JP2002152540A - Apparatus and method for processing color image signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an apparatus and a method for processing a color image signal capable of improving color reproducibility by executing an optimum color conversion in all hues. SOLUTION: An equal hue surface converter 3 regulates the lightness for the color image signal hue regulated by a hue converter 2, then color converts the signal along a conversion route corresponding to the signal, and converts the signal into a color image signal in a color reproducing area of the output unit. For the color image signal having, for example, higher lightness than that of the maximum chroma color of the color reproducing area of the output unit and out of the color reproducing area of the output unit, a conversion route along a straight line formed at a prescribed angle from a chroma axis out of the color reproducing area of the output unit and along a straight line for maintaining the lightness of the intersection between the previous line and the color reproducing profile of the output unit in the color reproducing area of the output unit is set. Thus, the color conversion is executed according to the conversion route bent outside the color reproducing area to prevent decrease in the lightness and decrease in the chroma, and the optimum color conversion is executed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a color image signal processing apparatus and a color image signal processing method for converting an input color image signal into an output color image signal within a color gamut reproducible by an output device. is there.

[0002]

2. Description of the Related Art Various devices have been developed as output devices that can handle color digital images, such as display devices such as CRTs and color LCDs, and printing devices such as printers. In these output devices, colors to be reproduced and a reproducible color range (color gamut) are different due to a difference in each output method. Therefore, a color conversion process is performed to convert an input color image signal into an output color image signal of a color in a color reproduction range in which color reproduction can be performed by the output device.

Conventionally, various methods have been attempted as color conversion methods for converting the color of an input color image signal into a color reproducible in an output device. For example, the color output by inputting a certain color image signal is measured, and the color image signal used for the output is associated with the input color image signal corresponding to the measured color, and the color image signal is There is a method of converting to an output color image signal that is associated when input. However, there are cases where the color indicated by the input color image signal cannot be reproduced by the output device. In such a case, it is necessary to perform a process of replacing the image signal input in some form with a color reproducible by the output device.

As a process of converting the input color image signal into an output color image signal of a color reproducible by an output device, for example, a method of changing one of saturation and brightness has been considered. However, in the method of changing the saturation while fixing the brightness, a bright color with high brightness loses its color and becomes white. Also, with the method of changing the brightness by fixing the saturation, the bright colors become extremely dark,
Color reproducibility decreases.

Further, as described in, for example, Japanese Patent Publication No. 6-36548, when the range of saturation and brightness of an input color image signal is larger than the reproduction range of an output device, the input color image signal is There is a technique for performing compression mapping within the color reproduction range of an output device so as to preserve gradation for both saturation and brightness. However, in this method, when the processing of compressing the saturation and the brightness between the greatly different color reproducible regions is performed, there is a problem that the color having the high brightness and the high saturation is reduced in both the brightness and the saturation.

In order to solve such a problem, for example, as described in Japanese Patent Application Laid-Open No. 2000-184222, the output device is divided into a high lightness region and a low lightness region from the maximum chroma lightness of the color reproduction range of the output device. A method has been proposed in which a target point is set in both regions and compression mapping is performed in a linear direction toward the target point. FIG. 12 is a diagram illustrating an example of a conventional color conversion process. As shown in FIG. 12A, in an area where the brightness is higher than the brightness of the maximum saturation in the color reproduction range of the output device, the brightness is stored and the saturation is changed, and the input color image signal is output from the output device. It is converted into an output color image signal within the color reproduction range. Also, in an area where the brightness is lower than the maximum chroma lightness of the color reproduction range of the output device, the lightness point on the lightness axis corresponding to the lightness of the maximum chroma color of the color reproduction range of the output device is set as the target point. The transformation is in the direction toward the point. As a result, a decrease in lightness is suppressed for high-brightness colors, and for low-brightness colors, the brightness and saturation are changed to prevent a decrease in saturation to some extent, thereby improving apparent color reproducibility.

However, in a region where the brightness is higher than the maximum saturation brightness of the color gamut of the output device, the brightness is preserved and the saturation is changed. Therefore, depending on the shape of the color gamut, as described above. However, there is a problem that the color saturation is greatly reduced. For example, FIG. 12B shows a yellow hue plane, and shows an example of a color gamut of an input color image signal and a color reproduction range of an output device. When the above-described color conversion is performed between the color gamut and the color gamut having such a shape, the saturation of high-saturation yellow greatly decreases, and the reproducibility deteriorates.

