JP2003259134A - Color image processing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To perform delicate color matching processing and to easily obtain sufficient color reproducibility. <P>SOLUTION: On the basis of a given reference point on the achromatic color axis of a color solid, a new reference point on the achromatic color axis is calculated from the ratio of the luminance component and saturation component of the directional vector of mapping of a color point to be processed by using an inputted parameter and mapping is carried out on the straight line connecting the new reference point and the color point to be processed. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image processing method.

[0002]

2. Description of the Related Art Conventionally, when reproducing the same color image between devices having different color reproduction ranges, such as a color display such as a CRT and a color image forming apparatus such as a color printer or a color copying machine. A color image processing method for performing color mapping processing is adopted. For example, if a color image reproduced by a CRT is reproduced as it is by a color copier, it generally has a bluish tint. Therefore, a color mapping process is applied to the color image to make the tint look similar. be able to.

Here, as the color mapping processing, CIE Lc * advocated by CIE (International Commission on Illumination) is proposed.
h, CIECAM97sJCh, etc., which is a color space that is visually perceptually uniform for humans, is selected as the standard coordinate system, and an algorithm that performs mapping processing between each hue section of the color gamut on the input side and the output side is used. Mapping (Gam
ut Mapping is known (Jan Mo
rovic, "ToDevelop a Univer
sal Gamut Mapping Algorit
hm ", http: //color.derby.a
c. uk. / ~ Jan / gamut_mapping.
html and JP-A-10-285420).

As a color image processing method adopting the gamut mapping, after the lightness widths of both the input and output color gamuts are matched by mapping processing in the lightness direction of the input color gamut, there is no Many methods have been proposed in which some reference point is set on the chromatic axis and the processing target color is mapped within the output system color reproduction range on a straight line connecting the processing target color and the reference point. In this case, the reference point and the intersection of the straight line and both hue sections are used as parameters. In general, the position of the reference point is often determined by the hue section on the input side or output side to be adopted and the color to be processed.

As a typical method for the gamut mapping, CA of LLIN, GCUSP, CARISMA method is used.
LCUSP etc. as SE2 are known. here,
In the LLIN, a point on the achromatic color axis having the same brightness as the color to be processed is adopted as the reference point. Further, in the LCUSP and GCUSP, a point on the achromatic color axis determined from the hue section is adopted as a reference point in the hue section. Further, in the LCUSP and GCUSP, when the lightness of the color to be processed is close to the lightness of the reference point, the mapping in the saturation direction is strong, and when there is a difference in lightness, the influence of the mapping in the lightness direction is strong. The ratio of mapping between the saturation direction and the lightness direction is determined by the reference point and does not depend on the position of the color to be processed.

Also, in another color image processing method in which a reference point is set on the achromatic color axis for mapping, the reference point on the achromatic color axis depending on the shape or positional relationship of the hue cross section on the input side or the output side is set. In most cases, it is adopted or a reference point on the achromatic color axis that depends on the brightness of the color to be processed is adopted.

[0007]

However, in the above-mentioned conventional color image processing method, it is difficult to perform delicate color matching processing, and it may not be possible to easily obtain sufficient color reproducibility.

For example, when the above-mentioned GCUSP or LCUSP is adopted, the color reproducibility of the output image is evaluated, and it is found that the color reproducibility is improved by increasing the saturation-dependent ratio. It is also conceivable to adopt a saturation direction mapping method such as LLIN. However, the L
Since only the saturation direction is mapped in LIN, if the saturation direction mapping method such as LLIN is adopted,
The effect of the mapping in the saturation direction may be too strong. Therefore, it is difficult to perform delicate color matching processing, and sufficient color reproducibility cannot be easily obtained.

Further, when the color to be processed is far from the reference point in the lightness direction, the above-mentioned GCUSP and LCUSP are used.
In the above method, there is no parameter that reduces the influence of mapping in the lightness direction and strengthens the influence in the saturation direction. For this reason, the influence in the lightness direction cannot be reduced and the influence in the saturation direction cannot be adjusted so that sufficient color reproducibility cannot be easily obtained.

The present invention provides a color image processing method which solves the problems of the conventional color image processing method described above and can perform delicate color matching processing and can easily obtain sufficient color reproducibility. The purpose is to provide.

