KR19990042024A - Image-dependent color gamut mapping - Google Patents

Abstract

The image-dependent gamut mapping method is for printing a color image displayed on a monitor to a more similar color image on a printer. The image-dependent color gamut mapping method is used to convert an RGB space of a color image displayed on a monitor into a predetermined color space. The method may further include adjusting a brightness value of a color image in a range of brightness values of a printer in the color space, and including a gamut between a monitor and a printer to detect a change in saturation value according to a change in brightness value generated in the adjusting step. Determining an anchor point according to the presence or absence, performing color gamut mapping based on the determined anchor point, and printing a color image by determining the number of dots for dyes of the printer according to the gamut mapping result. There is a gist.

Description

Image-dependent color gamut mapping

The present invention relates to an image-dependent gamut mapping method, and more particularly, to an image-dependent gamut mapping method using variable anchor points based on a change in brightness value.

In order to meet the needs of users, the functions of color display devices are diversified, high quality and low price.

However, since the range of colors that these devices can support is device dependent, a phenomenon in which colors reproduced for the same color image appear differently occurs.

For example, when the same color image is output through the monitor and the printer device at the same time, the reproduced two images show a significant difference.

The cause of this phenomenon can be attributed to the influence of ambient lighting and the difference in color correction between devices. However, the biggest influence is caused by the difference in the range of color support, that is, the color gamut between devices. Can be.

When a color image of a device with a wide gamut such as a monitor is reproduced by a device with a narrow gamut such as a printer, colors that fall outside the printer's gamut are generated, which causes color differences because they are not reproduced in the original accurate colors.

Therefore, it is necessary to process colors outside the gamut into the gamut to reduce the perceived color difference.

This conversion of the color of the color image into a color gamut that can be reproduced by the color output device is called gamut mapping.

The gamut mapping method is largely divided into image independent gamut mapping and image dependent gamut mapping.

First, image independent gamut mapping means mapping between the gamut of the original device and the gamut of the device to be reproduced.

The image-dependent gamut mapping refers to the mapping between the gamut of a given color image and the gamut of a device to be reproduced, and the mapping process may proceed differently depending on the content of a given color image.

There are several color gamut mapping methods according to the prior art.

Commonly used mapping methods include color gamut clipping, linear compression, and nonlinear compression.

Here, the gamut cutting method is a method of mapping the color outside the gamut into the boundary region of the gamut to be mapped.

Using this method, since no change occurs in the color inside the color gamut to be mapped, non-linearity with the color inside the color gamut occurs when the color outside the gamut is reproduced.

However, there is an advantage of maintaining the maximum saturation value of the mapped gamut.

In addition, the linear compression method is a method of linearly converting the maximum and minimum of the brightness or saturation values of the original color gamut to the maximum and minimum of the brightness or saturation values of the color gamut to be mapped.

The nonlinear compression method finds a nonlinear function used for mapping and maps it according to that function.

Hereinafter, the color gamut mapping method according to the prior art will be described.

1 is a view showing the brightness mapping and saturation mapping of the gamut mapping method according to the prior art, Figure 1a is a function of the brightness or saturation mapping, Figure 1b is a diagram showing the saturation mapping while maintaining the brightness value.

Here, the dotted line shown in FIG. 1A represents linear compression, the solid line is nonlinear compression, and the bright part is a method of maintaining a slight clipping and the remaining part of the original brightness, and FIG. 1B is linear at a constant color angle and brightness value. Shows how to compress.

1A and 1B map the saturation of the color to be expressed without setting an anchor point to the color boundary point of the output device by simple cutting without changing brightness and saturation.

2A to 2C are diagrams illustrating various mapping methods in which brightness and saturation are changed according to the related art.