[0010] Of course, if the color conversion is performed so as to preserve the saturation in an area having a higher lightness than the lightness of the maximum saturation in the color reproduction range of the output device, in the case shown in FIG. Can be converted. However, in that case, a significant decrease in brightness occurs in other colors.
Thus, color conversion in a single direction cannot improve color reproducibility in all hues.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and provides a color image signal processing apparatus capable of performing optimum color conversion in all hues and improving color reproducibility. It is an object of the present invention to provide a color image signal processing method.

[0010]

SUMMARY OF THE INVENTION The present invention relates to a color image signal processing apparatus and a color image signal processing method for converting an input color image signal into an output color image signal within a color reproduction range reproducible by an output device. A conversion path is set according to the input color image signal. At this time, when the conversion path passes through the predetermined position, the conversion direction is changed at the predetermined position to set the conversion path. Then, the color image signal input along the set conversion path is converted.

For example, a predetermined position is defined as the outline of a color gamut reproducible by an output device, and outside the color gamut, a straight line having an angle with a saturation axis corresponding to the hue of an input color image signal is defined as a conversion path. In the reproduction range, a conversion path in which the conversion direction is changed can be set outside the color reproduction range so that a straight line parallel to the saturation axis is used as the conversion path. Thus, for example, when the high-saturation color has high lightness, the angle with the saturation axis is increased to prevent a decrease in saturation, and when the high-saturation color has low lightness, the angle with the saturation axis is reduced. It is possible to prevent a decrease in lightness and also prevent a decrease in lightness within the color reproduction range. And if such angle adjustment is performed for each hue,
Optimum color conversion can be performed for all hues, and color reproducibility can be improved.

Of course, it is also possible to divide the area according to the brightness of the maximum chroma color of the color gamut of the output device and to set different conversion paths in each area, as in the prior art. A color transformation that changes the direction of the path at a given location can be applied to one or both.

Also, a color gamut on the input side is defined in a color space,
By setting a conversion path for converting the color gamut on the input side to a color gamut reproducible by the output device, the conversion accuracy in the color gamut on the input side can be improved. At this time, a conversion path for converting the color gamut on the input side to a color reproduction gamut reproducible by the output device can be extended and used as a conversion path other than the color gamut on the input side. Alternatively, it is possible to change the direction of the conversion path as the outline and the predetermined position of the color gamut on the input side, and set another conversion path outside the color gamut on the input side.

[0014]

FIG. 1 is a block diagram showing an embodiment of the present invention. In the figure, 1 is an input color space conversion unit,
Reference numeral 2 denotes a hue conversion unit; 3, an in-hue in-plane conversion unit; and 4, an output color space conversion unit. The input color space conversion unit 1 converts a color space of an input color image signal into a device-independent color space used in subsequent processing. For example, if an input color image signal is an RGB color space and a device-independent color space is a CIELab color space, a C
Performs color space conversion to the IELab color space. If the input color image signal is a signal in a color space that does not depend on the device, the input color space conversion unit 1 is unnecessary.

The hue converter 2 shifts the hue of the input color image signal. For example, when an input color image signal is in a device-dependent color space, for example, an RGB color space, a signal of (R, G, B) = (0, 0, 100) indicates “blue”. When converted to an independent color space, the hue is not necessarily "blue". Similarly, even when the hue is “blue” in a device-independent color space, the color reproduced in the output device may not always be “blue”. For example, if the input color image signal is
Even a signal in a device-independent color space is at least affected by color misregistration in this output device. In order to correct such a color shift between the input side and the output side, the hue converter 2 shifts the hue of the input color image signal. Further, by performing such a hue shift, it is possible to convert the color into a more suitable color when performing color conversion to the color reproduction range of the output device. The moving amount and moving direction of the hue can be set for each hue. For example, in the case of blue, it is better to move largely to green. Also,
It is better to move the color closer to blue for green or cyan and to red for magenta. Yellow and red do not need to be moved as much.

The in-plane conversion unit 3 converts the color image signal after the hue conversion into an output color image signal within a color reproduction range reproducible by the output device. At this time, a conversion path corresponding to the hue of the input color image signal after the hue conversion is set in the hue plane, and the color conversion is performed along the conversion path. As an example of the conversion path, here, as an example, the area is divided according to the brightness of the maximum chroma color of the color reproduction range of the output device, and a conversion path suitable for each area is set. For areas where the brightness is lower than the maximum chroma color of the output device color gamut, the target point is the point on the lightness axis corresponding to the maximum chroma color brightness of the output device color gamut. A straight line connecting the color and the target point is set as a conversion path.

