JP2010154095A - Color conversion device, printing device, and color conversion processing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress generation of a chromatic color in a gray area of a print result upon converting colors from an RGB system to a CMYK system. <P>SOLUTION: A color conversion device for converting dot image data of the RGM system into dot image data of a CMYK system and outputting them to a print mechanism includes: a print condition acquisition section for acquiring a print condition used upon printing the dot image data of the CMYK system by a printing device; and a color conversion processing section for selectively performing first conversion operation to express gray with CMY without using K from a lowest density to a first reference density and second conversion operation to express gray with CMY without using K from the lowest density to a second reference density higher than the first reference density on the basis of the print condition used upon printing the dot image data of the CMYK system by the print mechanism according to a correspondence relationship between the density of gray expressed with the RGB system and the amount of use of CMYK. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a color conversion device, a printing device, and a color conversion processing method for converting dot image data in RGB format into dot image data in CMYK format.

  Ink jet printers, laser printers, and the like widely used as color printing apparatuses express colors using C (cyan), M (magenta), and Y (yellow) ink or toner. In such a color printing apparatus, black dots can be generated by overlapping CMY, and gray gradation can be expressed by the density of black dots.

  As described above, the color printing apparatus can generate black dots by superimposing CMY. However, as described in Patent Document 1, in order to reduce the consumption of CMY ink and toner, In addition, K (black) ink and toner are often used. FIG. 9 is a diagram illustrating an example of the discharge amount of each CMYK ink in the case of expressing an achromatic gray level in an inkjet printer that performs printing using CMYK ink.

  This figure shows the gray scale density expressed in RGB (red, green, blue) format on the horizontal axis, and the vertical axis shows the discharge amount of each ink when printing gray of that density with CMYK ink. The content of color conversion from RGB format to CMYK format for gray is shown.

As shown in the drawing, in the region where the gray density is the lowest, gray is expressed by CMY ink without using K ink, and use of K ink is started from a density D1 having a relatively low density. As the density increases, the usage amount of K and the ink usage amount of CMY are increased. Thereafter, in an area higher than D2, which has a relatively high concentration, the usage amount of K is greatly increased while the usage amount of CMY is decreased. With such ink discharge amount control, gray gradation expression with reduced CMY ink usage can be performed.
JP 2006-319961 A

  When the ink discharge amount control as shown in FIG. 9 is performed, there is a problem that chromatic colors are likely to occur in a relatively light portion of a region expressing gray in CMYK. In other words, if the positions of the CMYK dots match, black dots are generated, so no problem occurs. However, if any of the CMY dots deviates from the K dots, the color of the shifted dots is present. It occurs as a color and is recognized in gray. This problem is remarkable in a line head type ink jet printer in which the tendency of dot deviation for each color is fixed. As shown in FIG. 9, when dot deviation occurs, particularly in a low gray density portion. The occurrence of chromatic colors will be noticeable.

  Accordingly, an object of the present invention is to suppress the occurrence of chromatic colors in a gray region of a printing result when performing color conversion from RGB format to CMYK format.

  In order to solve the above problems, a color conversion device according to a first aspect of the present invention is a color conversion device that converts dot image data in RGB format into dot image data in CMYK format and outputs the dot image data to a printing mechanism. The minimum density according to the correspondence relationship between the printing density acquisition unit for acquiring printing conditions when printing the dot image data in the CMYK format with the printing apparatus, and the gray density expressed in the RGB format and the amount of CMYK used The first conversion operation for expressing gray in CMY without using K from the first reference density to the first reference density, and CMY without using K from the lowest density to the second reference density higher than the first reference density. And a second conversion operation for expressing gray in a selective manner based on printing conditions when printing the CMYK format dot image data with the printing mechanism. Characterized by comprising a color conversion processing unit.

  In the present invention, the generation of chromatic colors in the gray region of the printing result is suppressed by switching the density at which the use of K in the gray gradation starts according to the printing conditions.

  Specifically, the color conversion processing unit can select any one of the conversion operations based on a printing paper type included in the printing conditions. In this case, the color conversion processing unit can select the second conversion operation if the printing paper type has a dot gain of a predetermined value or less. This is because, when the dot gain is small, the occurrence of chromatic colors in the gray region is conspicuous.

