JP2007235661A - Image processor, program, and color conversion condition setting method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To restrain a reduction of an image quality of an image to be formed, while carrying out a color conversion so that a color material quantity upon an image formation is further reduced. <P>SOLUTION: Such a direction is searched that a reduction amount of Y is maximized with respect to a position on an L*a*b* color space of an input L*a*b* value corresponding to an input CMYK value, based on an L*a*b*-Y characteristic representing an L*a*b* value obtained when Y of CMYK is each value. When the input L*a*b* value is changed to the corresponding direction, such an output Y value that a color difference is within a threshold value and the reduction amount of Y is over a predetermined value, and the corresponding output L*a*b* value are acquired, and an output CMK value corresponding to the output L*a*b* value when Y is the output Y value is acquired, and toner reducing conversion conditions are set so that the input CMYK value is converted into an output CMYK value. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an image processing device, a program, and a color conversion condition setting method, and more particularly to an image processing device that performs color conversion so that the amount of color material during image formation is reduced, and a computer to function as the image processing device. And a color conversion condition setting method for setting a color conversion condition used in the image processing apparatus.

  Technology to reduce consumption of color materials such as toner during image formation by an image forming apparatus for the purpose of reducing running costs in image forming apparatuses such as copiers, printers, or multifunction machines having these functions Various proposals have been made in the past, for example, in Patent Document 1, based on input image data, whether or not each color of a color material of a color printer and each color of a plurality of color components exist in an image represented by the input image data. A technique is disclosed in which a color printer performs image formation using only a minimum color plate necessary for detecting and expressing the detected color.

  Patent Document 2 discloses a technique for determining whether each area of an input image is a halftone area, a halftone dot area, or any other area, and whether it is a chromatic color or an achromatic color, and performing different toner save processing depending on the determination result. ing.

  In Patent Document 3, a toner total amount regulation value is set based on the image type of the image to be formed on the paper, the image density, the paper basis number, the smoothness, the paper feed direction, and the layout information in the page. A technique for converting RGB color space image data into YMCK color space image data so as not to exceed the regulation value is disclosed.

In Patent Document 4, a plurality of conversion data having different UCR amounts and toner consumption amounts are stored as conversion data set in the color conversion table, and the UCR adjustment amount and the toner consumption adjustment amount are designated by the user. And a technique for obtaining conversion data matching a designated UCR adjustment amount and toner consumption adjustment amount from a plurality of conversion data by interpolation.
JP 2004-155015 A JP 2004-157190 A JP 2004-191853 A JP 2004-243559 A

  As a technique for reducing the amount of color material when an image (full color image) is formed using CMYK color materials, for example, Patent Document 2 describes a technique for adjusting the luminance of the entire image. Describes a technique for regulating the total amount of toner. However, when image formation is performed by applying these techniques, there is a problem that a noticeable image quality that can be easily visually recognized is generated in the formed image. For this reason, to reduce the amount of color material when an image is formed using CMYK color materials, the amount of K color material is increased, and instead, the amount of CMY color materials is reduced. Techniques for reducing the amount of color material are exclusively used. Compared with each of the above-described techniques, this technique has an advantage that it is difficult to visually recognize a deterioration in the image quality of the formed image because it is in a colorimetric match state (color difference is approximately zero).

  However, for the image forming apparatus, there is a need for higher image quality of the formed image. On the other hand, depending on the type of formed image, a certain level of image quality is ensured while the color material amount (running cost) is further increased. There is a need to reduce this, and a technology that satisfies this need has been awaited.

  The present invention has been made in consideration of the above facts, and an image processing apparatus and image capable of performing color conversion so as to further reduce the amount of color material at the time of image formation while suppressing deterioration in the image quality of the formed image. The purpose is to obtain a processing program.

  It is another object of the present invention to obtain a color conversion condition setting method that can obtain a color conversion condition that can further reduce the amount of color material during image formation while suppressing deterioration in the image quality of the formed image.

  In order to reduce the total color material amount, instead of specifying the total color material amount, specify the color difference, which is a factor that affects the image quality, and maintain continuity and smoothness within the range of the color difference. Thus, if the combination of the color material amounts that minimizes the total color material amount is determined, the image quality deterioration can be suppressed. Furthermore, when image formation is performed using color materials of CMYK colors that are combinations of color materials used for general image formation, color materials of CMY colors other than K among the color materials of CMYK colors are compared. , Y color is less involved in color formation than other colors (C color and M color) (many colors are less involved in Y color). Even if the amount is reduced, it is possible to suppress the deterioration of the image quality of the formed image as compared with the case where the amount of the color material of other colors is reduced. The inventor of the present application has conceived the above matters and made the present invention.

  Based on the above, the image forming apparatus according to the first aspect of the invention is a color space in which each color value after conversion corresponding to the amount of color material at the time of image formation by the image forming device does not depend on the device for the input color value. The apparatus includes color conversion means for converting the input color value under the condition that the total color value is reduced by allowing the above color difference to be larger than 0 and less than or equal to the threshold value. Yes.

  In the present invention, the device-independent color space includes a uniform perceptual color space such as an L * a * b * color space recommended by the CIE (Commission Internationale de l'Eclairage), and a CAM02 space. However, other color spaces such as CIEXYZ tristimulus values or a color space that takes frequency characteristics into account like iCAM may be applied. In addition, the color difference in a color space that does not depend on the device can be obtained by various methods, for example, using a CIE color difference formula or a unique evaluation function. The image processing apparatus according to the present invention can be incorporated in an image forming apparatus that forms an image using, for example, CMYK color materials. However, the image forming apparatus is instructed to transfer image data to the image forming apparatus. It is also possible to cause an information processing apparatus such as a computer to function as the image processing apparatus according to the present invention.

