JP2002344755A - Color correction method - Google Patents

Abstract

(57) [Summary] [PROBLEMS] To automatically set a color correction parameter for a color printer or the like which can reproduce a color image in good color. SOLUTION: A specific color (target color) in HLS space
Is converted to Lab values in the LAB space and set as target values Lab_t. Further, the grid point data of the target color is used as input data, and after processing such as color correction using color correction parameters, HLS → CMY conversion, and image processing, is converted into a Lab value in a LAB space to obtain a predicted output value Lab_e. . Then, the target value Lab_t and the predicted output value Lab
The difference ΔELab with respect to _e is obtained, and the color is adjusted so that the difference ΔELab is minimized while satisfying the constraint condition (H, L, S) _CMY ≤ (H, L + ΔL, S) _CMY that the gradation is not inverted by color correction. Automatically set correction parameters.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color correction method for a printing device such as a color printer, and more particularly, to setting of a color correction parameter.

[0002]

2. Description of the Related Art With the development of digital technology, the printing industry has been greatly influenced, and in recent years, printers have been widely used in ordinary households. And
While user demands for printer performance are increasing, manufacturers are also improving printer engine performance, adding new functions to printer drivers, and improving performance.

[0003]

As described above, in an ink jet printer and the like, new functions are added to a printer driver and the performance is improved in addition to the improvement of the engine performance. Color reproducibility is important, and further improvement in this aspect is desired.

SUMMARY OF THE INVENTION The present invention has been made in view of such a demand, and an object of the present invention is to provide a color correction method for automatically setting a color correction parameter for a color printer or the like which enables good color reproduction of a color image. To provide.

[0005]

In order to achieve the above object, the present invention provides an input color image signal represented in an HLS color space by performing color correction using color correction parameters, space conversion, and predetermined image processing. A color correction method for obtaining an output color image signal based on a CMY color space, wherein a predetermined specific color is set as a target color, and CIELAB is obtained from grid point coordinate data in the HLS color space with the target color.
The Lab value in the space is obtained as the target value, the grid point coordinate data in the HLS color space for this specific color is used as input data, and the CMY colors obtained by performing color correction, color space conversion, and predetermined image processing on this input data From output data based on space to C
A Lab value in IELAB space is obtained as a predicted output value, and a color correction parameter is automatically set so that a difference value between the target value and the predicted output value is reduced.

In the HLS color space, a constraint condition is set that the gradation of the input data before and after the correction by the color correction parameter is not inverted. The constraint condition is satisfied, and the target value and the predicted output are satisfied. The color correction parameters are automatically set so that the difference between the values is minimized.

The target value is a target value for a natural image of a color printer.

In setting the target value for the natural image, a gamut compression process is used.

The target value is a target value for an artificial image of a color printer.

[0010]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a flowchart showing one embodiment of a color correction method according to the present invention.

In this embodiment, an input color image signal is subjected to color conversion to obtain an output color image signal for printing. In such processing, a color image for a color printer or the like for improving color reproduction of a color image is provided. The correction parameters can be automatically set. As a conversion method, a triangular prism interpolation method is used. For this purpose, grid point parameters in an HLS color space (H = hue, L = brightness, S = saturation) are required.

In FIG. 1, first, grid points are created in the HLS color space (step 100).

FIG. 2 shows a color model of the HLS color space. Such a color space forms a bi-hexagonal pyramid subspace with the L axis as the central axis, and one vertex (L
= 1) is white (W) and the other vertex (L = 0) is black (B)
k), and gray between them (0 <L <1).
The hue H is represented by a direction viewed from the L axis, and the L
The directions of the vertices of the hexagonal plane perpendicular to the axis are R (red), Y (yellow), G (green), C (cyan), B
(Blue) and M (magenta). Saturation degree S
Is represented by the distance from the L axis, all grays are S = 0, and the maximum saturation of each hue is S = 1, L = 0.5.

In this embodiment, as the input data, the HLS color space is equally divided to generate the above-mentioned grid points, and the coordinate data (HLS data) of the grid points is used (ie, FIG. 1). Step 100).

The HLS data is corrected using color correction parameters. This is because grid point coordinate data in the HLS color space is converted and grid point coordinate data (H
'L'S' data) (step 10).
1). This H'L'S 'data is in the CMY color space (C = cyan,
(M = magenta, Y = yellow) is converted to coordinate data (CMY data) (step 102), and BG / UCR (Under Color Reduction) processing, γ correction processing, and total amount control processing are performed on the CMY data. C´M´ suitable for printing
Grid point coordinate data (C′M′Y) in Y′K ′ color space (K = black)
'K' data) is generated (step 103).