In a region where the brightness is higher than the brightness of the maximum chroma color in the color reproduction range of the output device, a conversion path is set in a direction in which the lightness is preserved in the color reproduction range of the output device. Outside the color reproduction range of the output device, first, a straight line is set so that the angle formed by the saturation axis is an angle corresponding to the hue of the color image signal. Further, a straight line in a direction in which lightness is preserved is set in the color gamut of the output device from a point where the straight line intersects the outline of the color gamut of the output device. Then, a conversion path that is formed by these two straight lines and that is bent outside the color gamut of the output device is set. As described above, in the present invention, in addition to setting the conversion path in a certain direction, in order to further enhance color reproducibility, color conversion is performed along the conversion path using the conversion path whose direction has been changed in the middle.

The conversion processing along the above-described conversion path converts, for example, a color gamut of an input color image signal to a color reproduction range of an output device. For example, if the input color image signal is generated using a display device such as a CRT or an LCD, a color in a color gamut that can be expressed on the display device is used as the color image signal. When an image input device such as a scanner or a digital camera is used, the color is expressed in the color gamut of the device.
Therefore, when a color image signal created using such an input device is input, the color gamut becomes a partial space in a device-independent color space. Also, the color gamut of the output device described above is also a partial space in a color space independent of the device, and the conversion may be performed between the two according to the above-described conversion path.

It is preferable that the equi-hue in-plane conversion unit 3 adjusts the color gamut on the input side in the lightness direction in advance. For example, the lightness range in the color gamut on the input side is matched with the lightness range of the color reproduction gamut of the output device, or the lightness of the maximum chroma color for each hue in the color gamut on the input side is determined by the color reproduction range of the output device in that lightness. It is preferable to perform a process such as approaching the lightness of the maximum chroma color. As a result, the two color gamuts are close to each other, and more optimal color conversion can be performed.

The output color space conversion section 4 converts the output color image signal color-converted by the in-plane conversion unit 3 into a color space depending on the output device, for example, a CMYK color space. Of course, if the output color image signal can be in a device-independent color space, the output color space conversion unit 4 is not necessary.

FIG. 2 is a flowchart showing an example of the operation according to the embodiment of the present invention. First, in S11, the color gamut of the color image signal on the input side and the color gamut of the output device are obtained in advance. Here, it is assumed that the color space is determined in a device-independent color space, for example, a CIELab color space. FIG. 3 is a conceptual diagram illustrating an example of a color gamut or a color gamut. Generally, the color gamut on the input side and the color gamut of the output device are not uniform, and have a complicated three-dimensional shape as shown in FIG. The inside of the solid shown in FIG. 3 is a region where a color image signal is input or a region where color reproduction can be performed in an output device. Here, the outer surfaces of the color gamut of the input device and the color gamut of the output device are determined.

In S12, a color image signal for performing color conversion is input. Here, if the color space of the input color image signal is different from the CIELab color space such as an RGB color space, the input color space conversion unit 1 converts the input color image signal such as the RGB color space into a CIEL color space.
Convert to ab color space.

Next, in step S13, the hue conversion unit 2
The hue of the color image signal converted into the IELab color space is changed according to the color of the color image signal. FIG.
FIG. 4 is a conceptual diagram of a hue changing process in a hue conversion unit. In the CIELab color space, the hue change is L ^*
This is a process of rotating and moving around the axis. For example, FIG.
Is rotated by the hue conversion unit 2 and changed to the color of point a. The rotation angle and the rotation direction at this time are determined according to the color of the input color image signal. For example, as described above, magenta (M) is red (R)
To move in the direction approaching. Further, it is preferable that blue (B) is largely moved in a direction approaching cyan (C). Conversely, the hue of yellow (Y) hardly needs to be changed.

In the following S14 to S16, the lightness adjustment is performed in the equal hue in-plane conversion unit 3 before performing the color conversion. FIG.
FIG. 9 is an explanatory diagram of a specific example of a brightness adjustment process in a constant hue in-plane conversion unit. In the figure, the broken line indicates the color gamut on the input side, and the solid line indicates the color gamut of the output device. First, in S14, the input color gamut and the color reproduction gamut of the output device are cut off from the hue of the color image signal whose hue has been changed, on a plane having the hue and passing through the lightness axis (L ^* ). , The cross section of the color gamut on the input side and the color gamut of the output device in that hue. At this time, it is assumed that a cross section of the color gamut on the input side and the color gamut of the output device as shown in FIG. In the coordinate system of this cross section, the vertical axis is the L ^* axis indicating lightness, and the horizontal axis is the C ^* axis indicating saturation.

Next, in step S15, the brightness range of the color gamut on the input side, that is, the range from the maximum value of brightness to the minimum value of brightness, is adjusted to the brightness range of the color reproduction range of the output device. Thus, the color gamut on the input side is as shown by the broken line in FIG.