  Further, the color conversion processing unit may select any one of the conversion operations based on a resolution included in the printing condition. In this case, the color conversion processing unit can select the second conversion operation if the resolution is a predetermined value or less. This is because, when the resolution is low, the occurrence of chromatic colors in the gray region is conspicuous.

  The color conversion processing unit may select any one of the conversion operations based on whether or not the pretreatment agent is applied to the printing paper included in the printing conditions. In this case, the color conversion processing unit can select the second conversion operation when applying the pretreatment agent to the printing paper. This is because when the pretreatment agent is applied, the dot gain of the printing paper is reduced, and the occurrence of chromatic colors in the gray region is easily noticeable.

  In order to solve the above problems, a printing apparatus according to a second aspect of the present invention is a printing apparatus that converts dot image data in RGB format into dot image data in CMYK format and performs printing using a printing mechanism. In accordance with the correspondence relationship between the printing condition acquisition unit for acquiring the printing conditions when the dot image data in the CMYK format is printed by the printing mechanism, and the gray density expressed in the RGB format and the amount of CMYK used, the minimum The first conversion operation for expressing gray in CMY without using K from the density to the first reference density, and without using K from the lowest density to the second reference density higher than the first reference density. The second conversion operation for expressing gray in CMY is selectively performed based on the printing conditions when the dot image data in the CMYK format is printed by the printing mechanism. Characterized by comprising a color conversion processing unit for the line.

  Here, in the present invention, when the printing mechanism includes a line-type inkjet head, the line-type inkjet head further includes a plurality of nozzle blocks in which nozzle groups are formed arranged in the line direction. Can be applied particularly effectively.

  That is, in the case of the serial type ink jet method, even if there is a dot shift for each color, the tendency of the dot shift is not fixed, so the occurrence of chromatic colors in the gray gradation is not noticeable. Ink jet heads have a fixed tendency for dot shift. For example, if magenta and black dots are shifted in a certain nozzle block, the conventional technology has redness in the gray level, especially in the low density area, in that nozzle block. It will occur.

  In order to solve the above-described problem, a color conversion processing method according to a third aspect of the present invention converts a dot image data in RGB format into dot image data in CMYK format, and provides a printing mechanism including a line-type inkjet head. A color conversion processing method for outputting, wherein the CMYK format dot image data is printed in the RGB format based on the print condition acquisition step for acquiring the print condition when printing with the printing apparatus, and the acquired print condition In accordance with the correspondence relationship between the gray density and the amount of CMYK used, the first conversion operation for expressing gray in CMY without using K from the lowest density to the first reference density, and the first reference from the lowest density A color that selectively executes the second conversion operation for expressing gray in CMY without using K up to a second reference density that is higher than the density. And having a conversion processing steps.

  According to the present invention, when color conversion from the RGB format to the CMYK format is performed, it is possible to suppress the occurrence of chromatic colors in the gray region of the print result.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram illustrating a functional configuration of a color conversion apparatus 10 according to the present invention and a printing apparatus 20 connected to the color conversion apparatus 10. As shown in the figure, the color conversion apparatus 10 includes an RGB dot image input unit 110 for inputting RGB dot image data, a color conversion processing unit 120 for converting RGB image data into CMYK format image data, and the like. The printing apparatus 20 includes a printing condition acquisition unit 130 that acquires printing conditions and a CMYK dot image output unit 140 that outputs dot image data in CMYK format.

  The RGB dot image input unit 110 inputs, for example, RGB dot image data represented by 8 bits for each color from an external device or another function unit in the device. The color conversion processing unit 120 includes a plurality of types of lookup tables (LUTs) 122, and converts RGB dot image data into CMYK dot image data with reference to any one of the lookup tables 122. The look-up table 122 is a table in which the correspondence between RGB representative values and CMYK values is recorded. Which lookup table 122 is used to perform color conversion is set based on the printing condition acquired by the printing condition acquisition unit 130. Specifically, it can be set based on color / monochrome, type of printing paper, resolution, presence / absence of coating of coating agent, and the like.