[0011]

To this end, in the color image processing method of the present invention, the color to be processed is processed by using the input parameters based on a given reference point on the achromatic color axis in the color solid. A new reference point on the achromatic color axis is calculated from the ratio of the lightness component and the saturation component in the direction vector of the point mapping, and mapping is performed on the straight line connecting the new reference point and the processing target color point.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 2 is a diagram showing the configuration of the color image processing apparatus according to the first embodiment of the present invention.

In the figure, reference numeral 30 denotes a color image processing apparatus main body, a computer including a computing means 31 such as a CPU and MPU, an internal storage device 32 such as a magnetic disk and a semiconductor memory, and an input / output interface (I / O) 33. Is. Then, in the color image processing apparatus main body 30,
External storage device 35 including a magnetic disk, CD-RO
A recording medium interface 34 that accommodates a removable recording medium such as an M, a DVD, a memory card, and a magneto-optical disk, a communication device 36 for communicating with a device such as a server 37 disposed outside, a keyboard, a mouse, and the like. Input device 41 and display device 4 such as CRT and liquid crystal display
2 is connected.

In the present embodiment, the color image processing apparatus uses the color reproduction in order to perform the optimum color reproduction even when the image forming apparatus having different color reproducibility such as the color reproduction range is used. This is a device that performs color management as image processing. For example, when the same color image is reproduced between devices having different color reproduction ranges such as a color display such as a CRT and a color image forming apparatus such as a color printer or a color copying machine, the color image processing is performed. The device can be used for color management. Then, as the color management, a color image processing method for performing color mapping processing is adopted.

Here, the internal storage device 32 stores a computer program for causing the calculating means 31 to execute the series of operations of the color image processing. Then, the arithmetic means 31 is adapted to execute the color image processing by sequentially reading and executing the computer programs stored in the internal storage device 32.

The recording medium interface 34 is also provided.
Contains a removable recording medium that stores the computer program, various data, and the like. And
The computer program stored in the recording medium,
Various data and the like are read out, transferred to the external storage device 35, and stored therein. As a result, the computer program can be transferred from the external storage device 35 to the internal storage device 32 when necessary, such as when the color image processing apparatus main body 30 is activated. The computer program may be directly transferred from the recording medium to the internal storage device 32. Further, the result of the color image processing executed by the arithmetic means 31 can also be stored in the internal storage device 32, the external storage device 35, or the recording medium contained in the recording medium interface 34.

Further, by communicating with the server 37 by the communication device 36, the computer program, various data and the like received from the server 37 can be stored in the internal storage device 32 or the external storage device 35. Further, the result of the color image processing executed by the calculation means 31 can be transmitted to the server 37.

Next, the operation of the color image processing apparatus having the above construction will be described.

FIG. 1 is a diagram showing the operation of the color image processing apparatus according to the first embodiment of the present invention, and FIG. 3 is a chart showing the color image processing method according to the first embodiment of the present invention. In FIG. 3, the vertical axis represents the brightness (Lig
htness), and the horizontal axis shows saturation (Chro
ma) is taken.

FIG. 1 shows a configuration of the color image processing apparatus main body 30 from the viewpoint of operation or function. The color image processing apparatus main body 30 includes a reference point calculation processing unit 11 and a reference point storage unit. Have twelve. Here, in the gamut mapping process, the reference point calculation processing unit 11 performs the linear mapping reference point Pc [c, on the achromatic color axis of the color solid.
L] coordinates are given, the reference point Pc [c, L]
Based on, the new reference point Pc ′ [c ′, L ′] is calculated with the given parameter w.
Further, the reference point storage unit 12 includes the reference point calculation processing unit 11 described above.
The new reference point Pc '[c', L '] calculated by
Stores the coordinates of. Then, the new reference point Pc '
A gamut mapping process is performed using [c ', L'].

In the present embodiment, the gamut mapping process itself is a known color mapping process as described in "Prior Art", and therefore its description is omitted.

Here, the reference point Pc [c, L] input to the reference point calculation unit 11 is a cylindrical coordinate representing a CIELa * b * color coordinate system which is a color space (color solid) established by CIE. Is defined by the lightness L and the saturation c in the Lch coordinate system. The color coordinate system is CIEL.
It may be something other than a * b *, but in the present embodiment, the case of using the CIE La * b * color coordinate system will be described.

