JP2007060558A - Color processing method and apparatus therefor, and image processing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology of attaining improvement of total image quality including reduction of the grainness, extension of the color gamut, the color constancy, and the color stability. <P>SOLUTION: A color processing method disclosed herein is configured such that when color separation data corresponding to a coloring material including chromatic colors and a black color different from the density are produced, the color separation data corresponding to the coloring material are produced at a high brightness region between a highest brightness and a brightness corresponding to a highest saturation reproducible by the coloring material at a hue corresponding to the coloring material, and color separation data corresponding to the chromatic colors and a black color at a low density are produced at an intermediate brightness region with a brightness lower than that of the high brightness region. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to color processing for generating color separation data corresponding to a color material including black and chromatic colors and different densities.

  When the color material used by the recording apparatus is, for example, cyan C, magenta M, yellow Y, and black K ink or toner, image processing for generating print data uses RGB image data corresponding to each color material C, M, Y, K Color separation into data. This color separation is usually performed using a look-up table (LUT). That is, this LUT defines a method for generating C, M, Y, and K data that indicates how to combine color materials with respect to a combination of R, G, and B data.

  A recording apparatus using three colors of CMY or four colors of CMYK reduces lightness by using complementary colors on the lower lightness side than the maximum saturation portion. However, if complementary colors are used, the saturation tends to decrease. In addition, since three color materials having greatly different hues are mixed, it is not possible to suppress color misregistration in the hue angle direction due to variations in the ink discharge amount and the toner transfer amount.

  On the other hand, in the printing field, in order to expand the color gamut that can be reproduced by the recording apparatus, the use of special colors in addition to the commonly used basic colors C, M, and Y and K is known. For example, special colors such as red R, orange O, green G, blue B, and violet V are used for the purpose of reproducing higher saturation.

  Japanese Patent Laid-Open No. 2003-011432 discloses that, when a spot color is used in a hue of a spot color, color separation is performed using both the spot color and the base color when the saturation is lower than a combination of the base colors in a certain lightness region. To do. For example, when using high lightness green, the color gamut is expanded by using a combination of cyan and yellow in the low lightness region of the hue of green. However, since a mixed color of two basic colors is used for reproduction of the low brightness region, it is impossible to suppress a color shift in the hue angle direction due to fluctuations in the ink discharge amount and the toner transfer amount. Further, it is necessary to use complementary colors in order to improve the graininess in the low brightness region, and the problem of lowering the saturation cannot be solved by using complementary colors.

  Japanese Laid-Open Patent Publication No. 2005-059360 describes a color separation example in which a dark portion of a special color hue is formed using a complementary color of a basic color in a low brightness area of the special color hue. In other words, the low brightness range is reproduced by using the complementary color of the basic color for the hue of the special color and the complementary color of the special color for the hue of the basic color. However, it cannot be said that the color mixture of complementary colors lowers the saturation and sufficiently expands the color gamut. In addition, it is not preferable to use a spot color with high saturation as a complementary color from the viewpoint of graininess.

  On the other hand, in order to improve image quality such as graininess, color constancy, and color stability, it is known to use a gray color material in addition to the commonly used basic color and black color material. Japanese Patent Application Laid-Open No. 2004-250519 describes an ink set including both spot colors and gray. However, specific color reproduction and color separation methods are not described, and the deterioration of color stability due to an increase in the number of colors is unsolved.

Japanese Patent Laid-Open No. 2003-011432 JP 2005-059360 JP JP 2004-250519 A

  The inventor has developed a method for appropriately combining chromatic color materials including special colors and gray color materials, thereby reducing graininess and expanding the color gamut, as well as color constancy and color stability. We tried to improve the total image quality including

  An object of the present invention is to achieve a total improvement in image quality including reduction of graininess, expansion of color gamut, color constancy, and color stability.

  The present invention has the following configuration as one means for achieving the above object.

  The color processing according to the present invention can reproduce the color material in the hue corresponding to the color material of the chromatic color when generating color separation data corresponding to the color material including black color having a different chromatic color and density. The high lightness region from the lightness corresponding to the highest saturation to the highest lightness region generates the color separation data corresponding to the color material, and the intermediate lightness region having a lightness lower than the high lightness region is the chromatic color and the low density. The color separation data corresponding to the black color material is generated.

  In the image processing according to the present invention, when a table for generating color separation data is created, a part of the signal values of the basic three colors is set to black so that the signal value of one of the three basic colors becomes zero. The signal value of any one of the remaining three colors among the three basic colors is replaced with a signal value of a special color, the signal value of the black color is subjected to grayscale separation, and the result obtained by the replacement and the grayscale separation is obtained. Signal values of one basic color, one special color, and two black colors are stored in the table.