  The invention described in claim 1 allows each color value after conversion to have a color difference in a device-independent color space with respect to the input color value within a range greater than 0 and less than or equal to a threshold value. Therefore, there is a possibility that a change in the color of the formed image (decrease in the image quality of the formed image) with respect to the input color value may occur slightly as compared with the technique for achieving a colorimetric match (a state where the color difference is approximately zero). However, according to the first aspect of the present invention, the color difference of each color value after conversion in the color space independent of the device is limited to the above range with respect to the input color value, and the total amount of each color value becomes small. Since the color values input under the conditions are converted, the total color material amount during image formation on the image forming device can be further reduced, and the image quality of the formed image is reduced for the total color material amount reduction amount. Can be suppressed. Therefore, according to the first aspect of the present invention, it is possible to perform color conversion so as to further reduce the amount of color material at the time of image formation while suppressing deterioration in the image quality of the formed image.

  In the image processing apparatus according to the second aspect of the present invention, the converted CMYK color values corresponding to the color material amounts of the CMYK colors at the time of image formation by the image forming device depend on the device for the input color values. A color for converting the input color value into each color value of CMYK so that a combination of K color value is as large as possible and Y color value is as small as possible within a range where the color difference in the color space is not less than a threshold value. The conversion means is included.

  According to a second aspect of the invention, there is provided color conversion means for converting an input color value into CMYK color values corresponding to the color material amounts of the CMYK colors at the time of image formation. The input color value may be a CMYK color value (color value in the CMYK color space), a reproduced color in a device-independent color space, or in another color space. It may be a color value. According to a second aspect of the present invention, the color conversion unit according to claim 2 performs the conversion on a color space in which the converted CMYK color values corresponding to the color material amounts of the CMYK colors at the time of image formation do not depend on a device for the input color values. In such a range, the K color value is as large as possible and the Y color value is as small as possible.

  As described above, by increasing the K color value in the converted CMYK color values as much as possible, the converted CMY color values are reduced, and the color material amounts of the CMY colors at the time of image formation, and thus at the time of image formation. Total colorant amount is reduced. In addition, the color change on the formed image can be achieved by reducing the Y color value in each color value of CMYK after conversion within a range where the color difference in the device-independent color space with respect to the input color value is within a threshold value or less. The color material amount of Y color can be further reduced while suppressing the image quality within the allowable range, and the total color material amount during image formation can be further reduced. Therefore, according to the second aspect of the present invention, image formation is suppressed even when compared with the case where the color material amount of each color of CMY is reduced instead of increasing the color material amount of K color while suppressing the deterioration of the image quality of the formed image. Color conversion can be performed so that the amount of color material at the time is further reduced.

  According to the first and second aspects of the invention, as the threshold value is increased, the total color material amount during image formation is reduced while the image quality of the formed image is also reduced. For this reason, in the invention described in claim 1 or 2, the threshold value may be fixedly set. However, as described in claim 3, for example, the threshold value is set according to the specified color material amount reduction degree. The color conversion means may be configured to switch between the two. In this case, by specifying the color material amount reduction degree, it is possible to select whether priority is given to reducing the color material amount during image formation or securing the image quality of the formed image. It is possible to switch between giving priority to reducing the color material amount and ensuring the image quality of the formed image according to the above.

  According to the second aspect of the present invention, the input color values are converted into the CMYK color values so that the converted CMYK color values are a combination of the K color values as large as possible and the Y color values as small as possible. As a result, the reproduced colors in the color space that does not depend on the device of the CMYK color values after conversion are within the range below the threshold with respect to the reproduced colors in the color space that does not depend on the device of the input color value. The color conversion means moves, but the color reproduction of the converted CMYK color values on the device-independent color space is within the range where the color difference is equal to or smaller than the threshold value as described in claim 4, for example. The conversion may be performed so as to move in a direction in which the saturation is reduced and, for example, as described in claim 5, the K color value or the Y color value is within a range where the color difference is equal to or less than a threshold value. In the direction where the amount of change is as large as possible The conversion may be performed for to movement. In particular, when the direction in which the change amount of the K color value or the Y color value is as large as possible is adopted as the movement direction of the reproduced color in the color space independent of the device, the color difference from the input color value (device The amount of reduction of the color material with respect to the amount of reproduction color movement in a color space that does not depend on the color) increases, so that it is possible to achieve both high-level reduction in the quality of the formed image and reduction in the amount of color material during image formation.

  According to a sixth aspect of the present invention, there is provided an image processing program for causing a computer to use a color space in which each color value after conversion corresponding to the amount of color material at the time of image formation by the image forming device does not depend on the device for the input color value. Under the condition that the total amount of each color value is small within a range where the above color difference is greater than 0 and less than or equal to the threshold value, it functions as a color conversion means for converting the input color value.

  Since the image processing program according to the sixth aspect of the invention is a program for causing a computer to function as the color conversion means, the computer executes the image processing program according to the sixth aspect of the invention. Will function as the image processing apparatus according to the first aspect of the present invention, and in the same manner as the first aspect of the present invention, the color material amount at the time of image formation is further reduced while suppressing the deterioration of the image quality of the formed image. Color conversion can be performed.

  According to a seventh aspect of the present invention, there is provided an image processing program on a color space in which the converted CMYK color values corresponding to the color material amounts of the CMYK colors at the time of image formation do not depend on a device for the input color values. Function as color conversion means for converting the input color values into CMYK color values so that the K color value is as large as possible and the Y color value is as small as possible within a range where the color difference in the range is less than or equal to the threshold value. Let

  Since the image processing program according to the seventh aspect of the invention is a program for causing a computer to function as the color conversion means, the computer executes the image processing program according to the seventh aspect of the invention, so that the computer Will function as the image processing apparatus according to claim 2, and, similarly to the invention according to claim 2, the color material amount at the time of image formation is further reduced while suppressing deterioration in the image quality of the formed image. Color conversion can be performed.

  A color conversion condition setting method according to an eighth aspect of the present invention provides a K color that can be taken within a reproduction color range in which a color difference on the color space is equal to or less than a threshold for a specific reproduction color on a device-independent color space. After selecting at least one of the largest possible K color value and the smallest possible Y color value in the value range and the Y color value range, the K color value and the Y color are selected as the unselected color values among the CMYK color values. A value corresponding to a reproduction color that has changed in accordance with the selection of at least one of the values is obtained for each of the plurality of reproduction colors on the color space, so that the reproduction color on the color space and the reproduction color A correspondence relationship with a combination of CMYK color values where the K color value is as large as possible and the Y color value is as small as possible within a range where the color difference on the color space is equal to or smaller than the threshold is obtained, and based on the obtained correspondence relationship, Input color value Setting the color conversion condition for converting into CMYK respective color values.