In this embodiment, in such a color correction method, the color correction parameters used in step 101 can be automatically set. In addition, as the color correction parameters to be set, the processing in step 101 is performed. This is to prevent the inversion of the density of each color when performed.

Therefore, when setting the color correction parameters, a specific color is determined in advance, and
The above steps 100 to 1 using HLS data as input data
Target the color to be reproduced when performing the process of 03
(target) color, and the CIE (Commission
International de l'Eclairage: International Commission on Illumination) LAB
A Lab value (L = brightness, a = substantially red, b = substantially yellow), which is a colorimetric value in space, is set in advance as a target value Lab_t (step 105).

Using the HLS data of this specific color as input data, the above-described steps 101 to 103 are performed, and the obtained C'M'Y'K 'data is prepared in advance. By passing it through a printer simulator for 'M'Y'K' → Lab conversion, the Lab of this C'M'Y'K 'data
The value is obtained as a predicted output value (Estimate Lab) Lab_e (step 104).

Next, a difference value between the obtained predicted output value Lab_e and the target value Lab_t, that is, a color difference value ΔELab is obtained (step 106), and a color correction parameter corresponding to the difference is generated (step 107). The color correction processing is performed on the HLS data of the specific color, which is the input data, by using this, and the color correction parameters are set so that the color difference value ΔELab is minimized. In addition, the following constraints are added to the setting of the color correction parameters. Hereinafter, step 107 will be described in more detail.

The color correction parameter is adjusted by the color difference value ΔELab, whereby the input data, that is, the coordinate data changes in the HLS color space. At this time, the gradation (L) is changed by the change. This is a color correction parameter that does not cause inversion. In other words, the HLS data is corrected by the color correction parameter corresponding to the color difference value ΔELab. At this time, the correction amount of the brightness (gradation, density) L is ΔL, and the data amount of the HLS data before correction is Assuming that (H, L, S), the data amount when the brightness L is corrected by + ΔL is (H, L + ΔL, S). The grid points in the HLS color space before and after the correction are shown in FIG.
, The HLS data on the iso-hue & iso-saturation line (lightness direction) as shown as an example Color correction parameters so that the gradation is not inverted.

Here, the determination as to whether or not to perform the inversion is performed by calculating the CMY data by performing an inverse operation on the predicted output value Lab_e obtained in step 104 and using the CMY data. Data value of (H, L, S) _CMY
Assuming that the correction amount of the brightness by the color correction parameter obtained for the color difference value ΔELab is + ΔL, the range in which the gradation of the output value (CMYK) does not reverse on the equal hue & equal saturation line in the HLS color space is (H, L, S) _CMY ≤ (H, L + ΔL, S) _CMY , and the color difference value ΔELab within a range satisfying such a constraint condition.
Is set to minimize the color correction parameter. As a result, in the same HLS color space, the grid point after correction by the color correction parameter does not invert in the L-axis direction with respect to the grid point before correction, and the grid point parameter is optimized and color reproduction is performed. Will be improved.

FIG. 3 is a flowchart showing one specific example of the method of setting the target value Lab_t in FIG. 1, and this specific example relates to the setting of the target value for a natural image such as a photograph.

The HLS color space is equally divided to obtain target grid point data (HLS data) of a specific color (step 200), which is converted into grid point data (RGB data) of an RGB color space (step 200). 201). Next, assuming a reference monitor, the RGB data is converted to grid point data (X
YZ data) (step 202), and a Lab value in the LAB space is obtained from the XYZ data (step 203). Then, the Lab value is subjected to gamut compression processing using the gamut information of the printer, and the target value L'a'b '(= Lab_
t) is obtained (step 204).

FIG. 4 is an explanatory diagram of the gamut compression method.

This is done in step 202 of FIG.
Since the XYZ data is created assuming the reference monitor, the Lab values obtained in step 203 are those when the saturation range is displayed on the reference monitor. Gamut compression compresses this saturation range to a range suitable for a printer. That is, in FIG. 4, A indicates the limit (outermost) of the saturation range for the lightness L on the reference monitor, and B indicates the limit (outermost) of the saturation range on the printer. is there. The gamut compression is to compress the saturation range A of the reference monitor into the saturation range B of the printer. Each of the saturation ranges A and B has a maximum saturation point.

