be used as common reference value information for each color. In this way, it is possible to output a smooth image having high dispersibility as the sum of all ink colors.

However, this embodiment does not necessarily have to use the threshold matrix as the reference value information. The reference value information may be prepared separately from the threshold matrix as integers from 1 to 16 (or 0 to 15) as in the first embodiment. In this case, the maximum value used for normalizing the reference value is Rmax=16 (or 15).

FIGS. 17A to 17C are diagrams for comparing the case of performing quantization processing by the method of Patent Document 1 with the present invention. Similar to FIG. 12A, FIG. 17A shows tone data of the second color (cyan) corresponding to the processing target area. On the other hand, FIG. 17B shows a threshold matrix for cyan data obtained by shift processing according to the method of Patent Document 1. FIG.

FIG. 17(b) subtracts each gradation value I indicated by the gradation data of FIG. 11(a) from each threshold of the matrix shown in FIG. 11(c), and subtracts 255 only when the value becomes negative. It is a matrix obtained by addition. Using the threshold matrix of FIG. 17(b) thus obtained and dithering according to the tone data of FIG. 17(a), dot arrangement data as shown in FIG. 17(c) is obtained. . Comparing the dot arrangement data shown in FIG. 17(c) with the dot arrangement data for the second color (cyan) of the present embodiment shown in FIG. 12(i), FIG. It can be seen that exclusivity with respect to the dot arrangement data of the first color shown is inferior to that in FIG. 12(i).

In the method of Patent Document 1, when the gradation values of the first color to be quantized first are uniform and the gradation values of the second color to be subsequently quantized are also uniform, The dots of the first color and the dots of the second color can be arranged completely exclusively. However, in an actual image, the gradation value of each pixel often varies to some extent as shown in FIG. 11(a). It is difficult to record under a relationship.

On the other hand, in this embodiment, if the sum of the number of dots of the first color and the second color does not exceed the number of pixels (16) in the unit area, As shown, the dots of the first color and the dots of the second color can be arranged in a completely exclusive relationship within the unit area. Then, such an exclusive relationship can be maintained in any unit area. That is, according to the present embodiment, it is possible to further suppress the frequency of overlapping printing of dots of different colors on the same position on the printing medium as compared with Japanese Patent Application Laid-Open No. 2002-200014, and to output an image with even higher color development. becomes.

11 and 12 used in the present embodiment are compared with FIGS. 17(a) to 17(c) of Patent Document 1 to explain the effects of the present embodiment. The same can be obtained in the first and second embodiments.
(Other embodiments)
In the above embodiment, the case of converting the gradation value I into binary data of recording (1) or non-recording (0) has been described. can also be applied. When the quantization value is three or more, the quantization value of each pixel can be expressed by adjusting the number and size of dots recorded in the area on the recording medium corresponding to one pixel.

For example, a case where the quantization value is three values will be described in detail. In this case, Imax is first adjusted to obtain a normalized value (I/Imax) between 0 and 2 from the gradation value I. Then, for a pixel with a normalized value (I/Imax) greater than 1, 1 dot is first placed on the pixel. Further, 1 is subtracted from the normalized value (I/Imax) of the pixel, and the obtained value is used as the new normalized value of the pixel.

After that, based on the new normalization value, the evaluation value Ev is obtained by the method described in the above embodiment together with the pixels to which dots have not yet been placed, and dots are placed again. As a result, the quantization value of the pixel on which two dots are arranged is set to "2", and two dots are printed in the corresponding area on the print medium. Also, the quantization value of the pixel where one dot is arranged is set to "1", and one dot is printed in the corresponding area on the printing medium. Furthermore, the quantization value of pixels for which dots are not arranged is set to "0", and no dot is printed in the corresponding area on the printing medium. In this case, the accumulated dot information of the first and second embodiments and the dot arrangement history information of the third embodiment may store the quantized values from 0 to 2 as they are.

As a method of printing two dots in an area corresponding to one pixel, for example, in the print head 102 shown in FIG. It suffices to prepare two columns each in the x direction. Also, a multi-pass printing method may be employed in which the print head 102 performs multiple print scans on the same image area of the print medium. Furthermore, by applying index expansion processing to the quantized values of three or more values, binary data with higher resolution may be generated and used as the final recording data.