Further, in S16, the point having the maximum saturation in the color gamut on the input side (CUSPi) after the adjustment of the lightness range in S15 and the point having the maximum saturation in the color reproduction range of the output device. The color gamut on the input side is adjusted by adjusting the point (CUSPi) by a predetermined function within the range of the difference from the lightness of (CUSPo). For example, FIG.
The brightness of the point (CUSPi) having the maximum saturation is adjusted from the color gamut on the input side indicated by the broken line in FIG.
Are adjusted so that the color gamut indicated by the broken line is obtained. In accordance with the color gamut adjustment on the input side, the brightness of the color image signal after the hue change is adjusted. As a result of such brightness adjustment processing, the color image signal after the hue change is performed as shown in FIG.
A color image signal within the color gamut indicated by the broken line in FIG.

In the following steps S17 to S21, the conversion path is set in the in-plane conversion unit 3 and the color conversion is performed along the set conversion path. First, in S17, it is determined whether the brightness of the color image signal after the brightness adjustment is higher or lower than the brightness of the point (CUPo) having the maximum saturation in the color reproduction range of the output device. If the brightness is the same as that of the point (CUSPo), any of the methods may be adopted.

If the brightness of the color image signal after the brightness adjustment is higher than the brightness of the point (CUSPo), S1
8, if the color image signal after the brightness adjustment is outside the color reproduction range of the output device, the angle between the color axis and the saturation axis is an angle corresponding to the hue of the color image signal, and the color of the color image signal after the brightness adjustment is adjusted. Find a straight line that passes through Then, an intersection between the straight line and the outline of the color reproduction range of the output device is obtained. Further S1
In step 9, a straight line that saves the brightness of the intersection obtained in S18, that is, a line segment connecting the intersection with the intersection having the same brightness as the intersection on the brightness axis is obtained. Then, a conversion path bent at the intersection obtained by the line segment and the straight line set in S18 is set. If the color image signal after the brightness adjustment is within the color gamut of the output device, the conversion path may be set in the direction in which the brightness is preserved, and the processing in S18 is not necessary.

FIG. 6 is an explanatory diagram of an example of setting of a conversion path and color conversion processing when the brightness of a color image signal after brightness adjustment is higher than the brightness of a point (CUSPo). Here, a case where the color image signal after the brightness adjustment is out of the color reproduction range of the output device is shown.

First, a straight line passing through a point (●) indicating the color image signal after the brightness adjustment and forming a predetermined angle with the saturation axis is considered, and an intersection (black) of the straight line with the outline of the color reproduction range of the output device is considered. Triangle). In addition, an intersection (similarly, a black triangle) with the outline of the color gamut on the input side after brightness adjustment for color conversion to be described later is also obtained. Note that the angle between the straight line and the saturation axis may be set according to, for example, the hue of the color image signal after the hue and brightness adjustment.

Further, the intersection (the black triangle) of the straight line forming a predetermined angle with the above-mentioned saturation axis and the outline of the color reproduction range of the output device.
, A straight line (this straight line preserves lightness) orthogonal to the lightness axis (L ^* axis) is considered. This straight line is a line segment connecting the intersection with the point on the lightness axis having the same brightness as the intersection. In this way, the light passes through the color of the color image signal after the lightness adjustment, goes to the intersection with the outline of the color reproduction range of the output device at a predetermined angle, and goes from the intersection in the direction orthogonal to the lightness axis. A bending conversion path is obtained.

When the conversion path is obtained in this way, S
At 21, a color conversion process is performed on the color image signal after the brightness adjustment according to the set conversion path. Here, as an example, color conversion is performed using a linear compression method. FIG.
FIG. 4 is an explanatory diagram of a linear compression method. When compressing the color gamut of a color image signal after brightness adjustment using a linear compression method to the color reproduction range of an output device, the distance from the brightness axis to the color gamut contour of the color image signal after brightness adjustment according to the compression path. Lin
And the distance L from the brightness axis to the outline of the color gamut of the output device
According to the ratio of out, the color conversion of the color image signal after the brightness adjustment is performed. The distance from the lightness axis to the color (indicated by ●) of the color image signal after lightness adjustment according to the compression path is represented by L′ i.
n, when the distance from the lightness axis to the color of the output color image signal (shown by ○) is L'out, L'out
Can be obtained by L'out = (Lout / Lin) × L'in. This calculation process may be performed separately for lightness and saturation.