  In the invention described in claim 8, for a specific reproduction color in a device-independent color space, first, a K color value range that can be taken within a color value range in which a color difference in a device-independent color space is equal to or less than a threshold value; In the Y color value range, at least one of a K color value as large as possible and a Y color value as small as possible is selected. Next, an unselected color among CMYK color values (CMY colors when only K color values are selected, CMY colors when only Y color values are selected, and CM colors when K color values and Y color values are selected) ) Is obtained as a value corresponding to the reproduced color that has changed with the selection of at least one of the K color value and the Y color value. In this way, by obtaining at least one of the K color value and the Y color value and then obtaining the value of the unselected color, the color difference in the device-independent color space for the specific color value is equal to or less than the threshold value. Within the range, a combination of CMYK color values having the largest K color value and the smallest Y color value is obtained.

  In the invention described in claim 8, the above processing (processing for obtaining a combination of CMYK color values corresponding to a specific reproduction color) is performed for each of a plurality of reproduction colors in a color space independent of the device, so that it does not depend on the device. A combination of a reproduced color on the color space and CMYK color values where the K color value is as large as possible and the Y color value is as small as possible within a range where the color difference in the device-independent color space for the reproduced color is less than or equal to the threshold value. Since the color conversion condition for converting the input color value to each color value of CMYK is set based on the obtained correspondence, the amount of color material at the time of image formation is suppressed while suppressing the deterioration of the image quality of the formed image Can be obtained. The color conversion means according to the invention described in claim 2 can be realized by setting the color conversion condition obtained by applying the invention described in claim 8, for example, in a conversion table such as LUT (LookUpTable).

  In the invention described in claim 8, the selection of the K color value as large as possible is specifically when the K color value of the CMYK color values is a predetermined value as described in claim 9, for example. The color difference in the color space with respect to a specific reproduction color is a threshold value based on reproduction color-K information representing a reproduction color in a color space that does not depend on a device that can be taken in each case when the K color value is each value. The determination can be performed by determining a K color value range that can be taken within the following reproduction color range and selecting a K color value in which the increase amount of the K color value is equal to or greater than a predetermined value from the determined K color value range. it can.

  In addition, in the invention described in claim 8, when selecting a Y color value as small as possible, specifically, as described in claim 10, for example, the Y color value among the CMYK color values is a predetermined value. The color difference in the color space with respect to a specific reproduction color based on the reproduction color-Y information that represents the reproduction color in the color space that does not depend on the device that can be taken at each time when the Y color value is each value. This is performed by determining a Y color value range that can be taken within a reproduction color range that is less than or equal to a threshold value, and selecting a Y color value that causes a decrease amount of the Y color value to be a predetermined value or more from the determined Y color value range. Can do.

  Further, in the invention according to claim 9 or claim 10, a K color value range or a Y color value that can be taken within a color value range in which a color difference in a color space independent of a device for a specific color value is equal to or less than a threshold value. Specifically, the determination of the range is, for example, as described in claim 11, with the brightness being constant and the saturation being lowered with reference to a position in a color space that does not depend on a device having a specific color value. This is performed by determining a K color value range or a Y color value range in which the color difference in the color space is equal to or smaller than a threshold value along the direction or the direction in which the change amount of the K color value or the Y color value is as large as possible. be able to.

  As described above, according to the present invention, each color value after conversion corresponding to the amount of color material at the time of image formation by the image forming device has a color difference in a device-independent color space with respect to the input color value from 0. Since the input color value is converted under the condition that the total amount of each color value is small while allowing it to be within the range of less than or equal to the threshold value, the color material at the time of image formation is suppressed while suppressing the deterioration of the image quality of the formed image. The color conversion can be performed so that the amount is further reduced.

  In the present invention, the converted CMYK color values corresponding to the color material amounts of the CMYK colors at the time of image formation are such that the color difference on the device-independent color space with respect to the input color values is within a range below the threshold value. The input color values are converted into CMYK color values so that the combination of the K color value is as large as possible and the Y color value is as small as possible. The color conversion can be performed so that the amount is further reduced.

  Further, the present invention provides a K color value that is as large as possible within a K color value range and a Y color value range that can be taken within a color value range in which a color difference in a color space that does not depend on a device is a threshold value or less with respect to a specific color value. After selecting at least one of the color value and the smallest possible Y color value, it corresponds to the color value changed with the selection of at least one of the K color value and the Y color value as the value of the unselected color among the CMYK color values. By calculating each value for a plurality of color values in a device-independent color space, the color value in the device-independent color space and the color difference in the device-independent color space are equal to or less than the threshold value. In this range, the K color value becomes as large as possible and the Y color value becomes as small as possible with the combination of CMYK color values, and the input color value is converted into the CMYK color values based on the obtained correspondence relationship. Color Since setting the conversion conditions, the color material amount at the time of image formation can be obtained color conversion conditions can be further reduced while suppressing degradation in image quality forming an image, having an excellent effect that.

  Hereinafter, an example of an embodiment of the present invention will be described in detail with reference to the drawings. FIG. 1 shows a color conversion processing unit 10 according to the present embodiment. The color conversion processing unit 10 corresponds to the image processing apparatus and the color conversion unit according to claims 1 to 5. 1 shows a functional block diagram of the color conversion processing unit 10. The color conversion processing unit 10 is actually a CPU of a computer not shown, or a printer connected to the computer via a communication line. This is realized by executing a predetermined color conversion processing program by the CPU built in the computer. The color conversion processing program corresponds to the image processing program according to claims 6 and 7.