In this case, the input reference point Pc
The coordinates [c, L] are, for example, the coordinates of the midpoint between the highest lightness color point and the lowest lightness color point in the intersection plane area in a device such as a color display or a color image forming apparatus to be simulated. In this case, first, of the color points existing on the L axis in the intersecting surface area, the achromatic color point having the highest lightness, that is, the highest lightness color point and the color points existing on the L axis, The achromatic color point having the lowest lightness, that is, the lowest lightness color point is obtained. Subsequently, the midpoint of the line segment connecting the highest lightness color point and the lowest lightness color point is obtained, and the coordinates of the midpoint are input as the coordinates of the reference point Pc [c, L].

In the present embodiment, in order to perform mapping using the calculated new reference points, the hue cross section given by the gamut mapping process is
It is necessary to satisfy the conditions of (c). (A) The achromatic range in the input system color reproduction range and the output system color reproduction range match. (B) The highest lightness color point on the achromatic color axis (L axis) is the highest lightness point in the input system color reproduction range and the output system color reproduction range. (C) The lowest lightness color point on the achromatic color axis is the lowest lightness point in the input system color reproduction range and the output system color reproduction range.

Further, in the present embodiment, the gamut mapping process used must satisfy the following conditions (d) to (f). (D) A mapping using a hue section. (E) It is a linear mapping having a reference point on the achromatic color axis, that is, the saturation of the reference point is 0. (F) The gamut mapping processing alone does not perform mapping in the hue direction.

Here, the hue section is a section of the same hue including the achromatic axis in the color reproduction range.

The linear mapping having a reference point on the achromatic axis is specifically expressed by the following equations (1) and (2).

[0029]

[Equation 1] Here, as shown in FIG. 3, the coordinates of the point indicating the processing target color (processing target color point) are set to (Co, Lo), and the coordinates of the reference point Pc [c, L] are set to (Cc, Lc). To do. A straight line connecting the points (Co, Lo) indicating the color to be processed and the reference points (Cc, Lc) intersects the input color reproduction range and the output color reproduction range, that is, two points, that is, 2 points. The coordinates of the intersections are (Co0, Lo0) and (Co1, Lo1), and (Cr0, Lr0) and (C
r1, Lr1). In addition, the point that the subscript is the same on the boundary between the input color gamut and the output color gamut is
This is a point that should be addressed by mapping. The point to be linearly mapped is (Cr, Lr).

Subsequently, the reference point calculation unit 11 can bias the ratio of the lightness component and the saturation component in the direction vector of the mapping of the color point to be processed in the saturation direction by using the input parameters. Calculate a new reference point on the possible achromatic axis. In this case, as the input parameters,
Using the given parameter w that is a parameter representing the size of the mapping in the saturation direction, the reference point Pc [c, L]
From the new reference point Pc ′ [c ′, L ′] to the following equation (3)
Calculate by

[0031]

[Equation 2] The coordinates of the point indicating the color to be processed are (Co, Lo). Then, the calculated new reference point Pc ′ [c ′,
The linear mapping using L ′] is the same as the linear mapping when the saturation coordinate c = Cc = 0 of the original reference point is changed as shown in the following equation (4) as shown in FIG. is there.

[0032]

[Equation 3] Here, the calculated new reference point Pc ′ [c ′, L ′]
The coordinates of are stored in the reference point storage unit 12. Then, the mapping is performed on the straight line connecting the new reference point Pc ′ and the point indicating the processing target color.

As described above, in the present embodiment, in the gamut mapping process, when the coordinates of the reference point Pc [c, L] of the linear mapping are given on the achromatic axis, the reference point Pc [c] is given. , L] based on the given parameter w
A new reference point Pc ′ [c ′, L ′] is calculated by Therefore, in the color image processing method of the present embodiment, as compared with the conventional Gamut mapping process of simply performing a linear mapping by giving a reference point on the achromatic color axis, the lightness component of the direction vector of the mapping is compared. The ratio of the saturation component can be biased in the saturation direction as a whole, and delicate color matching processing can be easily performed to obtain sufficient color reproducibility.