When quantized values of three or more values are represented by dot size instead of the number of dots, pixels with a quantized value of "2" are assigned large dots, and pixels with a quantized value of "1" are assigned a large dot. A small dot should be placed in each pixel, and no dot should be placed in a pixel whose quantization value is "0". In this case, the target number of dots set in S503 of FIG. 5 or S1003 of FIG. A target value corresponding to the amount of ink is obtained. A target dot number setting unit 403 in FIG. 4 serves as a target value deriving unit for deriving and setting such a target value.

In the above description, the target dot number D in the processing target area is set by comparing the average value of the gradation values in the gradation data with the threshold values of the threshold matrix, but the method for setting the target dot number is not limited to this. For example, the target dot number D can be derived according to (Equation 3) using the sum SUM of the plurality of tone values I corresponding to the processing target area and a randomly set natural number N (1≦N≦255). can.
D=0 when SUM<N
When SUM≧N D=INT((SUM-N)/255)+1 (Formula 3)

Here, INT(x) is a function that returns a natural number not exceeding x. For example, in the gradation data of FIG. 7A shown in the first embodiment, SUM=1756. Here, if N=128 and the target number of dots is calculated using (Equation 3), SUM≧N, so
D=((1756-128)/255)+1=7
becomes. Although N is set to the central value 128 of the gradation value I here, it is preferable that N is set randomly to such an extent that it does not deviate greatly from such a central value. In this way, the target dot count of the processing target area can be obtained without storing a large size threshold matrix in the memory.

In the above, according to the color order stored in the quantization order information 408, the first color is black, the second color is cyan, the third color is magenta, and the fourth color is yellow. It is not limited to such a form. However, the later the ink color is, the more difficult it is for the reference value information to be reflected in the evaluation value information as compared to the exclusion value, so the dispersibility tends to be impaired. For this reason, in the present embodiment, processing is performed in order of the optical density of the dots printed on the printing medium, and yellow, which has the lowest optical density, is used as the fourth color, the dispersibility of which is most likely to be impaired.

However, the color order for processing may be changed for each unit area. In particular, when distributed reference value information is used, if all unit areas are processed in the same color order, repeated patterns and textures may be visually recognized. In such a case, by switching the color order of processing for each unit area, it is possible to make these patterns and textures inconspicuous in the entire image. At this time, for example, the visually inconspicuous yellow may be fixed as the fourth color, and the first to third colors may be sequentially switched among black, cyan, and magenta.

Furthermore, the color order of the above processing may be set based on the image data. For example, when the hue of the processing target area indicated by the R'G'B' data obtained in S301 of FIG. 3 is close to G (green), the first color is cyan, the second color is yellow, and the third color may be black, and the fourth color may be magenta.

Further, in the flowcharts described with reference to FIGS. 5 and 10, after setting the processing target area, the processing target color is set for the set processing target area, but these setting orders may be reversed. That is, first, a color to be processed may be set, quantization processing for the set color to be processed may be performed on all unit areas, and then the color to be processed may be switched to the next ink color. However, in this case, it is necessary to prepare a memory area for storing cumulative dot number information and dot arrangement history information for all unit areas. Therefore, it can be said that the order according to the flow charts described in FIGS. 5 and 10 is preferable in that such a memory area can be saved.

In the above description, the reference value information for the processing target area is acquired based on the threshold matrix and reference value information stored in advance in the memory. It may be generated based on gradation data or the like. For example, gradation data corresponding to the region to be processed may be referred to, and reference values from 1 to 16 may be assigned to pixels in descending order of gradation values. Further, after applying predetermined edge processing or filtering processing to the gradation data, reference values 1 to 16 may be assigned to the pixels in descending order of the gradation value. In this manner, the reference value R is generated according to parameters such as image features (edges and ink amount), and then exclusion processing is performed based on this reference value R. This makes it possible to alleviate poor color development due to overlapping of dots while reproducing the same.

In the above, referring to FIG. 6, 4 pixels×4 pixels are associated with the unit area, but the unit area of the present invention is of course not limited to such size and shape. The unit area may have a larger size, and the number of pixels in the x direction and the number of pixels in the y direction may not be equal. The smaller the number of pixels included in the unit area, the smaller the amount of memory used for processing, and the easier it is for the gradation value I and thus the characteristics of the image to appear in the output image. On the other hand, as the number of pixels corresponding to the unit area is increased, exclusivity within the unit area and accompanying color development can be improved, and graininess can be reduced.

