JP2007184736A - Color gamut generation device, color gamut generation method, and color gamut generation program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a color gamut generation device, a method and a program which can generate a color gamut capable of obtaining an appropriate color reproduction in all areas of the color gamut. <P>SOLUTION: A color gamut generation section divides a second color gamut generated based upon a first color gamut in a color space of an input color image signal into a plurality of color areas, corrects color gamut formation data of the second color gamut by the divided areas, and generates a third color gamut based upon the corrected color gamut formation data. The color gamuts after the correction processing are passed to a multi-dimensional lookup table generation section 3, which sets values after color conversion at respective grating points. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a color gamut creation device, a color gamut creation method, and a color gamut creation program, and in particular, a color gamut creation device and a color gamut creation for creating a color gamut as a basis for color conversion using a multidimensional lookup table. The present invention relates to a method and a color gamut creation program.

  Conventionally, there is a color conversion process as a process for a color image signal. For example, a color image signal depending on an input device is converted into a color image signal corresponding to an output device. In this case, as an input-side device, a CRT used when creating a color image, an image reading device such as a scanner, or the like can be considered. In these input-side devices, the color gamut that can be handled is limited according to the respective characteristics. The same applies to the output device. For example, in the case of a printer, the color gamut that can be reproduced is limited by the color material used.

  As described above, both the input-side device and the output device have their respective color gamuts, and a color image signal created using the input-side device has a color gamut corresponding to the output device. After being converted into a color image signal, it is output by an output device. Such color conversion may be performed directly, but once the color image signal on the input side is converted into a color space independent of the device, it is also converted into a color space dependent on the output device. .

  In the case of conversion to an intermediate device-independent color space, colors that can be expressed in the input-side device are only a part of the color gamut in the device-independent color space. Similarly, the colors that can be reproduced in the output device are also part of the color gamut of the color space that does not depend on the device. Although both color gamuts overlap with the lightness axis as the center, colors that can be represented by the input device may not be reproducible by the output device, and in that case, it is necessary to convert them to colors that can be reproduced by the output device. is there. This processing is called gamut compression processing, and various methods are known. When such gamut compression processing is performed, a color gamut expressed by the input device and a color gamut reproducible by the output device are set in advance. And color conversion between the two is defined.

  On the other hand, when color conversion is performed on a color image signal, a three-dimensional calculation is generally required. For example, in various color conversion processes represented by the above-described gamut compression process and the like, it is often impossible to perform good color conversion by uniform conversion on a color space such as matrix conversion. Therefore, recently, a multi-dimensional lookup table is used.

  When color conversion is performed, a color image signal is input to such a multidimensional lookup table, and a corresponding output color image signal is obtained. If the output color image signal is stored in the multi-dimensional look-up table corresponding to all possible values of the input color image signal, color conversion can be performed accurately. However, the number of combinations of values that the color image signal can take is enormous, and an enormous memory capacity is required to store the corresponding value of the output color image signal. For this reason, in the normally used multidimensional lookup table, the color space that can be taken by the input color image signal is divided into a plurality of colors, the output color image signal is stored only at the grid points of the divided color space, and the other color images When a signal is input, the value of the output color image signal is obtained from neighboring grid point data by interpolation or the like. In the following description, the interpolation process may be referred to as a multidimensional lookup table.

Consider performing color conversion processing including gamut compression processing using a multi-dimensional lookup table as described above. Here, it is assumed that the input color image signal is created using an input-side device such as a CRT and is converted into the CIELab color space as a device-independent color space. The multidimensional lookup table is also created on the assumption that a color image signal in the CIELab color space is input as the input side. FIG. 10 is an explanatory diagram of an example of lattice points in the multidimensional lookup table. FIG. 10 shows an example in which the L ^* axis, a ^* axis, and b ^* axis in the CIELab color space are divided into four equal parts, and ● is a lattice point. As shown in FIG. 10, lattice points are scattered throughout the CIELab color space. When a color image signal is input, interpolation calculation is performed using data of neighboring grid points (eight points).

  FIG. 11 is a conceptual diagram showing an example of the color gamut of the input color image signal. The input color image signal has a limited color gamut expressed by the device used when creating the color image or the device used when reading the color image as described above. When expressed in the CIELab color space, for example, as shown in FIG. 11, it is a partial space in the CIELab color space.