Here, the distance Lin from the lightness axis to the color gamut outline of the color image signal after the lightness adjustment and the distance L'in from the lightness axis to the color of the color image signal after the lightness adjustment follow the conversion path. Distance. For example, as shown in FIG. 6, when a conversion path bent at the intersection on the outline of the color reproduction area of the output device is set, the distance Lin is the intersection of the lightness axis and the outline of the color reproduction area of the output device. Is the sum of the distance Lout between the line and the intersection and the distance Ld between the line of the predetermined angle with the saturation axis and the outline of the color gamut on the input side. That is, Lin = Lout + Ld. Similarly, the distance L'in from the brightness axis to the color of the color image signal after brightness adjustment
Is the sum of the distance Lout between the lightness axis and the intersection on the outline of the color reproduction area of the output device, and the distance between the intersection and the color of the color image signal after the lightness adjustment. Thus, the distance Lin and the distance L'in along the conversion path are obtained, and the distance Lo is obtained.
The saturation of the output color image signal may be obtained in accordance with the above expression together with ut. The lightness of the output color image signal is the lightness at a point intersecting the lightness axis in this conversion path, and the hue is the hue of the hue plane on which this conversion path is set. Note that these chroma and hue are obtained as a ^* and b ^* values in the CIELab color space.

When the color image signal after the brightness adjustment is a high chroma color, a straight line passing through this color and forming a predetermined angle with the chroma axis may not intersect the outline of the color reproduction range of the output device. In such a case, a straight line (point (CUSPo)) that maintains the brightness of the maximum chroma color (CUSPo) in the color reproduction range of the output device.
An intersection between Po) and a point on the lightness axis having the same lightness and a straight line passing through the point (CUSPo)) is obtained, and a conversion path is set using the intersection to perform color conversion in the same manner as described above.

By performing color conversion along the conversion path in this manner, it is possible to suppress a decrease in saturation as compared with performing color conversion by preserving brightness, and to perform color conversion linearly toward a target point. Rather than performing, it is possible to suppress a decrease in lightness for a high lightness color. Further, the amount of suppression of the decrease in saturation and the amount of suppression of the decrease in lightness can be adjusted by adjusting the angle of a straight line passing through the color image signal after the lightness adjustment. FIG. 8 is an explanatory diagram of an example of setting the angle between the cross-sectional shape of the color gamut on the input side and the color reproduction gamut of the output device and the linear saturation axis. FIG. 8A shows a case where the lightness of a high chroma color is very high, as shown in FIG. 12B, for example. In such a case, if the color conversion is performed while maintaining the brightness as in the related art, the saturation is significantly reduced. Therefore, in a hue (for example, yellow) having a cross-sectional shape of such an input-side color gamut and an output device color reproduction gamut, a straight line passing a color of a color image signal outside the color reproduction range of the output device with respect to a saturation axis. Increase the angle. FIG. 8A shows an example in which compression is performed at an angle almost 90 °. By performing such a color conversion, the color conversion can be performed without causing a large decrease in chroma and without causing a significant decrease in lightness.

Conversely, as shown in FIG. 8B, in the case of a hue of a high chroma color having a low lightness (for example, blue), a straight line passing through the color of the color image signal outside the color reproduction range of the output device is used. If the angle with respect to the saturation axis is increased, the brightness is greatly reduced. Therefore, the angle is reduced in such a hue. As a result, a decrease in brightness can be suppressed.

Returning to FIG. 2, if it is determined in S17 that the brightness of the color image signal after brightness adjustment is lower than the brightness of the point (CUSPo), in S20, an achromatic color having a predetermined brightness is targeted. And a conversion path is set in the direction of a straight line connecting the color of the color image signal after brightness adjustment and the target point,
In S21, color conversion is performed according to the set conversion path. By such a color conversion process, a color with low brightness can be apparently converted to a similar color. The target point may be, for example, an achromatic (ie, on the L ^* axis) point having the same lightness as the lightness of the point (CUSPo). By setting such a target point, the point (CU
Color continuity between the color conversion result for a color image signal higher than the brightness of SPo) can be guaranteed.

FIG. 9 is an explanatory diagram of an example of the color conversion processing when the brightness of the color image signal after the brightness adjustment is lower than the brightness of the point (CUPo). Also in this case, the brightness and the saturation are converted using the linear compression method. As shown in FIG. 9, a straight line passing through the point indicating the color image signal after the brightness adjustment and the target point is set as a conversion path, and the straight line and the outline of the color gamut on the input side and the outline of the color gamut of the output device are set. Find the intersection of In FIG. 9, the intersections are indicated by black triangles. The distance from the intersection of the straight line and the outline of the color gamut on the input side after brightness adjustment to the L ^* axis is Lin, and the distance from the intersection of the straight line and the outline of the color gamut of the output device to the L ^* axis is Lout.
And Further, as shown in FIG. 7 described above, the distance from the point (indicated by a black circle) indicating the color image signal after the brightness adjustment to the L ^* axis is L′ in, and the point indicating the output color image signal after the conversion ( Assuming that the distance from the ( ^* ) to the L ^* axis is L'out, L'out can be obtained by L'out = (Lout / Lin) × L'in. This calculation process may be performed separately for lightness and saturation. Thus, an output color image signal after color conversion is obtained.