  The printing condition acquisition unit 130 acquires printing conditions set by the user. As described above, the printing conditions include color / monochrome, type of printing paper, resolution, presence / absence of coating of an application agent, and the like. However, some of them may be used. The print condition may be received by the print condition acquisition unit 130 from the user, or the print condition acquisition unit 130 may acquire the print condition received by the other functional unit from the user.

  The CMYK dot image output unit 140 performs halftone processing or the like on the dot image data converted into the CMYK format and outputs the result to the printing apparatus 20. Here, the halftone process is, for example, a process for expressing a CMYK dot image represented by 8 bits for each color in a gradation that can be printed by the printing apparatus 20.

  As shown in the figure, the printing apparatus 20 includes a CMYK dot image input unit 210 that inputs dot image data in CMYK format, a printing mechanism 220, and a paper supply / discharge mechanism 230 that performs a paper supply / discharge operation for printing paper. I have.

  In this embodiment, the printing mechanism 220 includes a plurality of line-type print heads that extend in a direction perpendicular to the paper conveyance direction and have a large number of nozzles, and discharge black or color ink from each print head. An ink jet method for printing is adopted. However, other methods may be adopted.

  The color conversion apparatus 10 and the printing apparatus 20 as shown in FIG. 1 can be specifically realized as a system configuration as shown in FIG. FIG. 2A shows an example in which the system is configured by the PC 40 provided with the printer driver 410 and the ink jet printer 50 provided with the line head type ink jet printing mechanism 510. In this system, the printer driver 410 includes a color conversion unit 412, and RGB format image data created by the application of the PC 40 is developed into a dot image on the PC 40 side, and color conversion processing is further performed in the CMYK format to perform the inkjet printer 50. Output to. In this case, the PC 40 functions as the color conversion device 10 and the ink jet printer 50 functions as the printing device 20.

  FIG. 2B shows an image reading apparatus 60, a PC 70 having a printer driver 710, an ink jet printer 80 having a dot image developing unit 810, a color converting unit 820, and a line head type ink jet printing mechanism 830. A configured example is shown. In this system, the RGB image data read by the image reading device 60 is subjected to color conversion by the color conversion unit 820 of the ink jet printer 80 and printed by the line head type ink jet printing mechanism 830. Alternatively, the page description language output from the printer driver 710 of the PC 70 is developed into a dot image by the dot image development unit 810 of the ink jet printer 80, and the color conversion unit 820 performs color conversion into the CMYK format, thereby performing a line head type ink jet printing mechanism. Printing is performed at 830. In this case, the inkjet printer 80 functions as the color conversion device 10 and the printing device 20.

  FIG. 3 is a diagram illustrating a nozzle surface of the nozzle block 23 constituting the print head provided in the printing mechanism 220 of the printing apparatus 20. The printing mechanism 220 has an ink ejection mechanism (not shown) provided for each nozzle, and performs printing by ejecting ink from the nozzle. The ink ejection mechanism can be configured using, for example, a piezo element. In the nozzle block 23, two rows of nozzle rows are formed in the line direction so as to be obliquely arranged in a predetermined unit (three in this example) in order to increase the resolution in the main scanning direction. Each series is formed with a slight shift in the main scanning direction in order to fill the space between the nozzles.

  FIG. 4 is a diagram illustrating the arrangement of the print head 22. As shown in the figure, four print heads 22 (22c, 22k, 22m, 22y) corresponding to four colors of inks of cyan C, black K, magenta M, and yellow Y convey the printing paper 25. They are arranged side by side. That is, the conveyance direction of the printing paper 25 intersects the line direction perpendicularly. In the example of this figure, each print head 22 has six nozzle blocks 23 arranged in a staggered state in the main scanning direction.

  Since the line type print head 22 is fixed, the ejection position of each nozzle is determined based on the arrangement of the print head 22 and the nozzle block 23. Since dots are formed on the printing paper depending on the ejection positions of the nozzles, depending on the placement accuracy of the print head 22 and the nozzle block 23, the positions of the dots may be shifted for each ink color. Since the nozzle pitches formed in the nozzle block 23 are the same, the positional deviation of dots tends to be the same in the nozzle block 23.