However, when the size of the unit area is n pixels×m pixels, the number of pixels in the x direction and the y direction of the image data is preferably a multiple of n and m. Therefore, if such a condition is not satisfied, white data is added to predetermined regions at the ends of the image data in the x and y directions so that the number of pixels in the x and y directions is a multiple of n and m. is preferably accompanied by

In the above description, all the image processing shown in FIG. 3 is performed by the image processing apparatus (host PC) 200 shown in FIG. 2, but the present invention is of course not limited to such a form. Part of the image processing shown in FIG. 3 may be performed by the host PC and the rest of the processing may be performed by the printing apparatus 100, or the printing apparatus 100 may perform all the processing. Further, the host PC may perform the steps up to the middle of the quantization processing described with reference to FIGS. 5 and 10, and the remaining steps may be performed by the printing apparatus. In any case, when the device for performing the quantization processing of the present invention is the image processing device of the present invention, and the quantization processing is performed in cooperation with the host device and the printing device, the entire print system can be the image processing device of the present invention. become a device.

The present invention supplies a program that implements one or more functions of the above-described embodiments to a system or device via a network or a 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 processing to It can also be implemented by a circuit (for example, ASIC) that implements one or more functions.

200 image processing device 403 target dot number setting unit 405 evaluation value setting unit 406 dot arrangement unit

Claims (22)