  According to the present invention, it is possible to achieve a total improvement in image quality including reduction of graininess, expansion of color gamut, color constancy, and color stability.

  Hereinafter, image processing according to an embodiment of the present invention will be described in detail with reference to the drawings.

  In the following, it is assumed that the printer performs printing with eight color materials of red R, green G, blue B, and gray Gy in addition to the basic colors of cyan C, magenta M, yellow Y and black K. .

[Use of gray color material]
First, how to use R, Gy, and K color materials, in other words, how to set color separation data corresponding to R, Gy, K color materials at LUT grid points, and, as a result, How to improve image quality will be described.

  Figure 1 shows the amount of each color material used between CMYK four colors, white, red, and black. The vertical axis represents the amount of color material used (e.g., equivalent to ink discharge amount or toner transfer amount). Indicates the signal level. That is, FIG. 1 shows a part of a table for color-converting RGB data to CMYK data.

  FIG. 2 is a diagram showing the amount of R, Gy, and K color material used between white, red, and black in Example 1, as in FIG. FIG. 2 shows a part of a table for color-converting RGB data to CMYKRGBGy data, as in FIG.

  First, the high brightness range from white W (R, G, B = 255, 255, 255) to red R (R, G, B = 255, 0, 0) changes the amount of R color material used. Realized. Therefore, pure red (R, G, B = 255, 0, 0) is reproduced only with the R color material (high brightness range).

  Next, in the brightness range from red (R, G, B = 255, 0, 0) to black (R, G, B = 0, 0, 0), first the G color material is applied to the R color material. In addition, the brightness is reduced and the color is reproduced (first medium brightness range). When the total usage amount of the color material reaches the limit value (indicated by reference numeral 201), the usage amount of the R color material is decreased and the usage amount of the Gy color material is increased (second intermediate lightness range).

  In addition, when the Gy color material reaches the specified usage amount (indicated by reference numeral 202), the Gy color material usage amount is reduced (added to both the R and Gy color materials) and the K color material is added. Reduce and reproduce the vicinity of black K (low brightness range).

  In this way, in the middle lightness region, by using only the R color material as the chromatic color and adding the Gy color material, the color misregistration in the hue angle direction due to the variation in the ink discharge amount and the toner transfer amount can be eliminated. Also, in other hues, color misregistration can be reduced by keeping the number of chromatic color materials used to a minimum.

  In the high brightness range, the combination of the M and Y color materials shown in FIG. 1 is replaced with the R color material. Furthermore, in the low brightness range, the combination of M, Y, C, and K color materials is replaced with the combination of R, Gy, and K color materials. Therefore, the saturation and graininess are improved as compared with the case of CMYK four colors.

  Furthermore, it is possible to balance saturation and graininess by adjusting the ratio of R, Gy, and K color materials. That is, by reducing the Gy color material and increasing the R and K color materials, the saturation can be improved instead of reducing the graininess. Of course, the reverse is also possible.

  The image quality improvement using the Gy color material has been described above for the red hue, but the image quality can also be improved by using the Gy color material for the other special color hues.

  The “special color” refers to a color having a hue different from the hues of yellow Y, magenta M, and cyan C, which are basic colors of the color material. In addition, the lightness and saturation that can be expressed on the recording medium can be expressed on the recording medium by combining two of the basic colorants of magenta M, yellow Y, and cyan C. It is preferable that the brightness and saturation be higher.

[Print system overview]
FIG. 3 is a block diagram illustrating a configuration example of a printing system according to the embodiment, which includes a printer 102 using spot color ink and a host device (computer or image processing device) 101.

  A printer (recording apparatus) 102 performs printing with C, M, Y, and K inks, R, G, and B special color inks, and Gy ink. Therefore, the recording head 10 that discharges these eight colors of ink is used.

  On the operating system (OS) of the host device 101, the application program 1 and the printer driver 11 are run. The application program 1 creates and edits image data to be printed by the printer 102.

  Image data or image data before editing can be input to the host device 101 via various media. For example, JPEG image data captured by a digital camera may be input via a memory card, TIFF image data read by a scanner, or image data recorded on a CDROM may be input. Of course, it may be downloaded from a server or Web site arranged on a network such as the Internet. The host device 101 displays the input image data on a monitor (not shown). The user of the host apparatus 101 edits and processes the image data using the application program 1 and instructs printing. In response to this instruction, the application program 1 (or OS) converts the image data into, for example, sRGB standard image data (8 bits for each color) and passes it to the printer driver 11.