Here, FIG. 2A is a method of mapping toward the center of brightness of the device gamut to which colors outside the gamut are to be reproduced, and FIG. 2B is toward the center if a value outside of the gamut is greater than the center of brightness of the gamut of the device to be reproduced. 2C is a method of linear mapping at a constant brightness value, and FIG. 2C is based on the brightness of the maximum saturation value according to each color angle, and if the value is closer than this value, maps toward the brightness value of the maximum saturation value. As shown in 6b, the linear mapping is performed at a constant brightness value.

6A and 6B select an anchor point as the median value 50 of the CIELab coordinate system and map the color to be expressed as a linear gamut boundary value for this intermediate value. FIG. 6A shows a monitor outside the lozenge printer gamut. Maps the gamut to the median value 50 of the CIELab coordinate system, and FIG. 6B maps the color gamut to the median value in the color gamut having a brightness greater than or equal to the median value 50 of the CIELab coordinate system and to any color gamut having a brightness less than or equal to the median value. Is mapped by simple color gamut as it is.

FIG. 6C illustrates that the anchor point is changed according to each color, and the subsequent process is the same as that of FIG. 6B.

However, the image independent gamut mapping method according to the prior art has a problem in that it is impossible to consider compliance of the human vision to the color image shown because the same gamut mapping is performed for all color images.

Accordingly, an object of the present invention is to provide an image dependent color gamut mapping method that enables a printer to print a color image displayed on a monitor as a similar color image.

FIG. 1-A diagram illustrating the brightness mapping and the saturation mapping of the gamut mapping method according to the prior art, FIG. 1A-A function of brightness or chroma mapping, and FIG. 1B-A diagram showing the mapping of saturation while maintaining the brightness value.

2A to 2C are diagrams illustrating various mapping methods of varying brightness and saturation according to the related art.

3 is a view showing a coloring sequence by dither matrices used in a typical printer.

FIG. 4 is a diagram illustrating a tetrahedron divided in one hexahedron during a forward tetrahedral interpolation process according to the present invention. FIG. 4A is a diagram illustrating a tetrahedral position in a hexahedron, and FIG. 4B is a diagram illustrating an equation of a state of each tetrahedron.

FIG. 5 is a diagram illustrating a tetrahedron coinciding in two different color spaces during color gamut mapping according to the present invention, and FIG. 5A is a diagram illustrating an RGB space, and FIG. 5B is a diagram illustrating a CIELab space.

6 is a diagram showing an intersection state between a triangular boundary and a straight line connecting two points according to the present invention;

FIG. 7 is a diagram illustrating color gamut mapping according to the present invention, and FIG. 7a is a diagram illustrating linear brightness mapping, and FIG. 7b is a diagram illustrating changing gamut and anchor point by an algorithm according to the present invention, and FIG. A diagram showing gamut mapping by gamut and anchor point

FIG. 8 is a diagram illustrating color gamut mapping according to the present invention, and FIG. 8A is a diagram illustrating that a narrow gamut is included in a wide gamut, and FIG. 8B is a diagram illustrating when a narrow gamut is not included in a wide gamut.

A feature of the image-dependent color gamut mapping method according to the present invention for achieving the above object is, in the image-dependent color gamut mapping method for converting the RGB space of the color image displayed on the monitor to a predetermined color space, the color in the color space Adjusting the brightness value of the image to the range of the brightness value of the printer, and to determine the anchor point in accordance with the presence or absence between the gamut of the monitor and the printer to detect a change in the saturation value according to the change in the brightness value generated in the adjustment step And color gamut mapping the color image according to the determined anchor point, and printing the color image by determining the number of dots for the dye of the printer according to the gamut mapping result.

Hereinafter, an image dependent color gamut mapping method according to the present invention will be described with reference to the accompanying drawings.

3 is a diagram illustrating a coloring sequence by dither matrices used in a typical printer.

4 is a view showing a tetrahedron divided in one hexahedron during the forward tetrahedral interpolation process according to the present invention, Figure 4a is a view showing the position of the tetrahedron in a cube, Figure 4b is a diagram showing the state of each tetrahedron FIG. 5 is a diagram illustrating a tetrahedron coinciding in two different color spaces in color gamut mapping according to the present invention, FIG. 5A is a diagram showing an RGB space, and FIG. 5B is a diagram showing a CIELab space.