The degree of adjustment of the ratio between the lightness component and the saturation component of the direction vector of the map can be determined by adjusting only the parameter w. That is, when the ratio is w = 0, the ratio is the original reference point Pc.
The ratio is the same as [c, L], and when w = 1, the ratio is the same as the mapping in the saturation direction when a point on the achromatic color axis having the same brightness as the color to be processed is the reference point. , W between 0 and 1, an intermediate ratio is obtained. Therefore, in the present embodiment, in the gamut mapping process,
The intermediate effect between the method of mapping only in the saturation direction such as LLIN described in “Prior Art” and the method of simultaneously mapping lightness and saturation toward a reference point on an achromatic color axis like GCUSP has an effect. Obtainable. Therefore, in the color image processing method according to the present embodiment, delicate color matching processing can be performed, and sufficient color reproducibility can be easily obtained. Further, the color image processing method according to the present embodiment is Any mapping method can be adopted as long as it is a mapping method that sets a reference point on the achromatic axis. Further, if the mapping by the adopted mapping method is continuous, it is guaranteed that the mapping by the color image processing method in the present embodiment is also continuous.

In this embodiment, the equation (1)
Although the case where the linear mapping is represented by (2) and (2) has been described, the straight line connecting the points (Co, Lo) indicating the processing target color and the reference points (Cc, Lc) is the input system color reproduction range and the output system color. Reproduction range and two intersecting points (Co
0, Lo0) and (Co1, Lo1), and (Cr0,
The linear mapping can also be expressed by the following equations (5) and (6) using Lr0) and (Cr1, Lr1) as parameters.

[0036]

[Equation 4] Next, a second embodiment of the present invention will be described. The configuration of the color image processing apparatus according to the present embodiment is similar to that of the first embodiment, and therefore will be described with reference to FIG.

FIG. 4 is a diagram showing the operation of the color image processing apparatus according to the second embodiment of the present invention.

FIG. 4 shows the configuration of the color image processing apparatus main body 30 viewed from the viewpoint of operation or function.
In the gamut mapping process, when the coordinates of the reference point Pc [c, L] of the linear mapping is given on the achromatic axis, the parameter w is given based on the reference point Pc [c, L]. The reference point calculation processing unit calculates a new reference point Pc ′ [c ′, L]. Reference numeral 22 is an input-system color gamut boundary storage unit that stores representative points on the boundary of the input-system color gamut, and 23 is a new reference point Pc ′ [c calculated by the reference-point calculation processing unit 21. , L] is a boundary parameter calculation processing unit that calculates a parameter for identifying whether a linear mapping can be defined for an arbitrary processing target color within the input system color reproduction range. Further, reference numeral 24 is a reference point correction processing unit for clipping the reference point Pc ″ [c ”, L] in an area where a linear mapping can be defined for an arbitrary processing target color within the input system color reproduction range, Reference numeral 25 is an achromatic color axis reference point calculation processing unit that sets a new reference point Pc ″ ′ [c ”, L ′] on the achromatic color axis, and 26 is the achromatic color axis reference point calculation processing unit 25. New reference point Pc "'calculated by
The reference point storage unit stores [c ", L '].

Here, the reference point Pc [L, c] input to the reference point calculation processing unit 21 is a cylindrical coordinate representing the CIELa * b * color coordinate system, as in the first embodiment. Is defined by the lightness L and the saturation c in the Lch coordinate system. The color coordinate system may be other than CIELa * b *, but in the present embodiment, CIELa * b * is used.
The case where the * b * color coordinate system is used will be described.

In the present embodiment, in order to perform mapping using the calculated new reference point, the hue cross section given by the gamut mapping processing is
It is necessary to satisfy the condition of. (A) The achromatic range in the input system color reproduction range and the output system color reproduction range match.

Further, in the present embodiment, the gamut mapping process used must satisfy the following conditions (b) and (c). (B) It is a linear mapping having a reference point on the achromatic color axis, that is, the saturation of the reference point is 0. (C) The gamut mapping processing alone does not perform mapping in the hue direction.

The linear mapping having a reference point on the achromatic axis is specifically expressed by the following equations (7) and (8).