Here, when the brightness of the color image signal after the brightness adjustment is lower than the brightness of the point (CUSPo), it has been described that the color conversion is performed linearly toward the target point. The brightness of the subsequent color image signal is point (C
Similarly to the case where the brightness is higher than USPo), a conversion path that bends at a predetermined position may be set. For example, it is also possible to maintain the brightness up to the outline of the color reproduction range of the output device or to convert the brightness along a straight line having a certain angle with respect to the brightness axis. The angle in the latter case may be different from the case where the brightness of the color image signal after the brightness adjustment is higher than the brightness of the point (CUSPo).

In the above example, the linear compression method was used in the color conversion in S21. However, the present invention is not limited to this, and it is a matter of course that the brightness and the saturation may be converted using a method other than the linear compression method.

Returning to FIG. 2, finally, in S22, the output color space conversion unit 5 converts the output color image signal into a color space required by the output device as necessary. For example, if a device on the output side requests a color image signal in the YMCK color space, color space conversion processing from the CIELab color space to the YMCK color space may be performed. Of course, if the CIELab color space used in the internal processing should be output as it is, this S2
Step 2 is not required. Thus, the process ends.

In the above description, it has been described that the input color image signal exists in the color gamut on the input side.
However, when a color image signal created using a device different from the device on the input side when the color gamut on the input side is set is input, or a color image signal for which color values are forcibly set, for example, In the case of, a color image signal outside the color gamut on the input side may be input. In such a case, the conversion path and the conversion function from the color gamut on the input side to the color gamut of the output device are extended and used, and color image signals outside the color gamut on the input side are also used within the color gamut of the output device. The conversion may be performed as follows.

FIG. 10 is an explanatory diagram of an example of setting a conversion path and color conversion for a color image signal outside the color gamut on the input side. FIG. 10A shows a case where the brightness of the color image signal after the brightness adjustment is higher than the brightness of the point (CUSPo), and FIG. 10B shows the case where the brightness of the color image signal after the brightness adjustment is the point (CUSPo). The case where the brightness is lower than the brightness is shown. When the brightness of the color image signal after the brightness adjustment is higher than the brightness of the point (CUSPo), similarly to the case in the color gamut on the input side, it passes through the point (●) indicating the color image signal after the brightness adjustment, A straight line that forms a predetermined angle with the saturation axis is considered, and an intersection (black triangle) between the straight line and the outline of the color reproduction range of the output device is obtained. Also in this case, the angle between the straight line and the saturation axis is set according to, for example, the hue. Further, a straight line that passes through the intersection of a straight line that forms a predetermined angle with the saturation axis and the outline of the color reproduction range of the output device and that is orthogonal to the L ^* axis (this straight line preserves lightness) is considered.

Each distance is calculated according to the path obtained at the intersection, which is obtained as described above. For a color image signal outside the color gamut on the input side, L'in is defined as the value from the intersection of Lout and the outline of the color reproduction range of the output device on a straight line that forms a predetermined angle with the saturation axis from the color image signal. The sum with the distance Ld '. That is, L'in
= Lout + Ld '. Note that Lin and Lout
Is the same as the above-described processing for the color image signal in the color gamut on the input side. The distance between the color image signal after color conversion by the linear compression method and the L ^* axis is L'o.
When ut, the distance L'out can be obtained by L'out = (Lout / Lin) × L'in as described with reference to FIG. Here, (Lout / L
The value of (in) is the same as in the color gamut on the input side, and is a compression ratio when the color gamut on the input side is compressed to the color reproduction range of the output device. Therefore, the color image signal after linear compression may be out of the color reproduction range of the output device. In that case, after performing color conversion by the linear compression method, the second conversion is performed so as to be a point within the color reproduction range of the output device. As the second conversion method, for example, a color difference minimum method can be used. Thus, the color outside the color gamut on the input side is 2 in this example.
The color is converted into a color within the color reproduction range of the output device by the stepwise color conversion. Of course, not limited to this method, for example, Lin =
Various methods can be applied, such as changing the compression ratio as L'in and mapping to the outer colors of the color reproduction range of the output device by one linear compression method.