  As shown in FIG. 1, the color conversion processing unit 10 is connected to an image forming engine 12 that forms (prints) a color image on paper using color materials of CMYK colors built in the printer. In this embodiment, the image forming engine 12 uses a configuration in which a color image is formed by electrophotographic method using toners of CMYK colors and then transferred and fixed on paper. However, the present invention is not limited to this. It is also possible to adopt a configuration in which an image is formed on a sheet by another method such as an ink jet method in which inks of CMYK colors are ejected as recording droplets from nozzles to form a color image on the sheet. Furthermore, the present configuration can be adopted not only for the four colors of CMYK but also for multicolor printing. Further, when image formation by the printer (printing of an image on a sheet) is instructed via a computer, the color conversion processing unit 10 receives image data 14 representing an image to be printed. In the present embodiment, as this image data 14, an image representing the color of each pixel of the image to be printed with a reproduction color (L * a * b * value) in the L * a * b * color space recommended by the CIE. A mode in which data is input will be described.

  The color conversion processing unit 10 includes a color space conversion 3D-LUT (three-dimensional lookup table) 16, 1D-LUT (one-dimensional lookup table) 18C, 18M, 18Y, and 18K for each color of CMYK, and conversion data setting. Unit 20 and a conversion data storage unit 22. The color space conversion 3D-LUT 16 performs color space conversion for converting a reproduction color (L * a * b * value) in the L * a * b * color space into a CMYK value. The C 1D-LUT 18C is an LUT that complements the conversion from the L * a * b * value to the C value by the color space conversion 3D-LUT 16, and converts the input C value to another C value. Similarly, the 1D-LUT 18M for M converts the inputted M value into another M value, the 1D-LUT 18Y for Y converts the inputted Y value into another Y value, and the 1D-LUT 18K for K inputs The converted K value is converted into another K value.

  In the conversion data storage unit 22, conversion data for performing the above conversion in each LUT 16, 18C, 18M, 18Y, 18K by setting in each LUT 16, 18C, 18M, 18Y, 18K, and toner consumption at the time of image formation Toner reduction conversion conditions for reducing the amount (this toner reduction conversion condition is generated by a toner reduction conversion condition setting process described later) are stored. The conversion data storage unit 22 stores a plurality of types of conversion data corresponding to different printing conditions as the conversion data, and corresponds to different toner reduction degrees as the toner reduction conversion conditions. A plurality of types of toner reduction conversion conditions are stored.

  The conversion data setting unit 20 is connected to the conversion data storage unit 22 and the LUTs 16, 18C, 18M, 18Y, and 18K. When the printing (image formation) is instructed via the computer, the conversion parameter setting unit 20 receives the printing parameters designated at the same time. Although details will be described later, the conversion data setting unit 20 reads the conversion data for each of the LUTs 16, 18C, 18M, 18Y, and 18K that matches the input print parameters from the conversion data storage unit 22 and, if necessary, The conversion conditions for toner reduction are also read from the conversion data storage unit 22, and after setting the conversion data corresponding to each LUT 16, 18C, 18M, 18Y, 18K, the conversion of the input image data 14 is performed for each LUT 16, 18C, 18M, 18Y, 18K. As a result, color conversion is performed on the image data 14 input to the color conversion processing unit 10 to convert L * a * b * values representing the colors of the respective pixels into CMYK color values. Data is output to the image forming engine 12.

  Next, generation of toner reduction conversion conditions to be stored in the conversion data storage unit 22 will be described as an operation of the present embodiment. The conversion condition for toner reduction according to the present embodiment is a conversion condition for converting each CMYK color value representing the color of each pixel of an image into a CMYK value having a smaller toner consumption amount (that is, the total value of each CMYK color value). As an example, as shown in FIG. 2, L * a * b * -Y characteristic data, L * a * b * -K characteristic data, and L * a * b * -CMYK conversion data are stored in the storage unit 30 of the computer. In this state, the CPU of the computer is caused to execute a predetermined program and function as the toner reduction conversion condition setting unit 32 (perform the toner reduction conversion condition setting process shown in FIG. 3).

For example, as shown in FIG. 4A, the L * a * b * -Y characteristic data is an L * a * b * color space that can be taken when the Y value among CMYK color values is a certain value. The above L * a * b * values are data representing a plurality of values having different Y values (in FIG. 4A, Y = Y ₁ , Y ₂ , Y in order to avoid confusion in the figure) ₃ (however, only Y ₁ > Y ₂ > Y ₃ is shown) corresponds to the reproduction color-Y information according to claim 10. Similarly, the L * a * b * -K characteristic data described above can be taken as L * a * that can be taken when the K value of CMYK color values is a certain value, as shown in FIG. 5A as an example. This is data representing the L * a * b * values in the b * color space for a plurality of values having different K values (in FIG. 5A, K = K ₁ , In the case of K ₂ and K ₃ (however, only K ₁ > K ₂ > K ₃ is shown), this corresponds to the reproduction color-K information according to claim 9.

  Hereinafter, the toner reduction conversion condition setting process performed by the toner reduction conversion condition setting unit 32 will be described with reference to FIG. First, in step 50, a color difference threshold to be applied when setting the toner reduction conversion condition is acquired. Although the details will be described later, the color difference threshold value has a correlation with the image quality of the output image, the degree of color change when the conversion is performed using the conversion condition for toner reduction, and the toner reduction amount. By executing the toner reduction conversion condition setting process a plurality of times, a plurality of toner reduction conversion conditions corresponding to different toner reduction degrees are respectively set (generated).

  In the next step 52, a specific combination is extracted as an input CMYK value from a plurality of combinations of predetermined CMYK values. The input CMYK values correspond to the vertices of each area when the CMYK color space is divided into a plurality of areas by dividing the coordinate axes of the CMYK colors defining the CMYK color space at a constant numerical interval, for example. CMYK values at locations can be used. In step 54, the input CMYK value is converted into an L * a * b * value (input L * a * b * value) with reference to the L * a * b * -CMYK conversion data stored in the storage unit 30. . In step 56, it is determined whether or not the input L * a * b * value obtained by the conversion in step 54 is a color value in a color region where the toner amount can be reduced.

  For example, the input L * a * b * value is a color value in a color region where the Y value is 0 or a value close to 0 and the K value is a maximum value or a value close to the maximum value. If it is almost impossible to reduce the toner consumption even after the operation, the determination in step 56 is negative and the process proceeds to step 58. After the input CMYK value is set as the output CMYK value, the input CMYK value is determined in step 88. Is stored in association with the output CMYK value. In this case, the toner reduction conversion condition is set so that the current input CMYK value is output without being changed.