FIG. 6 is a diagram illustrating an intersection state between a triangular boundary and a straight line connecting two points according to the present invention. FIG.

FIG. 7 is a diagram illustrating color gamut mapping according to the present invention, FIG. 7A is a diagram illustrating linear brightness mapping, and FIG. 7B is a view illustrating changing gamut and anchor point by an algorithm according to the present invention, and FIG. 7C is changed. A diagram showing gamut mapping by gamut and anchor point.

FIG. 8 is a diagram illustrating color gamut mapping according to the present invention, and FIG. 8A is a view showing that a narrow color gamut is included in a wide color gamut, and FIG. 8B is a view showing when a narrow color gamut is not included in a wide gamut.

The image dependent color gamut mapping method according to the present invention configured as described above will be described in detail with reference to the accompanying drawings.

First, in order to know the color characteristics of the monitor and the printer, the CIEXYZ values are measured with different measuring devices for each color.

At this time, the color is fixed.

That is, the sample patch of the monitor has 5 RGB values in 5 steps, and the number of sample patches of 5 * 5 * 5 (= 125) is made in the center of the monitor, and then the CIEXYZ value is measured. .

And the sample patch of the printer is made using the dither matrix of the AM (Amplitude Modification) method as shown in FIG.

The CMY values are made in four steps through the dither matrix to make 5 * 5 * 5 (= 125) patches, and a square color sample that can be output to a printer is measured to measure CIEXYZ values.

In addition, the gamut boundary value necessary for determining an anchor point in gamut mapping is detected from the measured values of the monitor and the printer, and is set based on the maximum saturation value that can be represented according to the hue angle. .

In addition, the values representing the measured values and gamut boundary values of the monitor and printer sample patches can be easily used in a mapping algorithm that creates and reproduces a look-up table.

In this case, the gamut boundary value is a maximum saturation value according to the brightness of the search result by searching for the brightness and saturation values of the measured colors.

The measured values have different white values because different measuring devices are used.

Therefore, a white shift converts colors based on the white of the monitor into colors based on the white of the printer.

Thereafter, the CIEXYZ values measured from the sample patches of the monitor and the printer are converted into CIELab values through Equations 1 to 3 below.

here, ego, ego, to be.

In this case, in order to convert the measured CIEXYZ value into a CIELab value, the RGB values displayed on the monitor are converted into CIELab values through Equations 4 and 5 below of forward tetrahedral interpolation.

That is, it converts RGB space, which is the color space of the monitor, to CIELab space where color gamut mapping will occur.

The forward tetrahedral interpolation refers to a change from a device dependent color space to a device independent color space. In the RGB coordinate system, one hexahedron is divided into six tetrahedrons around an axis connected to the furthest vertex as shown in FIGS. 4A and 4B. Take form.

First, the weights between the points to be interpolated in the RGB space and the vertex vector of the tetrahedron are calculated using Equation 4.

This is because the interpolation method obtains the interpolated points by applying the weight obtained in one color space in another color space.

At this time, in order for the point to be interpolated to belong to the tetrahedron, the weight must satisfy the following condition.

ego, ego, And to be.

The weight that satisfies the above condition calculates the value of the interpolated point using Equation 5 below used for the measured value corresponding to each vertex of the tetrahedron.

Here, the calculated parameter follows a value as shown in FIGS. 5A and 5B.

In addition, in order to compensate for the color outside the gamut in the gamut mapping, the intersection point between the boundary of the gamut and the straight line is calculated using Equations 6 and 7 as shown in FIG. 6.

That is, as shown in FIG. 6, a straight line connecting a point representing an arbitrary color outside the gamut and an achromatic point may have a predetermined color angle, and it is assumed that the triangle belongs to the boundary of the gamut.

The process of obtaining the intersection point is as follows.

here, If the value of is not 0, it means that the set straight line and the triangle cross each other.