[0043]

[Equation 5] Here, the coordinates of the point indicating the processing target color are (Co, Lo), and the coordinates of the reference point Pc [c, L] are (Cc, Lc). A straight line connecting the points (Co, Lo) indicating the color to be processed and the reference points (Cc, Lc) intersects the input color reproduction range and the output color reproduction range, that is, two points, that is, 2 points. The coordinates of the intersection points are (Co0, Lo
0) and (Co1, Lo1), and (Cr0, Lr0)
And (Cr1, Lr1). It should be noted that points on the boundary between the input color gamut and the output color gamut that have the same subscript are points to be mapped. The point to be linearly mapped is (Cr, Lr).

Next, the reference point calculation processing unit 21 uses the input parameter based on the given reference point Pc [c, L] to determine the saturation from negative infinity. The first step of calculating a new reference point that changes to 0 is performed. In this case, a given parameter w, which is a parameter representing the size of the mapping in the saturation direction, is used as the input parameter, and a new reference point Pc ′ [c ′, from the reference point Pc [c, L] is used. L ′] is calculated by the following equation (9).

[0045]

[Equation 6] The input system color gamut boundary storage unit 22 stores representative points on the input system color gamut boundary. In this case, as a representative point, at least the highest brightness color point (white point) on the achromatic axis
[0, Lw], lowest brightness color point (black point) on the achromatic axis
[0, Lb], a point on the color gamut boundary that is connected from the white point [c0, L0], and a point on the color gamut boundary that is connected from the black point [c1, L1] are stored. Need to be.

Then, the boundary parameter calculation processing section 23
Is a tangent line of the input system color reproduction range where the inclination at the highest lightness point on the achromatic color axis is the minimum, and the inclination at the lowest lightness point on the achromatic color axis is the maximum, based on the information indicating the input system gamut boundary. The second step of calculating the tangent line of the input system color reproduction range is performed. In this case, the input system gamut boundary storage unit 2
The white point [0, Lw] and the black point [0, Lb] stored in 2 and the points [c0, L0] and [c1, L1] on the color gamut boundary connected from the white point and the black point. Is the new reference point Pc ′ [c ′, L] calculated by the reference point calculation unit 21 based on the above, in a region where a linear mapping can be defined for any processing target color within the input system color reproduction range? Parameters [Aw, Lw] and [Ab, Lb] for identifying whether or not to calculate are calculated by the following equations (10) and (11).

[0047]

[Equation 7] Then, when the new reference point does not exist in the area surrounded by the two tangent lines and the achromatic color axis, the reference point correction processing unit 24 moves the new reference point in the saturation direction to move the two reference points. The third step of moving into the area surrounded by the tangent line and the achromatic axis is executed. In this case, the parameter [A
Based on w, Lw], [Ab, Lb], a new reference point Pc ′ calculated by the reference point calculation processing unit 21.
A process is performed so that [c ′, L] is contained within a region that can be defined by a linear mapping regarding an arbitrary processing target color within the input system color reproduction range. In addition, a new reference point Pc ′ [c ′, L] point that is not contained in the area is processed so as to be contained in the area by clipping in the saturation direction. Then, the reference point correction processing unit 24
Executes the following equation (12) or (13), and then
Formula (14) or (15) is executed.

[0048]

[Equation 8] As a result, a new reference point Pc ″ [C ″, L] is calculated.

Next, the achromatic color reference point calculation processing section 25.
Is a fourth step of calculating a new reference point on the achromatic color axis that can give the same lightness as the linear map based on the new reference point in the area surrounded by the two tangent lines and the achromatic color axis. To execute. In this case, the new reference point Pc ″ [C ”calculated by the reference point correction processing unit 24,
L], the lightness of the reference point on the achromatic color axis that gives the same effect as the linear mapping with respect to the color to be processed is expressed by the following equation (16).
Calculate by

[0050]

[Equation 9] In this case, the coordinates of the processing target color are (Co, Lo).

Subsequently, the reference point storage unit 26 stores the coordinates of the new reference point Pc "'[C", L'] calculated by the achromatic color axial reference point calculation processing unit 25.

Then, based on the new reference point Pc "'[C", L'] stored in the reference point storage unit 26, the gamut mapping process is continuously executed.