On the other hand, if the determination in step 56 is affirmed, the process proceeds to step 60 to determine the type of the color region to which the input L * a * b * value belongs, and branch according to the determination result. For example, in the color region to which the input L * a * b * value belongs, the K value is a maximum value or a value close to the maximum value (a value with no room for an increase in K toner), but the Y value is equal to or greater than a predetermined value If it is determined that the color area is a value that can be reduced), that is, an area where Y toner consumption can be reduced, the process proceeds from step 60 to step 62 and the storage unit 30 reads L * a * b * -Y characteristic data is captured, and in the next step 64, the input L * a * b * value is checked against the L * a * b * -Y characteristic data to obtain the input L * a * b * value. The Y value corresponding to is recognized. For example L * a * b * position of the input L * a * b * values on the space corresponds to between Y = Y ₁ curves and Y = Y ₂ curve as shown in FIG. 4 (B) when a position, Y value corresponding to the input L * a * b * values, depending on the proximity between the curve and Y = Y ₂ curves of the position and Y = Y ₁ of Y ₁ and Y ₂ It can be obtained by calculating a weighted average. The Y value corresponds to the consumption amount of Y color toner when the color represented by the input L * a * b * value is formed with CMYK toner.

In step 66, a search is made in the L * a * b * space for the direction in which the amount of reduction in the Y value is maximized with reference to the position of the input L * a * b * value in the L * a * b * space. . For example, when the position of the input L * a * b * value on the L * a * b * space is the position shown in FIG. 4B, the input L * on the L * a * b * space as the position of the a * b * values are shown in the input L * a * b * Y value is less than the Y value corresponding to the value Y = Y ₂ curves with orthogonal linear directions (see FIG. 4 (B) arrow Is extracted as the direction in which the amount of reduction in the Y value is maximized. In step 68, the color difference (movement distance in the L * a * b * space) when the input L * a * b * value is changed in the direction extracted in the search in step 66 is within the threshold value. A Y value (output Y value) with a reduction amount equal to or greater than a predetermined value is obtained, and a corresponding L * a * b * value (output color value) is recognized. Although in FIG. 4 (B) shows an example of setting the output Y value for Y _2, and a color difference = threshold amount reduction may be the output Y value the maximum Y value. Thus, an output color value (L * a * b * value) in which the color difference is within a threshold with respect to the input L * a * b * value and the reduction amount of the Y value is equal to or greater than a predetermined value, and the output color value are represented. An output Y value representing the amount of Y color toner when the color is formed by CMYK toner is obtained.

  In the next step 70, when the Y value is the output Y value obtained in step 68, a CMK value corresponding to the output color value (each color value of the CMK when the color of the output color value is formed by the CMYK toner: output) CMK value) is derived based on the L * a * b * -CMYK conversion data stored in the storage unit 30. Since the Y color is less involved in color formation than other colors, even if the Y value is reduced as described above, each CMK color value derived in step 70 is the input L * a. It hardly increases with respect to each color value of CMK corresponding to the * b * value, and the total value of each color value of CMYK becomes smaller by the decrease of the Y value. When the processing of step 70 is performed, the routine proceeds to step 88, where the input CMYK value is stored in association with the output CMYK value. As a result, the toner input conversion condition is set so that the current input CMYK value is converted to an output CMYK value in which the total value of the CMYK color values is further reduced by reducing the Y value. .

On the other hand, in the determination of the previous step 60, for example, the color region to which the input L * a * b * value belongs has a Y value of 0 or a value close to 0 (a value in which there is no room for Y toner reduction). It is possible to reduce the total consumption of each color toner by increasing the color region where the value is smaller than the maximum value by a predetermined value or more (a value with room to increase the amount of K toner), that is, the consumption amount of K toner. If it is determined that the region is a region where the K toner can be increased, the process proceeds from step 60 to step 72 and the L * a * b * -K characteristic data is fetched from the storage unit 30. By comparing the * b * value with the L * a * b * -K characteristic data, the K value corresponding to the input L * a * b * value is recognized. For example L * a * b * position of the input L * a * b * values on the space corresponds to between the curves of K = K ₂ curves and K = K ₃ as shown in FIG. 5 (B) If it is a position, the K value corresponding to the input L * a * b * value is the value of K ₂ and K ₃ depending on the proximity of the position to the K = K ₂ curve and the K = K ₃ curve. It can be obtained by calculating a weighted average. This K value corresponds to the consumption amount of the K color toner when the color represented by the input L * a * b * value is formed with CMYK toner.

In step 76, the direction in which the increase amount of the K value is maximized is searched for in the L * a * b * space with reference to the position of the input L * a * b * value in the L * a * b * space. . For example, if the position of the input L * a * b * value on the L * a * b * space is the position shown in FIG. 5B, the input L * on the L * a * b * space as the position of the a * b * values are shown in the input L * a * b * than the K value corresponding to the value perpendicular to the K value is larger K = K ₂ curve linear direction (see FIG. 5 (B) arrow Is extracted as the direction in which the amount of increase in the K value is maximized. In step 78, the color difference (movement distance in the L * a * b * space) when the input L * a * b * value is changed in the direction extracted in the search in step 76 is within a threshold value. A K value (output K value) whose increase amount is a predetermined value or more is obtained, and a corresponding L * a * b * value (output color value) is recognized. FIG. 4B shows an example in which the output K value is set to a value between K ₁ and K ₂ , but the color difference = threshold and the K value with the largest increase amount is the output K value. You may do it. As a result, the output color value (L * a * b * value) in which the color difference is within the threshold with respect to the input L * a * b * value and the increase amount of the K value is equal to or greater than the predetermined value and the output color value are represented. An output K value representing the amount of K-color toner when the color is formed with CMYK toner is obtained.