Intersection points satisfying the above condition are calculated using Equations 8 to 9 below.

Since the brightness value of the color image converted to the CIELab value exceeds the maximum and minimum ranges of the measured brightness value of the printer, the brightness value of the image should be adjusted to the brightness value range of the printer. To compensate for the change in brightness that occurs when the adjustment occurs, adjust the brightness of the monitor's gamut threshold as well.

As shown in FIG. 7A, the brightness value compensates the maximum (white) and minimum (black) values of the original image by matching the maximum (white, background of paper) and minimum (black) values of the printer.

Where max Original L * represents the RGB brightness of the monitor and Reproduction L * represents the brightness of the printer.

As shown in FIG. 7B, the color image compensated for the brightness value changes the brightness of the wide color gamut when the linear brightness changes to compensate for the difference between the brightness and the original image. This branch changes the position of the vertex, and a straight line with a point representing the changed maximum chroma value and a point representing the maximum chroma value in the narrow gamut.

Therefore, the anchor point, which is the point on the achromatic axis where these straight lines intersect, is transformed.

That is, FIG. 7C obtains a lower brightness value than the mapping using the anchor point where the mapped color is not changed.

Accordingly, in the gamut mapping, an appropriate criterion for the dark region of the adjusted brightness value cannot be set so that only a half of the brightness range of the narrow gamut is set to the dark region.

In addition, as shown in FIG. 8A, when a full coverage relationship between color gamuts is established, an anchor point, which is a point of an achromatic axis intersecting a straight line connecting the point indicating the changed maximum saturation value and the point indicating the maximum saturation value in the measured color gamut of the printer, is shown. To indicate that color gamut mapping occurs.

8B illustrates that gamut mapping occurs when the incomplete inclusion relationship between gamuts is satisfied when the changed maximum saturation value meets the measured gamut value of the printer.

Through the above process, the CIELab space is converted into the CMY space of the printer to detect the CMY value, which determines the number of dots for each color dye through reverse tetrahedral interpolation.

Reverse tetrahedral interpolation here refers to the conversion of device independent space into device dependent color space.

That is, the following equations (10) and (11) are used to convert the intersection points into CMY values, which are device dependent spaces.

Where the weight is , , Intersection if , , Is the intersection of the line and the triangle.

Accordingly, the printer prints a color image more similar to the color image of the monitor by determining the number of dots for the dye according to the converted CMY value.

As described above, the image-dependent color gamut mapping method according to the present invention interpolates the brightness and saturation of the color image displayed on the monitor through forward tetrahedral interpolation and reverse tetrahedral interpolation to determine the anchor point, thereby gamut mapping and thus more similar color image in the printer. Can print the effect.

Claims (4)

In the image-dependent color gamut mapping method for converting the RGB space of the color image displayed on the monitor to a predetermined color space, Adjusting a brightness value of a color image in the color space to a brightness value range of a printer; Determining an anchor point according to whether the monitor and the printer include color gamuts in order to detect a change in chroma value according to a change in brightness value generated in the adjusting step; Color gamut mapping a color image according to the determined anchor point; And determining the number of dots for the dye of the printer according to the color gamut mapping result to print a color image. The method of claim 1, In the anchor point determining step, when the gamut of the printer is included in the gamut of the monitor, the anchor point is determined by an achromatic axis point where a straight line connecting a point representing the maximum saturation value of the printer and a point representing the maximum saturation value of the converted monitor is crossed. An image dependent color gamut mapping method, further comprising the steps. The method of claim 1, The determining of the anchor point may further include determining an anchor point at a point where the maximum gamut value of the converted monitor meets the measured gamut value of the printer when the gamut of the printer is not included in the gamut of the monitor. Image dependent color gamut mapping method. The method of claim 1, The gamut mapping step is characterized in that the gamut above the determined anchor point is gamut mapped to the anchor point, and the gamut above the anchor point is chroma-cut.