As described above, in the present embodiment, unlike the first embodiment, a linear mapping based on the reference point is not required to directly calculate a new reference point, but an arbitrary mapping within the input system color reproduction range is possible. It is arranged to check whether a mapping can be defined for the color to be processed. Therefore, the gamut mapping process can be applied even when the input-system color reproduction range is such that the highest-brightness color point and the lowest-brightness color point are located on a portion other than the achromatic color axis. This allows
It is a mapping method such as GCUSP described in "Prior Art" that performs non-linear compression depending on saturation in the lightness direction, and the highest lightness color point and the lowest lightness color point are not always on the achromatic axis. A mapping method that generates an input system color reproduction range that does not exist can also be stably adopted.

Further, as the color image processing method according to the present embodiment, any mapping method can be adopted as long as it is a mapping method for setting a reference point on the achromatic color axis. Furthermore, if the mapping by the adopted mapping method is continuous, it is guaranteed that the mapping by the color image processing method of this embodiment is also continuous.

In this embodiment, the equation (7)
Although the case where the linear mapping is represented by (8) and (8) has been described, the straight line connecting the points (Co, Lo) indicating the processing target color and the reference points (Cc, Lc) is the input system color reproduction range and the output system color. Reproduction range and two intersecting points (Co
0, Lo0) and (Co1, Lo1), and (Cr0,
The linear mapping can also be expressed by the following equations (17) and (18) using Lr0) and (Cr1, Lr1) as parameters.

[0056]

[Equation 10] Further, the present invention is not limited to the above-mentioned embodiments, and various modifications can be made based on the spirit of the present invention, and they are not excluded from the scope of the present invention.

[0057]

As described in detail above, according to the present invention, in the color image processing method, the input parameters are used based on the given reference point on the achromatic color axis in the color solid. , A new reference point on the achromatic color axis is calculated by the ratio of the lightness component and the saturation component in the direction vector of the mapping of the processing target color point, and on a straight line connecting the new reference point and the processing target color point. Do the mapping.

In this case, delicate color matching processing can be performed, and sufficient color reproducibility can be easily obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing an operation of a color image processing apparatus according to a first embodiment of the present invention.

FIG. 2 is a diagram showing a configuration of a color image processing apparatus according to the first embodiment of the present invention.

FIG. 3 is a chart showing a color image processing method according to the first embodiment of the present invention.

FIG. 4 is a diagram showing an operation of a color image processing device according to a second embodiment of the present invention.

[Explanation of symbols]

11 Reference point calculation processing unit 12 Reference point storage

   ─────────────────────────────────────────────────── ─── Continued front page    F-term (reference) 5B057 BA25 CA01 CA08 CA12 CA16                       CB01 CB08 CB12 CB16 CE17                       CE18                 5C077 LL19 MP08 PP36 PQ12 PQ23                 5C079 HB08 HB12 KA01 LA03 MA04                       MA11 NA03 PA03 PA05

Claims (2)

[Claims] 1. Based on a given reference point on an achromatic color axis in (a) a color solid, using the input parameters,
A new reference point on the achromatic color axis is calculated from the ratio of the lightness component and the saturation component in the direction vector of the mapping of the processing target color point, and (b) a straight line connecting the new reference point and the processing target color point. A color image processing method characterized by performing the above mapping. 2. A color image processing method for: (a) mapping on a straight line connecting a reference point in a color solid and a color point to be processed; (b) inputting based on the given reference point. 0 from negative infinity to saturation using parameters
The first step to calculate a new reference point that changes to
(C) The tangent line of the input system color reproduction range where the inclination at the highest lightness point on the achromatic color axis is the minimum and the inclination at the lowest lightness point on the achromatic color axis is the maximum, based on the information indicating the input system gamut boundary. Second calculation of the tangent line of the input system color reproduction range
And (d) the new reference point calculated in the first step is not within the area surrounded by the two tangent lines calculated in the second step and the achromatic axis, the new reference point A third step of moving the point in the saturation direction into the area surrounded by the two tangent lines and the achromatic color axis; and (e) the new step in the area surrounded by the two tangent lines and the achromatic color axis. And a fourth step of calculating a new reference point on the achromatic color axis that can give the same lightness as the linear mapping with the reference point as a reference.