  In the next step 80, when the K value is the output K value obtained in step 78, the CMY value corresponding to the output color value (CMY color values when the color of the output color value is formed by CMYK toner: output) CMY value) is derived based on the L * a * b * -CMYK conversion data stored in the storage unit 30. Note that since the Y color is less involved in color formation than other colors, even if the Y value is decreased as described above, there is almost no increase in the color values of the CMK. Note that, by increasing the K value as described above, the CMY color values derived in step 80 are decreased with respect to the CMY color values corresponding to the input L * a * b * values. The total value becomes smaller. When the processing of step 80 is performed, the routine proceeds to step 88, where the input CMYK value is stored in association with the output CMYK value. As a result, for the current input CMYK value, by increasing the K value, the toner reduction conversion condition is set so that the total value of the CMYK color values is converted into the output CMYK value that is further reduced.

  Further, in the determination of the previous step 60, for example, in the color region to which the input L * a * b * value belongs, the Y value is equal to or greater than a predetermined value (there is room for Y toner reduction), and the K value is also greater than the maximum value. Is a color region that is smaller than a predetermined value (a value that has room to increase the amount of K toner), that is, by reducing the consumption amount of Y toner and increasing the consumption amount of K toner, the total consumption amount of each color toner can be reduced. When it is determined that the Y reduction / K increase possible region can be reduced, the process proceeds from Step 60 to Step 82, and the input is performed in the same manner as in Steps 62 to 68 and Steps 72 to 78 described above. An output color value (L * a * b * value) in which the color difference is within a threshold with respect to the L * a * b * value and the K value increase amount and the Y value decrease amount become as large as possible, and the output color value represents K when forming colors with CMYK toner And output K value and the output K of obtaining an output Y value, the Y value recognition process performed representing the amount of toner of the Y color.

  In the output K and Y value recognition processing in step 82, the direction in which the amount of increase in the K value is the maximum and the direction in which the amount of decrease in the Y value are the maximum do not necessarily match, but in this case, K or Y The output color value (L * a * b * value), the output K value, and the output Y value may be obtained so that one of the priorities is given priority, and the amount of increase or decrease of the one priority color is maximized. Based on the position of the input L * a * b * value in the L * a * b * space, the total value of CMYK color values (total consumption of CMYK color toners) is within a range where the color difference is equal to or less than the threshold value. The minimum color value may be searched, the color value extracted by the search may be used as the output color value, and the K value and Y value corresponding to the output color value may be used as the output K value and output Y value. When the process of step 84 is performed, the process proceeds to step 88, and the input CMYK value is stored in association with the output CMYK value. As a result, the current input CMYK value is converted into an output CMYK value in which the total value of the CMYK color values is further reduced by setting the K value as large as possible and the Y value as small as possible. Conversion conditions for toner reduction are set.

  When the processing of step 88 is performed, the routine proceeds to step 90, where it is determined whether or not there are CMYK values that are not extracted as input CMYK values in a plurality of combinations of predetermined CMYK values. If the determination is affirmative, the process returns to step 52, and steps 52 to 90 are repeated until the determination of step 90 is negative. As a result, a plurality of input CMYK values corresponding to the respective positions in the CMYK color space are associated with output CMYK values having a smaller sum of the CMYK color values. If the determination in step 90 is negative, data associating the input CMYK value with the output CMYK value is output as a toner reduction conversion condition, and the toner reduction conversion condition setting process is terminated.

  As described above, the toner reduction conversion condition setting process is executed a plurality of times using different threshold values, so that a plurality of toner reduction conversion conditions corresponding to different toner reduction degrees are set. In this embodiment, the plurality of toner reduction conversion conditions are respectively stored in the conversion data storage unit 22 (FIG. 1).

  Next, color conversion processing performed by the conversion data setting unit 20 will be described with reference to FIG. In step 100, print parameters designated at the same time when printing (image formation) is instructed via the computer are acquired. In the next step 102, the conversion data for each of the LUTs 16, 18C, 18M, 18Y, and 18K matching the print parameters acquired in step 100 is read from the conversion data storage unit 22, respectively. In step 104, the conversion data for 1D-LUTs 18C, 18M, 18Y, and 18K among the conversion data for each LUT read from the conversion data storage unit 22 is set in the 1D-LUTs 18C, 18M, 18Y, and 18K as they are. .

  In the next step 106, it is determined whether or not an instruction to reduce the toner consumption is instructed. In this embodiment, when the reduction of toner consumption is instructed at the time of printing instruction and the degree of reduction is also instructed, information indicating the instructed toner reduction degree is added to the print parameter acquired by the conversion data setting unit 20. Is done. Therefore, the above determination can be made by determining whether or not information indicating the toner reduction degree is added to the print parameter acquired in step 100. When the determination in step 106 is negative, the process proceeds to step 108, and the conversion data for 3D-LUT 16 read from the conversion data storage unit 22 is set in the 3D-LUT 16 as it is.

  When the process of step 108 is performed, the process proceeds to step 116, and the conversion of the image data 14 input to the color conversion processing unit 10 is performed by each LUT 16, 18C, 18M, 18Y, 18K. In this case, the image data 14 input to the color conversion processing unit 10 is converted into CMYK color values in which L * a * b * values representing the colors of the pixels correspond exactly to the L * a * b * values. The Then, the image data that has undergone the color conversion is output to the image forming engine 12, and the image formation is performed based on the image data, so that the CMYK toner consumption amount is not reduced, but is input to the color conversion processing unit 10. Thus, an image in which the color of the image represented by the image data 14 is accurately reproduced with toner of each color of CMYK is formed.

  On the other hand, if information indicating the designated toner reduction degree is added to the print parameter acquired by the conversion data setting unit 20, the determination in step 106 is affirmed and the process proceeds to step 110, and the designated toner is designated. The conversion condition for toner reduction corresponding to the degree of reduction is also read from the conversion data storage unit 22. In the next step 112, L * a * b * → CMYK conversion specified by the conversion data for 3D-LUT 16 read from the conversion data storage unit 22 and read from the conversion data storage unit 22 in the 3D-LUT 16. The conversion data for 3D-LUT 16 and the conversion conditions for toner reduction are integrated to generate new conversion data so that conversion that integrates CMYK → CMYK conversion defined by the conversion conditions for toner reduction is performed. In step 114, after the new conversion data for 3D-LUT 16 generated in step 112 is set in 3D-LUT 16, in the next step 116, conversion is performed on the image data 14 input to the color conversion processing unit 10. This is performed by the LUTs 16, 18C, 18M, 18Y, and 18K.