FIG. 12 is an explanatory diagram showing an example of the relationship between the color gamut of the input color image signal on a certain hue plane and the grid points of the multi-dimensional lookup table. When the CIELab color space shown in FIGS. 10 and 11 is cut along a hue plane including the L ^* axis, such as the a ^* axis and the b ^* axis, the result is as shown in FIG. 12, for example. In FIG. 12, the hatched portion is the color gamut of the input color image signal. Normally, a color image signal within this color gamut is input. However, as a multidimensional lookup table, grid points are arranged for the entire CIELab color space, and color image signals after color conversion need to be associated with each grid point, and the color gamut of the input color image signal The value of the output color image signal after conversion is also stored for other grid points.

  As an example, let us consider a case where the color gamuts on the input side and the output side match and the colors within the color gamut of the input color image signal are output as they are as the output color image signal. For the color gamut of the input color image signal, the lightness is stored here and converted to a color outside the color gamut of the input color image signal. In this case, the color value at the grid point is stored in the grid point in the color gamut of the input color image signal. Further, the color values outside the color gamut after storing the brightness and converting are stored in the grid points outside the color gamut.

  Assume that a color image signal (point P in FIG. 12) having the maximum saturation is input in the color gamut shown in FIG. In this case, in the multidimensional lookup table, eight neighboring lattice points are selected and interpolation calculation is performed. In FIG. 12, for the convenience of illustration, only four selected lattice points are indicated by ●. Of the grid points selected at this time, three (grid points u, v, w) are grid points outside the color gamut. Therefore, the color values on the color gamut outline after the lightness is saved and converted are stored, and the color is indicated by ◯ in FIG. That is, the value of the point x is stored in the lattice points v and w, and the value of the point y is stored in the lattice point u. Accordingly, an interpolation operation is performed using the grid point t in the color gamut, the point x (for two points), and the point y. As a result, the color of the point P ′ is output from the output color image signal.

  As described above, when color conversion is performed using a multidimensional lookup table, conventionally, a phenomenon has occurred in which a desired result cannot be obtained even if setting is performed based on a color gamut or the like. Such a phenomenon occurs not only in the above-described example but also in the vicinity of the outline of the color gamut. In particular, as shown by a point P in FIG.

  As described above, the color space of the input device and the shape of the color space after conversion may be different, which may cause a decrease in saturation.

  For example, in the ICC profile, various device color spaces can be reproduced by combining the monitor profile / scan profile and the printer profile. In such a situation, a color space having a color gamut of various sizes is input to the printer profile. For example, the color space set by the printer is an sRGB color space, and the input color space is In the case of a wide color space such as WideRGB, there is a problem that the color reproduction range between sRGB and WideRGB is crushed or turbid.

  In order to solve this problem, in Patent Document 1, the outline of a color gamut is determined from the color image signals from a plurality of input-side devices based on their maximum saturation, and the determined color gamut has a particularly high specific color. A technique has been proposed in which grid points of a multidimensional lookup table arranged in the vicinity are acquired and the color gamut is corrected so that these grid points become the color gamut.

Patent Document 2 proposes a method of setting a fixed input device and grasping the color gamut when grasping the color gamut of the input device.
JP 2002-152539 A JP 2000-101863 A

  However, although the technique described in Patent Document 1 can reduce the conversion error of the high saturation color, there is a problem that turbidity (mixing of other colors) occurs in the high brightness region.

  In the technique described in Patent Document 2, when an input device is fixed, color reproduction is optimal for a fixed device, but optimal color reproduction cannot be obtained for other input devices. There was a problem.

  The present invention has been made in consideration of the above facts, and a color gamut creating apparatus, a color gamut creating method, and a color capable of creating a color gamut that can obtain appropriate color reproduction in all the areas of the color gamut. The purpose is to obtain an area creation program.

  In order to achieve the above object, a color gamut creation device according to the first aspect of the present invention provides a second color gamut generated based on a first color gamut in a predetermined color space of an input color image signal. Dividing into a plurality of areas, and for each divided area, correction means for correcting the color gamut formation data of the second color gamut, and generation for generating a third color gamut based on the corrected color gamut formation data Means.