  FIG. 5 is a diagram for explaining generation of a chromatic color based on dot shift when printing gray which is an achromatic color. In general, for gray, black dots are formed by superimposing CMY in almost the same amount, and the gray gradation is expressed by controlling the number of dots. Alternatively, it is also widely practiced to reduce the amount of CMY ink used by further superimposing K ink on CMY.

  Here, in the case where the black dots are formed by overlapping the CMYK dots as shown in FIG. 5A, if the positions of the dots of the respective colors coincide with each other, as shown in FIG. Only the dots are formed. Further, as shown in FIG. 5C, when the CMY dots are evenly displaced from the K dots, the CMY color components are uniformly formed around the black dots. When such dots are dispersed, an achromatic ideal gray with no color is expressed. Here, when the ink of K overlaps with ink of other colors, there is an action of canceling the color.

  However, depending on the arrangement state of the print head 22 and the nozzle block 23, as shown in FIGS. 5D and 5E, the ink dots of one color in the CMY are compared with the dots of the other colors. There is a case where it deviates from K dots. Since such dots have the same tendency at least in the nozzle block 23, when the dots are dispersed, the color of the shifted dots becomes gray that appears as a color. For example, when the C dot deviates from the K dot as shown in FIG. 5D, the color becomes bluish gray, and the Y dot becomes a CM dot as shown in FIG. 5E. When compared with K dots, the color is yellowish gray. The occurrence of such chromatic colors becomes conspicuous particularly in the gray gradation from the low density to the medium density.

  FIG. 6 is a diagram for explaining the relationship between dot displacement of ink, dot gain, and resolution. In this figure, a case where the C dot is shifted from the K dot as compared with the MY dot is taken as an example. Here, the dot gain is an index representing the characteristic of the printing paper indicating the dot area for the same ink discharge amount. That is, it is the ease of spreading of dots on the printing paper. For example, mat paper has a low dot gain and plain paper has a high dot gain. The resolution is an index indicating the number of dots per unit area. In this embodiment, since a line-type inkjet head is used, the resolution in the main scanning direction is fixed, and the resolution is adjusted by the number of dots per unit length in the sub-scanning direction.

  As shown in this figure, at a low resolution, the dot shift becomes more conspicuous as the dot gain is lower. That is, since the center position of each dot is the same, if the dot gain is increased and the area of the dot is increased, the shift of the dot is relatively reduced and becomes inconspicuous. Further, at high resolution, regardless of the dot gain, the amount of deviation with respect to the dot density is relatively small, so that the occurrence of color due to dot deviation is less noticeable.

  For this reason, in this embodiment, two types of lookup tables as shown in FIG. 7 are prepared for gray gradation. FIG. 7 is a diagram showing the contents of color conversion corresponding to gray in the lookup table, and shows CMYK ink ejection amounts with respect to the RGB density indicated by the horizontal axis.

  The color conversion shown in FIG. 7A has the same contents as in the prior art. In the region where the gray density is the lowest, gray is expressed with CMY ink without using K ink, and the density is relatively low. Use of the ink of density D1 to K is started. As the density increases, the usage amount of K and the ink usage amount of CMY are increased. Thereafter, in an area higher than D2, which has a relatively high concentration, the usage amount of K is greatly increased while the usage amount of CMY is decreased.

  In the color conversion shown in FIG. 7B, gray is expressed with CMY inks from the lowest density region to the intermediate density D3 having a higher density than D1 without using K ink. Then, the use of the ink of K from the intermediate density D3 is started. As the density increases, the usage amount of K and the ink usage amount of CMY are increased. Thereafter, in an area higher than D4 having a relatively high density, the usage amount of K is greatly increased while the usage amount of CMY is decreased. However, the lookup table 122 is not limited to two types, and more types may be prepared.

  In the present embodiment, in the case of printing where the occurrence of color due to dot misalignment is not noticeable, such as when the dot gain is large or when printing is performed at a high resolution, the gray gradation density as shown in FIG. The amount of CMY ink used is reduced by using K ink from the lower part.

  On the other hand, in the case of printing in which the dot gain is small and the occurrence of color tone due to dot displacement is noticeable, as shown in FIG. Black is expressed by CMY without using K in a low density portion where the occurrence of coloring is conspicuous. This prevents the occurrence of color due to deviation between the K ink dots and the other ink dots.