In this case, the compression direction differs for each hue, and is determined in consideration of the gamut shape for each hue. In other words, the compression direction is kept constant with the maximum saturation point of the printer as the boundary. The positional relationship (brightness) between the monitor and the maximum saturation point of the printer
, The compression direction (θH, θS) is controlled for each hue. Further, the Lab value L'a'b 'of the target color for natural image is set by controlling the direction in which compression is performed on the same hue plane in the Marcel space as much as possible.

As described above, by using the gamut compression method for setting the Lab value of the target color for a natural image, the gamut compression direction (brightness / chroma components) on the highlight side common to all hues is used. Gamut compression direction on the shadow side (lightness
Saturation component) Highlight-side gamut compression direction according to brightness difference between input color space and maximum saturation point in printer (brightness / saturation component) According to brightness difference between input color space and maximum saturation point in printer The gamut compression direction (brightness / saturation components) on the shadow side can be controlled.

FIG. 5 is a flowchart showing another specific example of the method of setting the target value Lab_t in FIG.
This specific example relates to setting a target value for a population image such as graphics.

First, in the HLS color space as shown in FIG. 2 (step 300), six basic hues H are determined. Here, C (H = 180 °), M
(H = 300 °), Y (H = 60 °) are set with the hues of the primary colors of the printer, R (H = 0 °), G (H = 120 °), B
(H = 240 ゜), the hue of the secondary color is set.
Hues of colors other than colors are assigned by linear interpolation (step 301).

Next, setting of the gray axis (S = 0: L axis) and correction of the brightness of the saturated color (S = 1) (mapping to the outermost edge of the printer) are performed. By default, as shown in FIG. 6, on the gray axis, the lightness of gray (paper ground color (white) to Black-max) reproduced by the printer is linearly assigned, and the maximum saturated color (S) of each hue of the monitor is assigned. = 1, L = 0.5), the highest chroma color for each hue of the printer is assigned. For the saturated color (S = 1, L = 0-1) of each hue of the monitor, the chroma of the printer is assigned. In consideration of the characteristics, the gamut outermost colors of the printer are assigned so that the saturation is basically linear (step 302).

As described above, the gray axis (S = 0) and the saturated color of each hue (S = 1, L = 0 to 1) are determined. Referring to the brightness correction for each determined hue, the HLS color space is determined. Saturation S (= 0
1), the brightness is corrected by a linear operation (step 30).
3), set the Lab value of the target color for the artificial image.

As described above, the target color La for artificial images
When the b value is set, the tone reproduction of the saturated color can be adjusted by controlling the lightness of the representative hue with respect to the saturated color in accordance with the color reproduction characteristics of the printer.

More specifically, it is possible to adjust the gradation characteristics for a saturated color (S = 1). Referring to FIG. 5, the maximum saturated color of the monitor for each hue corresponds to the maximum chroma color of the printer, and the saturated color (S = 1, L
= 0 to 1), the outermost colors of the gamut of the printer (the highest chroma colors for each lightness) are assigned and set so that the chroma is linearly arranged from white (paper) to black. A representative color R (H = 0 ゜) defined in the RGB color space that is the input space,
Y (H = 60 °), G (H = 120 °), C (H = 180 °), B (H =
It is possible to adjust gradation characteristics for saturated colors of 240 °) and M (H = 300 °). The gradation characteristic (compression direction) for the saturated color of the representative hue can be adjusted by setting the lightness (L) within a range that does not reverse.

It is possible to adjust the saturation other than the saturated color (S = 1). Colors other than the saturated color (S = 1) are mapped within the reproduction range of the printer according to the saturation S in the HLS color space (RGB base). Representative colors R (H = 0), Y (H = 60), G (H = 120)
゜), C (H = 180 ゜), B (H = 240 ゜), M (H = 300 ゜)
S = 75%, S = 50%, S = 25%
(0-100%) can be set freely, and the saturation can be adjusted.

The hue (Hue) can be adjusted. Referring to FIG. 5, representative hues R (H = 0 °), Y (H = 60 °), and G (H = 120) defined in an RGB color space as an input space.
゜), C (H = 180 ゜), B (H = 240 ゜), M (H = 300 ゜)
Hue (0 ° to 360 °) in the device independent color space (CIELAB or Munsell) corresponding to the hue (hues other than the representative hue are determined by linear interpolation). If Hue = 400 is set only for the primary colors (Y = 0, C = 80, M = 300) in the printer, the hue in the device independent color space of the primary colors (Y, M, C) of the target printer is set. Is automatically set.