  The printer driver 11 performs color gamut mapping as the pre-stage process 2 on the input image data. In other words, the pre-processing 2 uses a three-dimensional LUT (3DLUT) indicating the relationship between the color gamut reproduced by the image data of the sRGB standard and the color gamut that can be reproduced by the printer 102 (printer color gamut), and interpolation calculation. Convert RGB data to RGB data in the printer color gamut.

  The post-processing 3 obtains color separation data YMCKRGBGy (8 bits for each color) corresponding to the ink combination that reproduces the color represented by the color gamut mapped RGB data. This processing is performed by using 3DLUT and interpolation calculation in the same manner as the previous processing 2. Details of the post-processing 3 will be described later.

  The gamma correction 4 performs gamma correction for converting the gradation value for each color data of the color separation data obtained by the post-processing 3. Specifically, the color separation data is converted to correspond to the gradation characteristics of the printer 102 using a one-dimensional LUT (1DLUT) corresponding to the gradation characteristics of each color ink of the printer 102.

  Halftoning 5 performs quantization to convert each color 8-bit color separation data YMCKRGBGy into 4-bit data using an error diffusion method. This 4-bit data becomes an index for indicating a dot arrangement pattern in the printer 102.

  The print data generation 6 creates print data obtained by adding print control information to 4-bit index data.

  The processes of the application program 1 and the printer driver 11 described above are realized by the CPU executing these programs. The program is loaded into a RAM from a ROM or hard disk (not shown) and executed. During execution, RAM is used as a work area for the CPU.

  On the other hand, the printer 102 performs dot arrangement patterning processing 7 and mask data conversion processing 8 on the print data input from the host device 101.

  The dot arrangement patterning process 7 performs dot arrangement according to a dot arrangement pattern corresponding to 4-bit index data (tone value information) for each pixel of the actual print image. In other words, a dot arrangement pattern corresponding to the gradation value of each pixel is assigned to each pixel represented by 4-bit data, and dot on / off for each of the multiple areas in the pixel is defined. Dispose data of “or” “0”.

  The mask data conversion process 8 performs a mask process on the 1-bit ejection data. That is, the ejection data for each scan for completing the recording of a scanning region (hereinafter referred to as “band”) having a predetermined width in the sub-scanning direction by the recording head 10 is a mask corresponding to each scanning. It is generated by processing using. The ejection data Y, M, C, K, R, G, B, and Gy for each scan are sent to the head drive circuit 9 at an appropriate timing. The head drive circuit 9 drives the recording head 10 so as to eject each ink according to the ejection data.

  Note that the above-described dot arrangement patterning process and mask data conversion process in the printer 102 are executed under the control of the CPU constituting the control unit of the printer 102 using a dedicated hardware circuit. Note that the CPU of the printer 102 may perform the above processing according to a program, or the printer driver 11 of the host device 101 may perform the above processing.

  The printer 102 uses red R, green G, and blue B as special color inks. It is preferable that these special color inks can express higher saturation and lightness than a secondary color formed by mixing two of the basic color inks of yellow Y, magenta M, and cyan C. However, the present invention is not limited to this, and it is sufficient that at least lightness higher than that of the secondary color can be expressed. That is, the “special color” suitable for the present embodiment is a color gamut expressed on the recording medium by a combination of any two of the basic color materials of magenta M, yellow Y, and cyan C. Is a color having high brightness and showing a hue angle in the color gamut. Furthermore, it is still preferable if the color can express a saturation higher than the color gamut.

  In this embodiment, Ky ink diluted with a predetermined ratio is used as Gy ink. However, the present invention is not limited to this, as long as it has at least a density lower than K ink and sufficiently low saturation. Good. That is, the same color material as that of the K ink may be used, a different color material may be used, or the color may be adjusted using a chromatic color material.

  Although a printer using ink as a color material will be described as an example, the present embodiment can also be applied to a printer, a copier or the like using another color material such as toner.

  The “pixel” is a minimum unit capable of gradation expression, and is a minimum unit to be subjected to multi-value data image processing, the above-described pre-processing, post-processing, gamma correction, halftoning, and the like. In addition, one pixel in the dot arrangement patterning process corresponds to a 2 × 4 pixel pattern, and each pixel in this pixel is called an area. An area is the smallest unit that can be defined as dot on / off.