  Here, D1, D2, D3, and D4 can be determined by the following procedure, for example. First, the ratio of the Max value (K100%, CMY *%) is determined. This can be obtained by printing a portion of 100% black in RGB data, and determining the ratio of CMY that has the highest surface density and the back surface density is within the specified value of the specification. This ratio varies depending on the paper. Next, a gray scale of 0-100% is printed. Here, 0% is paper white and 100% is the ratio defined above. At this time, D1, D2, D3, and D4 are determined so that the gray gradation is smooth. At this time, each value is determined in consideration of reducing the occurrence of chromatic colors in the gray gradation. When setting a curve as shown in FIG. 7, the smoothness is adjusted by visual observation, and the color of the printed material is measured to display an L * (lightness) curve. This curve is adjusted so that there is no unevenness such as popping out or falling down.

  FIG. 8 is a flowchart for explaining the operation of the color conversion apparatus 10. First, the RGB dot image input unit 110 inputs dot image data in RGB format (S101). The dot image data may be input from the outside of the color conversion device 10 or may be generated inside the color conversion device 10.

  Further, the printing condition acquisition unit 130 acquires the printing conditions set by the user (S102). Then, the color conversion processing unit 120 sets a lookup table 122 corresponding to the printing conditions (S103). In the present embodiment, the lookup table 122 is set based on the following criteria. Any of the following criteria may be used, or a plurality of criteria may be used in combination. Further, specific reference values and the like can be appropriately adjusted according to the characteristics of the printing apparatus 20 and the like.

Here, for the gray gradation, the look-up table includes a first look-up table (see FIG. 7A) that uses K ink from a low density portion, and K ink more than the first look-up table. One of the second look-up tables (see FIG. 7B) having a high use starting density is set. In order to reduce the amount of CMY ink used, it is assumed that the first look-up table having a relatively low K use starting density is initially set.
<Standard 1>
When printing paper having a dot gain of a reference value or less is used, the second look-up table having a relatively high use start density of K is set because chromatic colors are easily generated. Here, since the dot gain is determined for each type of printing paper, the type of printing paper can be associated with the lookup table in advance.
<Standard 2>
When printing is performed by performing pre-processing for reducing the dot gain, the second look-up table having a relatively high K use starting density is set because the occurrence of chromatic colors is easily noticeable. Here, the pretreatment for reducing the dot gain is, for example, a case where the coating agent is coated on the printing paper before ink ejection.
<Standard 3>
In the case of printing with a resolution equal to or lower than the reference value, the second look-up table having a relatively high use start density of K is set because chromatic colors are easily generated.

  In any reference, three or more types of lookup tables 122 may be provided for gray gradation, and a plurality of reference values may be provided corresponding to the types of lookup tables 122. Further, the reference value of dot gain or resolution can be determined as follows, for example. That is, when the first look-up table and the second look-up table are applied and the gray level is printed by changing the dot gain or the resolution, the second look-up table is applied. A value that can suppress the occurrence of coloring is obtained experimentally, and that value is used as a reference value.

  When the lookup table is set according to the above criteria, the color conversion processing unit 120 performs color conversion from the RGB format to the CMYK format using the set lookup table (S104).

  Then, the CMYK dot image output unit 140 performs halftone processing and the like on the dot image data converted into the CMYK format and outputs the result to the printing apparatus 20 (S105). The printing apparatus 20 performs printing based on the output dot image data, thereby performing printing processing that suppresses the occurrence of chromatic colors in the gray region.

  As described above, according to the present embodiment, a plurality of lookup tables in which the K ink use start density is adjusted in gray scale are prepared, and the dot gain, resolution, etc. of the printing paper determined by the printing conditions are prepared. By using the corresponding lookup table, it is possible to suppress the occurrence of chromatic colors in the gray region of the print result when performing color conversion from the RGB format to the CMYK format.