[0036]

As described above, according to the present invention,
In a color correction method in which an output color image signal based on a CMY color space is obtained by performing a color correction using a color correction parameter, a space conversion, and a predetermined image processing on an input color image signal represented in an HLS color space, The specified specific color is set as the target color, the Lab value in CIELAB space is calculated as the target value from the grid point coordinate data in the HLS color space for the target color, and the grid point coordinate data in the HLS color space for the specific color is input. CMY obtained by color correction, color space conversion, and predetermined image processing of input data
A Lab value in a CIELAB space is obtained as a predicted output value from output data based on a color space, and the color correction parameters are automatically set so that a difference between a target value and a predicted output value is reduced. Color correction parameters for improving the color reproduction can be automatically set.

Further, according to the present invention, in the HLS color space, the constraint condition that the gradation of the input data before and after the correction by the color correction parameter is not inverted is set, the constraint condition is satisfied, and the target condition is satisfied. Since the color correction parameter is automatically set so that the difference between the value and the predicted output value is minimized, color correction without inversion of gradation is possible, and excellent color reproduction of a color image can be realized.

Further, according to the present invention, since the above-mentioned target value is used as a target value for a natural image of a color printer, good color reproduction can be realized for a natural image such as a photograph.

When setting the target value for the natural image, the gamut compression processing is used, so that the gamut compression direction on the highlight side or the shadow side common to all hues, the input color space, and the maximum saturation point of the printer. It is possible to control the gamut compression directions on the highlight side and the shadow side according to the difference in brightness, and it is possible to adjust gradation reproduction of a saturated color.

Further, according to the present invention, the target value is used as a target value for an artificial image of a color printer.
Good color reproduction can be realized.

[Brief description of the drawings]

FIG. 1 is a flowchart illustrating an embodiment of a color correction method according to the present invention.

FIG. 2 shows a color model of an HLS color space.

FIG. 3 is a flowchart illustrating a specific example of a target value setting method in the embodiment in FIG. 1;

FIG. 4 is an explanatory diagram of a gamut compression method in FIG. 3;

FIG. 5 is a flowchart illustrating another specific example of a method for setting a target value in the embodiment in FIG. 1;

FIG. 6 is a diagram showing a method of allocating saturated colors of respective hues of the monitor in FIG. 5 to the highest saturated color for each hue of the printer.

[Explanation of symbols]

 W White Bk Black R Red Y Yellow G Green C Cyan B Blue M Magenta

Continuation of the front page F term (reference) 2C262 AA24 AB11 BA03 BA09 BC13 BC19 EA04 5B057 CE17 CE18 DB02 DB06 DB09 DC25 5C077 MP08 PP33 PP36 PP37 PP47 PQ20 TT02 5C079 HB02 HB08 HB12 LB02 NA03 NA18 PA03

Claims (5)

[Claims] 1. A color for obtaining an output color image signal based on a CMY color space by performing color correction, space conversion, and predetermined image processing on an input color image signal represented in an HLS color space by color correction parameters. In the correction method, a predetermined specific color is set as a target color, and CIELAB is obtained from grid point coordinate data in the HLS color space of the target color.
A Lab value in the space is obtained as a target value, grid point coordinate data in the HLS color space for the specific color is used as input data, and the input data is subjected to the color correction, the color space conversion, and the predetermined image processing. A Lab value in a CIELAB space is obtained as a predicted output value from the obtained output data based on the CMY color space, and the color correction parameter is automatically set so that a difference between the target value and the predicted output value is reduced. Characteristic color correction method. 2. A constraint condition according to claim 1, wherein in the HLS color space, a constraint condition that the gradation of the input data before and after the correction by the color correction parameter is not inverted is set, and the constraint condition is satisfied. And automatically setting the color correction parameter so that a difference value between the target value and the predicted output value is minimized. 3. The color correction method according to claim 1, wherein the target value is a target value for a natural image of a color printer. 4. The color correction method according to claim 3, wherein a gamut compression process is used when setting the target value for the natural image. 5. The color correction method according to claim 1, wherein the target value is a target value for an artificial image of a color printer.