The same applies when the brightness of the color image signal after the brightness adjustment is lower than the brightness of the point (CUSPo). For example, as shown in FIG.
Achromatic color (ie on the L ^* axis) with the same lightness as o)
Is set as a target point, and color conversion is performed along a straight line passing through the target point and the color image signal. In this case, L
The in and Lout are the same as those in the above-described color gamut on the input side. Further, the distance from the intersection of the straight line passing through the target point and the color image signal with the outline of the color reproduction range of the output device to the color image signal after the brightness adjustment is Ld '. At this time, L′ in = Lout + Ld ′, and the value L′ out of the color image signal after color conversion by the linear compression method can be obtained by L′ out = (Lout / Lin) × L′ in. Also in this case, if the color is not converted into the color within the color reproduction range of the output device by the linear compression method, the second color conversion may be performed. As the second color conversion method, for example, a color difference minimum method can be used.

In this manner, a color image signal outside the color gamut on the input side can also be converted into a color within the color gamut (outer) of the output device. At this time, by extending and using the conversion path and the conversion function used in the color gamut on the input side, the same relationship before and after the conversion as in the color gamut can be maintained outside the color gamut on the input side. In addition, by using the color difference minimization method after approaching the color gamut of the output device by the linear compression method, the color can be converted into a color that can be reproduced almost satisfactorily without a large decrease in saturation or lightness. become. As a method of performing color conversion on a color image signal outside the color gamut on the input side, a method other than the above-described linear compression method or a combination of the linear compression method and the minimum color difference method may be used.

FIG. 11 is an explanatory diagram of another example of setting a conversion path and color conversion for a color image signal outside the color gamut on the input side. Outside the color gamut on the input side, a conversion path and a conversion function used in the color gamut on the input side are used as in the above-described example, and a conversion path different from that in the color gamut on the input side may be set. For example, in the example illustrated in FIG. 11, a straight line in a direction in which lightness is preserved outside the color gamut on the input side, that is, a direction orthogonal to the lightness axis (parallel to the saturation axis) is assumed. An intersection between the straight line and the outline of the color gamut on the input side is obtained, and the above-described conversion path is set inside the intersection. As a result, a conversion path that bends twice at the outline of the color gamut of the output device and the outline of the color gamut of the output device is set. Then, according to such a conversion path, color conversion can be performed by using, for example, the linear compression method described in the example of FIG. As in this example, in the present invention, a plurality of bent portions may exist in the conversion path, and the conversion path can be set arbitrarily.

It is also possible to combine the example shown in FIG. 10 with the example shown in FIG. For example, a color gamut is set outside the color gamut on the input side so as to include the color gamut of a device that can be many input sides, and a conversion path as shown in FIG. 10 is set within the color gamut. In the outer region, the method shown in FIG. 11 in which the contour of the region is a bending point can be used. Of course, not limited to this, various conversion paths can be set.

In the above-described configuration, an example is shown in which the direction of the conversion path is changed in the outline of the color gamut or color gamut.
However, it is not limited to this. For example, the direction may be changed at a predetermined position even in a color gamut or a color gamut. The change in the direction is not limited to one time, but can be changed any number of times. In the above example, the color conversion method is different depending on whether the maximum chroma color is above or below the lightness. However, the same method may be used for all lightnesses, or the color conversion method may be further subdivided depending on the lightness.

In the above-described configuration, an example has been described in which a conversion path on the same hue plane is set, and color conversion is performed along the conversion path. In the present invention, it is also possible to set a conversion path for changing not only the hue plane but also the hue, and perform color conversion along the conversion path. Also in this case, a color conversion can be performed by setting a conversion path so as to change the direction when passing through a predetermined position.

[0051]

As is apparent from the above description, according to the present invention, when converting an input color image signal into a color image signal within the color reproduction range of the output device, the input color image signal By setting a conversion path according to the above, and when the conversion path passes through a predetermined position, by changing the conversion direction at the predetermined position, optimal color conversion corresponding to the outer shape of various color reproduction areas is performed. It can be carried out. Therefore, there is an effect that optimal color conversion can be realized for all hues.

As for the conversion path when a color image signal outside the color gamut on the input side set in advance is input, the same conversion path as in the color gamut on the input side is used, or the color path outside the color gamut on the input side is used. In such a case, the conversion direction at the time of color conversion can be freely set, for example, by using another path, and color conversion can be favorably performed even outside the color gamut on the input side.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a flowchart illustrating an example of an operation according to the embodiment of the present invention.

FIG. 3 is a conceptual diagram illustrating an example of a color gamut or a color gamut.

FIG. 4 is a conceptual diagram of a hue changing process in a hue conversion unit.

FIG. 5 is an explanatory diagram of a specific example of a lightness adjustment process in a constant hue in-plane conversion unit.

FIG. 6 shows a point (C) at which the brightness of the color image signal after the brightness adjustment is adjusted.
FIG. 11 is an explanatory diagram of an example of a setting of a conversion path and a color conversion process when the brightness is higher than USPo).