  In this case, the image data 14 input to the color conversion processing unit 10 has an L * a * b * color space in which an L * a * b * value representing the color of each pixel is the same as the L * a * b * value. When the above color difference is within the threshold corresponding to the designated toner reduction degree, the color values are converted into CMYK color values in which the total value of the CMYK color values is reduced. Then, the image data that has undergone the color conversion is output to the image forming engine 12, and image formation is performed based on the image data, whereby the color of the image represented by the image data 14 input to the color conversion processing unit 10 is changed. An image reproduced with a slightly different color (a color difference in the L * a * b * color space is within a threshold corresponding to the toner reduction degree) is formed. The total consumption amount of toner of each color of CMYK is reduced.

  In the above description, in the toner reduction conversion condition setting process, it is determined whether or not the input L * a * b * value is a color value within a color region in which the toner amount can be reduced (step 56). In this case, at least one of a K color value as large as possible (output K value) and a Y color value as small as possible (output Y value) within a range where the color difference in the L * a * b * color space is equal to or less than the threshold value is obtained. Although the embodiment has been described, the present invention is not limited to this, and the determination of step 56 is omitted, and the input L * a * b * value corresponding to the input CMYK value can hardly be expected to reduce the toner amount. Even if it is a color value in the color area, the total consumption of toner of each color of CMYK may be aimed at by obtaining at least one of the output K value and the output Y value.

  In the above description, the K color value as large as possible (output K value) or the Y color value as small as possible within the range where the color difference in the L * a * b * color space with respect to the input L * a * b * value is not more than the threshold value. In obtaining (output Y value), the direction in which the amount of increase in the K value or the amount of decrease in the Y value is maximized is searched, and the color difference is within the threshold when the direction is extracted in the search and the K value is within the threshold. In the above description, the output K value or the output Y value in which the amount of increase or the decrease in the Y value is greater than or equal to the predetermined value has been described. However, the present invention is not limited to this, and the output K value or the output Y value is obtained. As shown in FIG. 4C or FIG. 5C, the position of the output L * a * b * value with respect to the position of the input L * a * b * value in the L * a * b * color space. In a direction where the color difference in the L * a * b * color space is less than or equal to the threshold value and the brightness is constant and the saturation decreases (the direction approaching the L * axis) It may reduce the amount of increase in the amount or Y value of K values along the direction so as to obtain the output K value or output Y value equal to or larger than a predetermined value. In this case, although the increase amount of the K value and the decrease amount of the Y value with the same color difference are small (the toner reduction amount is small as compared with the change in the image quality of the formed image), the output K value and the output Y value are further increased. The effect that it can obtain | require easily is acquired.

  Also, in the above, the toner reduction conversion condition obtained by applying the present invention is integrated with the conversion data for the 3D-LUT 16, new conversion data for the 3D-LUT 16 is generated, and the generated new conversion data is set. Although an embodiment has been described in which the CMYK value in which the total consumption of toner of each color of CMYK is reduced is obtained by performing color space conversion of the image data using the 3D-LUT 16, the present invention is not limited to this.

  When generating conversion data (also referred to as a profile) to be set in an LUT that performs color space conversion, for example, as shown in FIG. 7A, a patch (referred to as a color standard) representing a color when the CMYK value is each value is used. Data that associates an L * a * b * value with a CMYK value for each color target by forming CMYK color toners on the paper and measuring the L * a * b * values for each color target on the paper. There is known a method of obtaining the data and using this data as conversion data. However, with this method, it is necessary to form a large number of color targets and to measure L * a * b * values for a large number of color targets, so that it takes a lot of trouble to generate conversion data. There is. For this reason, a method using a printer model is also used as another method for generating conversion data.

  Based on the correspondence relationship between a smaller number of L * a * b * values and CMYK values, the printer model receives an inputted L * a * b * value when the corresponding CMYK value is unknown. This is a program that estimates and outputs CMYK values corresponding to * b * values using various algorithms. As shown in FIG. 7A, conversion data generation using a printer model is performed by forming a color target with known CMYK values on a sheet of paper using toner of each color of CMYK ((1) in FIG. 7A). )), And by measuring the L * a * b * value of each formed color target, a data set for associating the L * a * b * value of each color target with the CMYK value is generated (((A) in FIG. 7A)). 2)). Next, after this data set is set in the printer model (see (3) in FIG. 7A), each L * a * b * value is sequentially input to the printer model, and is output in order from the printer model. Conversion data (profile) is generated by associating the CMYK value with the input L * a * b * value (see (4) in FIG. 7A). Then, by setting the generated conversion data in the LUT (see (5) in FIG. 7A), it is possible to perform color space conversion using the LUT. In the generation of conversion data (profile) using a printer model, the number of necessary color targets can be greatly reduced as compared to the case of generating conversion data (profile) directly from color targets. (Profile) Generation labor can be greatly reduced.

  Here, when the conversion data (profile) is generated by the printer model, as shown in (3) of FIG. 7B, the CMYK values of the respective color targets are determined in accordance with the toner reduction conversion conditions obtained by applying the present invention. Convert to CMYK values for toner reduction, generate a data set that associates the CMYK values for toner reduction after conversion of each color target with L * a * b * values of each color target, and set this data set to the printer model ( (See (4) in FIG. 7 (B)). By generating conversion data (profile) using this printer model (see (5) in FIG. 7 (B)), the input L * a * b * value is obtained. On the other hand, conversion data (profile) for outputting CMYK values for toner reduction is obtained. By performing color space conversion with this conversion data set in the LUT, CMYK values that reduce the total consumption of toner of each color of CMYK may be obtained.

  The printer model is a program having a conversion data generation algorithm. As shown in FIG. 7C, the CMYK value represented by the conversion condition based on the conversion condition for toner reduction obtained by applying the present invention. → An algorithm (CMYK correction algorithm) representing conversion of CMYK values for toner reduction is added to the printer model (see (1) in FIG. 7C), and conversion data is generated using this printer model. Conversion data (profile) that outputs CMYK values for toner reduction with respect to input L * a * b * values can be obtained. By setting this conversion data in the LUT and performing color space conversion, each color of CMYK can be obtained. CMYK values that reduce the total toner consumption are obtained. As described above, the toner reduction conversion condition obtained by applying the present invention is also applicable to the case where toner reduction conversion data (profile) is generated using a printer model.