An image processing device for determining dot arrangement for each of the plurality of types of colorants based on images corresponding to each of the plurality of types of colorants,
Acquisition means for acquiring, with respect to a predetermined region of the image, dot arrangement information corresponding to one of the plurality of types of coloring materials whose dot arrangement has already been determined;
target value deriving means for deriving, based on the image, a target value for a predetermined colorant for which dot arrangement has not been determined among the plurality of types of colorants for the predetermined region;
evaluation value derivation means for deriving an evaluation value of each pixel included in the predetermined area based on the arrangement information;
dot arrangement determination means for determining the arrangement of dots of the predetermined color material in the predetermined area based on the target value and the evaluation value;
has
The evaluation value is a value indicating a priority for arranging dots of the predetermined color material for each of a plurality of pixels included in the predetermined area;
The image processing apparatus , wherein the evaluation value deriving means sets the evaluation value such that the priority is higher for a pixel having a smaller number of already arranged dots in the predetermined area . The evaluation value deriving means sets the evaluation value of each pixel included in the predetermined area based on a predetermined reference value and the arrangement information for each pixel included in the predetermined area. The image processing apparatus according to claim 1 . An image processing device for determining dot arrangement for each of the plurality of types of colorants based on images corresponding to each of the plurality of types of colorants,
Acquisition means for acquiring, with respect to a predetermined region of the image, dot arrangement information corresponding to one of the plurality of types of coloring materials whose dot arrangement has already been determined;
target value deriving means for deriving, based on the image, a target value for a predetermined colorant for which dot arrangement has not been determined among the plurality of types of colorants for the predetermined region;
evaluation value deriving means for deriving an evaluation value of each pixel included in the predetermined area based on the arrangement information;
dot arrangement determination means for determining the arrangement of dots of the predetermined color material in the predetermined area based on the target value and the evaluation value;
has
The evaluation value is a value indicating a priority for arranging dots of the predetermined color material for each of a plurality of pixels included in the predetermined area;
The evaluation value deriving means derives an exclusion value indicating a degree to which the placement of the dots of the predetermined color material is suppressed for each pixel included in the predetermined region based on the placement information;
The image processing apparatus, wherein the evaluation value is derived such that the priority decreases as the exclusion value increases in the predetermined area. The exclusive value is obtained by multiplying the number of dots already arranged and a weighting factor for each color material for each of a plurality of pixels included in the predetermined area, and then adding the value obtained by the multiplication to the arrangement of the dots. 4. The image processing apparatus according to claim 3 , wherein is derived by adding for already determined colorants. 5. The image processing apparatus according to claim 4 , wherein the weighting coefficient value is greater for a colorant with a higher optical density of dots formed on the recording medium. 5. The image processing apparatus according to claim 4 , wherein the value of the weighting factor is larger for a color material with higher saturation of dots formed on the recording medium. The evaluation value deriving means calculates, based on the gradation value of each pixel included in the predetermined area, a predetermined reference value for each pixel included in the predetermined area, and the exclusion value, the 7. The image processing apparatus according to any one of claims 3 to 6 , wherein the evaluation value is derived for each pixel included. 8. The image processing apparatus according to claim 7 , wherein the evaluation value deriving means derives the evaluation value such that the evaluation value increases as the tone value of the pixel increases. The reference value is a value indicating a reference of priority for arranging dots of the predetermined color material for each pixel included in the predetermined area. 9. The image processing apparatus according to claim 7 , wherein the evaluation value is derived so as to increase the evaluation value. 10. The image processing according to claim 7 , wherein said evaluation value deriving means derives said evaluation value by subtracting said reference value and said exclusive value from said tone value. Device. The evaluation value derivation means normalizes the reference value in the same range as the gradation value and normalizes the exclusive value in the same range as the gradation value from the value obtained by normalizing the gradation value. 10. The image processing apparatus according to any one of claims 7 to 9 , wherein the evaluation value is derived by subtracting the calculated value. 12. The image processing apparatus according to any one of claims 1 to 11 , wherein the predetermined coloring materials are set in descending order of optical density from among the plurality of types of coloring materials. the predetermined area is set in order from among a plurality of unit areas obtained by dividing the image;
13. The image according to any one of claims 1 to 12 , wherein the order in which the predetermined color materials are set from among the plurality of types of color materials is changed for each of the plurality of unit areas. processing equipment. The target value deriving means derives the target value by comparing an average value of gradation values of pixels included in the predetermined region with threshold values of pixels corresponding to the predetermined region in a threshold matrix. 14. The image processing apparatus according to any one of claims 1 to 13 . 15. The image processing apparatus according to claim 14 , wherein said evaluation value deriving means derives said evaluation value based on said threshold matrix. 16. The image processing apparatus according to any one of claims 1 to 15 , wherein the target value is the number of dots of the predetermined color material to be arranged in the predetermined area. The target value corresponds to the amount of the predetermined coloring material to be applied to the area on the recording medium corresponding to the predetermined area, and the dot arrangement determining means selects the pixels to be arranged dots from the predetermined area. 16. The image processing apparatus according to any one of claims 1 to 15 , wherein the dot size is determined. 18. The image processing apparatus according to any one of claims 1 to 17 , wherein the plurality of types of colorants include black, cyan, magenta, and yellow. 19. The apparatus according to any one of claims 1 to 18 , further comprising recording means for recording dots on a recording medium based on the arrangement of the dots of the predetermined color material determined by the dot arrangement determination means. image processing device. An image processing method for determining dot arrangement for each of a plurality of types of colorants based on images corresponding to each of the plurality of types of colorants, comprising:
an obtaining step of obtaining, for a predetermined region of the image, dot arrangement information corresponding to one of the plurality of types of coloring materials whose dot arrangement has already been determined;
a target value deriving step of deriving, based on the image, a target value for a predetermined colorant for which dot arrangement has not been determined among the plurality of types of colorants for the predetermined region;
an evaluation value derivation step of deriving an evaluation value of each pixel included in the predetermined region based on the arrangement information;
a dot arrangement determination step of determining the arrangement of the dots of the predetermined color material in the predetermined area based on the target value and the evaluation value;
has
The evaluation value is a value indicating a priority for arranging dots of the predetermined color material for each of a plurality of pixels included in the predetermined area;
The image processing method , wherein the evaluation value deriving step sets the evaluation value such that the priority of a pixel having a smaller number of already arranged dots in the predetermined area is higher . An image processing method for determining dot arrangement for each of a plurality of types of colorants based on images corresponding to each of the plurality of types of colorants, comprising:
an obtaining step of obtaining, for a predetermined region of the image, dot arrangement information corresponding to one of the plurality of types of coloring materials whose dot arrangement has already been determined;
a target value deriving step of deriving, based on the image, a target value for a predetermined colorant for which dot arrangement has not been determined among the plurality of types of colorants for the predetermined region;
an evaluation value derivation step of deriving an evaluation value of each pixel included in the predetermined region based on the arrangement information;
a dot arrangement determination step of determining the arrangement of the dots of the predetermined color material in the predetermined area based on the target value and the evaluation value;
has
The evaluation value is a value indicating a priority for arranging dots of the predetermined color material for each of a plurality of pixels included in the predetermined area;
The evaluation value deriving step derives, based on the layout information, an exclusion value indicating the extent to which the placement of the dots of the predetermined color material is suppressed for each pixel included in the predetermined region;
An image processing method, wherein the evaluation value is derived such that the priority decreases as the exclusion value increases in the predetermined area. A program for causing a computer to function as each means of the image processing apparatus according to any one of claims 1 to 19 .

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