  In this connection, “image data” in the pre-stage process, the post-stage process, and the gamma correction represents a set of pixels to be processed, and each pixel is, for example, data having a gradation value of 8 bits. In addition, “pixel data” in halftoning represents the pixel data itself to be processed. By halftoning, the above 8-bit pixel data is converted into pixel data (index data) containing 4-bit gradation values. Is done.

[Post-processing]
FIG. 4 is a diagram showing the concept of 3DLUT used by post-processing 3.

  As described above, the post-process 3 reads the CMYKRGBGy data of the grid points with respect to the input RGB values of the 3DLUT shown in FIG. 4 configured by slicing the RGB space, performs interpolation processing, and converts the input RGB data to the color separation data CMYKRGBGy It is processing to convert to.

  Color separation data for realizing the characteristics shown in FIG. 2 is stored in each grid point of 3DLUT. Thereby, a desired gradation value or color gamut can be realized.

  For example, as in FIG. 2, if attention is paid to the hue of red R, the line segment from the white W grid point (255, 255, 255) to the red R grid point (255, 0, 0) shown in FIG. Data of YMCKRGBGy = (0, 0, 0, 0, r, 0, 0, 0) and 0 ≦ r ≦ 255 is arranged at the upper grid point (high brightness region). Furthermore, first, YMCKRGBGy = (0, 0, 0, 0, r, 0) representing the mixed color of red R and gray Gy at the grid points on the line segment from red R (255, 0, 0) toward black K , 0, gy), r = 255, and 0 ≦ gy <255 (first intermediate lightness range).

  Then, after the grid point that has reached the limit value of the total ink usage indicated by reference numeral 201 in FIG. 2, YMCKRGBGy = (0, 0, 0, 0, r, 0, 0, gy), 0 <r < Data of 255, 0 <Gy <255 is arranged (second intermediate brightness range). Then, after the grid point where the usage amount of the gray Gy indicated by reference numeral 202 in FIG. 2 reaches a predetermined value, YMCKRGBGy = (0, 0, 0, k, r, 0, 0, gy), 0 ≦ k ≦ Data of 255, 0 ≦ r <255, 0 ≦ gy <255 is arranged (low brightness region).

  For other hues Y, M, R, G, and B, data may be arranged at each lattice point in the same manner. Further, data other than Y, M, C, R, G, and B can be similarly arranged for hues that cannot be simply shown in FIG.

  That is, the conversion relationship indicated by 3DLUT represents the content of the implementation in the generation of color separation data in the first embodiment. Of course, the generation of color separation data is not limited to the form using 3DLUT as shown in FIG. For example, the color separation data may be calculated by performing an operation on the input RGB data in accordance with the above expression representing the conversion relationship each time.

[Create LUT]
The CMYKRGBGy eight-color LUT may be converted from a CMY three-color LUT created by a known method using a specific conversion formula. FIG. 5 is a diagram conceptually showing conversion from a three-color LUT to an eight-color LUT.

  As shown in Fig. 5, GCR (Gray Color Remove) processing is performed to reduce the amount of color material used for one of the three CMY colors, and the amount used for one of the remaining two colors (M in Fig. 5) Replace it with a spot color (R in Fig. 5) to make it 0. The total amount of color materials used in the color separation data obtained by combining the three basic colors, that is, one basic color, one special color, and black K, is reduced as compared with the CMY three-color data. Therefore, the K data is divided into two colors, black K and gray Gy, and the total amount of color material used is adjusted to a value close to that of the CMY three colors. As a result, a LUT composed of a total of four colors or less including one basic color, one special color, and two black colors in the entire color gamut is created.

[Reduction of color shift]
At the time of image output, the color material usage varies due to various factors such as temperature and color material aging, and a deviation from the color material usage specified in the post-processing 3 occurs.

  FIG. 6 is a conceptual diagram of a color shift caused by a shift in the amount of color material used in the high saturation range. FIG. 6 shows color reproduction by MY two colors, but if a complementary color is used to reduce the brightness, the color misregistration area is further expanded.

  FIG. 7 is a conceptual diagram of color misregistration caused by misregistration in the amount of color material used in Example 1. In the first embodiment, since a chromatic color material having a hue close to the reproduced color is used, the amount of color shift in the hue angle direction is reduced. In addition, since an achromatic gray color material is used to reduce the lightness, the color misregistration range does not expand even in a low lightness region.

  Further, the host device 101 that executes the processing illustrated in FIG. 3 is not limited to a computer, and the printer 102 may execute each processing of the host device 101, for example.