FIG. 7 is an explanatory diagram of a linear compression method.

FIG. 8 is an explanatory diagram of an example of setting an angle between a cross-sectional shape of a color gamut on an input side and a color reproduction gamut of an output device and a linear saturation axis.

FIG. 9 shows that the brightness of a color image signal after brightness adjustment is a point (C
FIG. 14 is an explanatory diagram of an example of color conversion processing when the brightness is lower than USPo).

FIG. 10 is an explanatory diagram of an example of setting of a conversion path and color conversion for a color image signal outside the color gamut on the input side.

FIG. 11 is an explanatory diagram of another example of setting of a conversion path and color conversion for a color image signal outside the color gamut on the input side.

FIG. 12 is an explanatory diagram of an example of a conventional color conversion process.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Input color space conversion part, 2 ... Hue conversion part, 3 ... Equal hue plane conversion part, 4 ... Output color space conversion part.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 5B057 CA01 CA08 CA12 CB01 CB08 CB12 CC02 CE17 CE18 CH18 DB02 DB06 DB09 5C077 LL19 MP08 PP21 PP27 PP28 PP31 PP32 PP33 PP36 PP37 PQ08 SS06 TT02 5C079 HB01 HB02 HB06 LA10B

Claims (14)

[Claims] 1. A color image signal processing device for converting an input color image signal into an output color image signal within a color gamut reproducible by an output device, wherein the color image signal is transmitted to a conversion path corresponding to the input color image signal. Color conversion means for converting the input color image signal along the color path, wherein the color conversion means changes the conversion direction at the predetermined position when the conversion path passes through the predetermined position. Image signal processing device. 2. The color image signal processing device according to claim 1, wherein the predetermined position is an outline of a color gamut reproducible by the output device. 3. The color image signal processing apparatus according to claim 1, wherein the conversion paths have the same hue in a color space. 4. The color image signal processing according to claim 1, wherein the conversion path can be set according to a hue of the input color image signal. apparatus. 5. The conversion path is divided into regions based on the lightness of the maximum chroma color in each hue of a color reproduction range reproducible by the output device, and different conversion paths can be set in each region. The color image signal processing device according to claim 1, wherein: 6. The input path outside the color reproduction range in which the brightness is higher than the lightness of the maximum chroma color in each hue of the color reproduction range reproducible by the output device and is outside the color reproduction range reproducible by the output device. 6. The color image signal according to claim 5, wherein an angle corresponding to the hue of the input color image signal is an angle up to the outline of the color gamut, and then the brightness is changed to a direction in which lightness is preserved. Color image signal processing device. 7. An input-side color gamut is defined in a color space, a conversion path for converting the input-side color gamut to a color gamut reproducible by the output device is set, and a color outside the color gamut is set. 7. The method according to claim 1, wherein a conversion path for converting the color gamut on the input side to a color gamut reproducible by the output device is extended and used as a conversion path for the image signal. 2. The color image signal processing device according to claim 1. 8. A color image signal processing method for converting an input color image signal into an output color image signal within a color gamut reproducible by an output device, wherein a conversion path corresponding to the input color image signal is provided. At this time, when the conversion path passes through a predetermined position, the conversion direction is changed and set at the predetermined position, and the input color image signal is converted into an output color image signal along the set conversion path. A color image signal processing method. 9. The color image signal processing method according to claim 8, wherein the predetermined position is an outline of a color gamut reproducible by the output device. 10. The color image signal processing method according to claim 8, wherein the conversion paths have the same hue in a color space. 11. The color image signal processing according to claim 8, wherein the conversion path can be set according to a hue of the input color image signal. Method. 12. The conversion path may be divided into regions based on the lightness of the maximum chroma color in each hue of a color gamut reproducible by the output device, and different conversion paths may be set for each region. Claims 8 to 1 characterized by the following:
2. The color image signal processing method according to claim 1. 13. The input device according to claim 1, wherein the conversion path is higher than the lightness of the maximum chroma color in each hue of the color reproduction range reproducible by the output device, and the input path is outside the color reproduction range reproducible by the output device. 13. The color image signal according to claim 12, wherein the color image signal has an angle corresponding to the hue of the input color image signal up to the outline of the color gamut, and thereafter changes to a direction in which lightness is preserved. Color image signal processing method. 14. A color gamut on the input side is defined in a color space, a conversion path for converting the color gamut on the input side to a color gamut reproducible by the output device is set, and a color outside the gamut is set. 14. A conversion path for converting a color gamut on the input side to a color reproduction gamut reproducible by the output device as a conversion path for an image signal, and is used by extension. 2. The color image signal processing method according to claim 1.