  Furthermore, in the above description, the aspect in which the present invention is applied in the case of performing color space conversion for converting L * a * b * values into CMYK values on the assumption that image formation is performed using color materials of CMYK colors. However, the input color value and the color value after conversion in the present invention are not limited to the above. For example, on the assumption that image formation is performed using a larger number of color materials, a larger number of CMYK values are used. It is also possible to apply when performing color conversion to convert values. Also in this case, priority is given to the amount of color material having a low degree of involvement in color formation as compared with other color materials, such as Y color in image formation using color materials of CMYK colors. It is desirable to reduce the total color material amount because the total color material amount can be effectively reduced. The invention described in claim 1 includes the above-described embodiment within the scope of the right. In the above description, the L * a * b * color space has been described as an example of the device-independent color space according to the present invention. However, the present invention is not limited to this, and a color appearance model such as a CAM02 space, It is also possible to apply a color space such as CIEXYZ tristimulus values or iCAM.

It is a functional block diagram which shows schematic structure of the color conversion process part which concerns on this embodiment. FIG. 11 is a functional block diagram of a computer that performs toner reduction conversion condition setting processing. 6 is a flowchart showing the contents of a toner reduction conversion condition setting process. It is a diagram for demonstrating the process which decreases Y value. It is a diagram for demonstrating the process which increases K value. It is a flowchart which shows the content of the color conversion process performed in a color conversion part. It is a conceptual diagram for demonstrating the other aspect which implement | achieves reduction of a toner consumption using the conversion conditions for toner reduction.

Explanation of symbols

10 color conversion processing unit 12 image forming engine 14 image data 20 conversion data setting unit 22 conversion data storage unit 30 storage unit 32 conversion condition setting unit for toner reduction

Claims (11)

  Each converted color value corresponding to the color material amount at the time of image formation in the image forming device is within a range where the color difference in the device-independent color space with respect to the input color value is greater than 0 and less than or equal to the threshold value. An image processing apparatus including color conversion means for converting the input color value under the condition that the total amount of each color value is reduced by allowing   The converted CMYK color values corresponding to the color material amounts of the CMYK colors at the time of image formation by the image forming device are within a range in which the color difference on the device-independent color space with respect to the input color values is equal to or less than the threshold value. An image processing apparatus including color conversion means for converting the input color values into CMYK color values so that a combination of K color values is as large as possible and Y color values are as small as possible.   The image processing apparatus according to claim 1, wherein the color conversion unit switches the threshold value in accordance with a specified color material amount reduction degree.   The color conversion unit is configured such that a reproduced color on the device-independent color space corresponding to each color value of CMYK after the conversion corresponds to a reproduced color on the color space of the input color value. 4. The image processing apparatus according to claim 2, wherein the conversion is performed so that the color difference moves in a direction in which the lightness is constant and the saturation decreases within a range where the color difference is equal to or less than a threshold value.   The color conversion unit is configured such that a reproduced color on the device-independent color space corresponding to each color value of CMYK after the conversion corresponds to a reproduced color on the color space of the input color value. 4. The image processing according to claim 2, wherein the conversion is performed so that the change amount of the K color value or the Y color value is as large as possible within a range where the color difference is equal to or less than a threshold value. 5. apparatus.   Each color value after conversion corresponding to the amount of color material at the time of image formation by the image forming device is such that the color difference in the device-independent color space with respect to the input color value is greater than 0 and less than or equal to the threshold value. An image processing program that functions as a color conversion unit that converts the input color value under a condition that the total amount of each color value becomes small within a range.   A range in which the CMYK color values after conversion corresponding to the color material amounts of the CMYK colors at the time of image formation by the image forming device are within the threshold value of the color difference in the device-independent color space with respect to the input color values An image processing program for functioning as color conversion means for converting the input color values into CMYK color values so that a combination of K color values is as large as possible and Y color values are as small as possible. K color as large as possible within a K color value range and a Y color value range that can be taken within a reproduction color range in which the color difference in the color space is not more than a threshold for a specific reproduction color in a device-independent color space After selecting at least one of a color value and a Y color value as small as possible, an unselected color value among CMYK color values corresponds to a reproduction color that has changed with the selection of at least one of a K color value and a Y color value By calculating each of the plurality of reproduction colors in the color space, a value to be obtained is determined within a range where the reproduction color in the color space and the color difference in the color space with respect to the reproduction color are within a threshold value or less. Find the correspondence with the combination of CMYK color values where the color value is as large as possible and the Y color value is as small as possible,
A color conversion condition setting method for setting a color conversion condition for converting an input color value to each color value of CMYK based on the obtained correspondence.   Based on the reproduction color-K information representing the reproduction color in the device-independent color space that can be taken when the K color value among the CMYK color values is a predetermined value, for each case where the K color value is each value, A K color value range that can be taken within a reproduction color range in which the color difference in the color space is equal to or less than a threshold for the specific reproduction color is determined, and an increase amount of the K color value from the determined K color value range 9. The color conversion condition setting method according to claim 8, wherein a K color value having a value equal to or greater than a predetermined value is selected as the K color value as large as possible.   Based on reproduction color-Y information representing the reproduction color in the device-independent color space that can be taken when the Y color value of each color value of CMYK is a predetermined value, for each case where the Y color value is each value, A Y color value range that can be taken within a reproduction color range in which a color difference in the color space is equal to or less than a threshold for the specific reproduction color is determined, and a decrease amount of a Y color value from the determined Y color value range 9. The color conversion condition setting method according to claim 8, wherein a Y color value having a value equal to or greater than a predetermined value is selected as the smallest possible Y color value.   The K color value range or the Y color value range is a direction in which the lightness is constant and the saturation decreases, or the K color value or the Y color value is based on a specific reproduction color in a color space that does not depend on the device. 11. The K color value range or the Y color value range in which the change amount is as large as possible and the color difference on the color space is equal to or less than the threshold value is determined. Color conversion condition setting method.

Images