  In this manner, color separation data corresponding to a color material set including a plurality of black color materials having different densities and used in the recording apparatus is generated for a predetermined color gamut. At this time, color separation data corresponding to the chromatic color material and the plurality of black color materials is generated for a lightness region lower than the maximum saturation portion of the hue of the chromatic color material in the predetermined color gamut. In this way, by reproducing the low brightness area of the hue of the chromatic color material using the chromatic color, gray and black, the use of complementary colors in the low brightness area is suppressed, and in addition to saturation and graininess, High color stability can be obtained. Further, by suppressing the use of a color material other than the chromatic color, it is possible to suppress color misregistration in the hue angle direction due to fluctuations in the ink discharge amount and toner transfer amount, and to obtain high color stability. Further, by using a gray color material, high graininess can be obtained even in a situation where no complementary color is used. In addition, the color gamut is expanded by using a gray color material instead of the complementary color.

[Other embodiments]
Note that the present invention can be applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, and a printer), and a device (for example, a copying machine and a facsimile device) including a single device. You may apply to.

  Another object of the present invention is to supply a storage medium (recording medium) that records software for realizing the functions of the above-described embodiments to a system or apparatus, and a computer (CPU or MPU) of the system or apparatus executes the software. Is also achieved. In this case, the software itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the software constitutes the present invention.

  In addition, the above functions are not only realized by the execution of the software, but an operating system (OS) running on a computer performs part or all of the actual processing according to the instructions of the software, and thereby the above functions This includes the case where is realized.

  In addition, the software is written in a function expansion card or unit memory connected to the computer, and the CPU of the card or unit performs part or all of the actual processing according to instructions of the software, thereby This includes the case where is realized.

  When the present invention is applied to the storage medium, the storage medium stores software corresponding to the flowchart described above.

A diagram showing the amount of each colorant used between white, red and black in CMYK four colors, In the examples, the amount of R, Gy, K color material used between white-red-black is shown in the same manner as FIG. A block diagram showing a configuration example of a print system of an embodiment, The figure which shows the concept of 3DLUT which the latter part processing uses, A diagram conceptually showing the conversion from LUT for three colors to LUT for eight colors, Conceptual diagram of color misregistration caused by misregistration of color material usage in high saturation range, It is a conceptual diagram of the color shift produced by the shift | offset | difference of the usage-amount of a coloring material in an Example.

Claims (9)

A color processing method for generating color separation data corresponding to a color material including black ink having different chromatic color and density,
In the hue corresponding to the color material of the chromatic color, the high brightness range from the brightness corresponding to the highest chroma that the color material can reproduce to the highest brightness level generates the color separation data corresponding to the color material,
The color processing method according to claim 1, wherein the color separation data corresponding to the chromatic color and the black color material having a low density is generated in an intermediate lightness region lower than the high lightness region. In the generation of the color separation data in the intermediate lightness range, the use of the chromatic color material is maximized with a decrease in lightness until the total use amount of the color material is reached. Increase the amount of color material used,
2. The amount of use of the chromatic color material is decreased and the amount of use of the low-density black color material is increased with the decrease in brightness after reaching the limit. The described color processing method.   Further, the generation of the color separation data in the low brightness range lower than the intermediate brightness range reduces the use amount of the chromatic color and the low brightness black color material as the brightness decreases, 3. The color processing method according to claim 1, wherein the usage amount of the black color material is increased.   The color according to any one of claims 1 to 3, wherein the generation of the color separation data in the high lightness region increases a usage amount of the chromatic color material with a decrease in lightness. Processing method. An image processing method for creating a table for generating color separation data,
A part of the signal values of the three basic colors is replaced with a black signal value so that the signal value of one of the three basic colors becomes zero, and the signal of any of the remaining two of the three basic colors Replace the value with the signal value of the spot color, separate the gray signal value into shades,
A color processing method comprising: storing in the table signal values of the basic color, the special color, and the black color obtained as a result of the replacement and density separation. Input means for inputting color data;
Conversion means for converting the color data into color separation data corresponding to a color material including a chromatic color and a black color;
In the hue corresponding to the color material of the chromatic color, the high-brightness region from the lightness corresponding to the highest chroma that can be reproduced by the color material to the high lightness range of the highest lightness is obtained by converting the color separation data corresponding to the color material. A color processing apparatus that generates and generates the color separation data corresponding to the chromatic color and the black color material having a low density in the intermediate lightness region having a lightness lower than that of the high lightness region.   5. A program that controls an image processing device to execute the color processing according to claim 1.   6. A program for controlling an image processing apparatus to execute the image processing according to claim 5.   9. A recording medium on which the program according to claim 7 or 8 is recorded.

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