FIG. 2 is a block diagram illustrating a functional configuration of a color conversion apparatus according to the present invention and a printing apparatus connected to the color conversion apparatus. It is a block diagram which shows the specific example of a system configuration. It is a figure which shows the nozzle surface of the nozzle block which comprises the print head with which the printing mechanism of a printing apparatus is provided. It is a figure which shows arrangement | positioning of a print head. It is a figure explaining generation | occurrence | production of the chromatic color based on the dot shift in the case of printing gray which is an achromatic color. It is a figure explaining the relationship between the dot deviation of ink, dot gain, and resolution. It is a figure which shows the content of the color conversion corresponded to the gray of a lookup table. It is a flowchart explaining operation | movement of a color conversion apparatus. It is a figure which shows the conventional content of the color conversion from RGB format in a gray gradation to CMYK format.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 ... Color conversion apparatus 20 ... Printing apparatus 22 ... Print head 23 ... Nozzle block 25 ... Printing paper 40 ... PC
50 ... Inkjet printer 60 ... Image reading device 70 ... PC
80 ... inkjet printer 110 ... RGB dot image input unit 120 ... color conversion processing unit 122 ... look-up table 130 ... printing condition acquisition unit 140 ... CMYK dot image output unit 210 ... CMYK dot image input unit 220 ... printing mechanism 230 ... supply / discharge Paper mechanism 410 ... Printer driver 412 ... Color conversion unit 510 ... Line head type inkjet printing mechanism 710 ... Printer driver 810 ... Dot image developing unit 820 ... Color conversion unit 830 ... Line head type inkjet printing mechanism

Claims (9)

A color conversion device that converts dot image data in RGB format into dot image data in CMYK format and outputs the dot image data to a printing mechanism,
A printing condition acquisition unit that acquires printing conditions when printing the CMYK format dot image data with the printing apparatus;
In accordance with the correspondence relationship between the gray density expressed in RGB format and the amount of CMYK used, the first conversion operation for expressing gray in CMY without using K from the lowest density to the first reference density, and the lowest density The CMYK format dot image data is printed by the printing mechanism using the second conversion operation for expressing gray in CMY without using K from the first reference density to the second reference density higher than the first reference density. A color conversion apparatus comprising: a color conversion processing unit that selectively executes based on printing conditions at the time. The color conversion device according to claim 1,
The color conversion apparatus, wherein the color conversion processing unit selects any one of the conversion operations based on a printing paper type included in the printing condition. The color conversion device according to claim 2,
The color conversion apparatus, wherein the color conversion processing unit selects the second conversion operation if the printing paper type has a dot gain of a predetermined value or less. The color conversion device according to claim 1,
The color conversion apparatus, wherein the color conversion processing unit selects any one of the conversion operations based on a resolution included in the printing condition. The color conversion device according to claim 1,
The color conversion apparatus, wherein the color conversion processing unit selects any one of the conversion operations based on whether or not a pretreatment agent is applied to printing paper included in the printing conditions. A printing apparatus that converts dot image data in RGB format into dot image data in CMYK format and performs printing using a printing mechanism,
A printing condition acquisition unit that acquires printing conditions when the dot image data in the CMYK format is printed by the printing mechanism;
In accordance with the correspondence relationship between the gray density expressed in RGB format and the amount of CMYK used, the first conversion operation for expressing gray in CMY without using K from the lowest density to the first reference density, and the lowest density The CMYK format dot image data is printed by the printing mechanism using the second conversion operation for expressing gray in CMY without using K from the first reference density to the second reference density higher than the first reference density. And a color conversion processing unit that selectively executes based on printing conditions at the time. The printing apparatus according to claim 6,
The printing apparatus includes a line-type inkjet head. The printing apparatus according to claim 7, wherein
The line-type inkjet head includes a plurality of nozzle blocks in which nozzle groups are formed arranged in a line direction. A color conversion processing method that converts dot image data in RGB format into dot image data in CMYK format and outputs the dot image data to a printing mechanism equipped with a line-type inkjet head,
A printing condition acquisition step of acquiring printing conditions when printing the dot image data in the CMYK format with the printing device;
Based on the acquired printing conditions, gray is expressed in CMY without using K from the lowest density to the first reference density according to the correspondence relationship between the gray density expressed in RGB format and the usage amount of CMYK. A color that selectively executes one conversion operation and a second conversion operation that expresses gray in CMY without using K from the lowest density to a second reference density that is higher than the first reference density. A color conversion processing method comprising: a conversion processing step.