  According to the present invention, the second color gamut generated based on the first color gamut in the predetermined color space of the input color image signal is divided into a plurality of areas, and the second color gamut is divided into the second area for each divided area. Since the color gamut is generated by correcting the color gamut formation data of the color gamut, it is possible to create a color gamut in which appropriate color reproduction can be obtained in all areas.

  In addition, as described in claim 2, the predetermined color space is a color space defined by brightness, saturation, and hue, and the correction unit is configured to adjust the saturation of the second color gamut. Correcting the color gamut formation data of the second color gamut based on a plurality of predetermined specific colors on the outline and the colors of the intermediate points between the maximum brightness and the minimum brightness of each of the specific colors can do.

  In addition, as defined in claim 3, the specific color includes at least a first specific color including a saturated saturated color of red, yellow, green, cyan, blue and magenta, and an intermediate color between the first specific color And the second specific color.

  According to a fourth aspect of the present invention, the plurality of areas may include a first specific hue area, a second specific hue area, a high brightness area, and a high saturation area.

  In addition, as described in claim 5, the input color image signal may be a color image signal depending on a plurality of devices.

  In this case, as described in claim 6, the correction means may correct the lightness to the maximum lightness while maintaining the saturation of the color gamut formation data of the first specific hue region.

  According to a seventh aspect of the present invention, the correction means has the same hue angle and the same brightness as the color of the color gamut formation data for the color gamut formation data of the second specific hue region. You may make it correct | amend to the minimum saturation among the color image signals of the said some apparatus.

  In addition, as described in claim 8, the correction unit has a plurality of colors having the same hue angle and the same brightness as the color of the color gamut formation data with respect to the color gamut formation data of the high brightness area. First correction means for correcting to the minimum saturation of the color image signal of the apparatus, and when the color space is divided into a plurality of grids corresponding to the multi-dimensional lookup table, after correction by the first correction means When the color of the corrected color gamut formation data is located near a specific grid point of the grid to which the color of the color gamut formation data belongs, the saturation of the color gamut formation data of the color around that color is set to the low saturation side. And a second correction means for correcting.

  In addition, as described in claim 9, when the correction unit divides the color space into a plurality of grids corresponding to a multidimensional lookup table for the color gamut formation data of the high saturation region, the color gamut formation is performed. A selection unit that selects a grid point having the maximum saturation among the grid points of the grid to which the color of the data belongs; a saturation correction unit that corrects the saturation of the color gamut formation data based on the position of the selected grid point; , May be included.

  According to a tenth aspect of the present invention, there is provided a color gamut creation method that divides a second color gamut generated based on a first color gamut in a predetermined color space of an input color image signal into a plurality of areas. For each of the areas, the color gamut formation data of the second color gamut is corrected, and a third color gamut is generated based on the corrected color gamut formation data.

  According to the present invention, the second color gamut generated based on the first color gamut in the predetermined color space of the input color image signal is divided into a plurality of areas, and the second color gamut is divided into the second area for each divided area. Since the third color gamut is generated by correcting the color gamut formation data of the color gamut, it is possible to create a color gamut in which appropriate color reproduction can be obtained in all regions.

  12. The color gamut creation program according to claim 11 divides a second color gamut generated based on a first color gamut in a predetermined color space of an input color image signal into a plurality of areas, For each of the areas, a process including correcting the color gamut formation data of the second color gamut and generating a third color gamut based on the corrected color gamut formation data is executed on a computer. It is characterized by making it.

  According to the present invention, the second color gamut generated based on the first color gamut in the predetermined color space of the input color image signal is divided into a plurality of areas, and the second color gamut is divided into the second area for each divided area. Since the third color gamut is generated by correcting the color gamut formation data of the color gamut, it is possible to create a color gamut in which appropriate color reproduction can be obtained in all regions.

  As described above, according to the present invention, there is an excellent effect that it is possible to create a color gamut in which appropriate color reproduction can be obtained in all areas of the color gamut.

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

  FIG. 1 is a block diagram showing an embodiment of a color gamut creating apparatus and a color gamut creating method according to the present invention. In the figure, 1 is a color gamut generation unit, 2 is a color gamut correction unit, and 3 is a multidimensional lookup table generation unit. The color gamut generation unit 1 generates a color gamut of a color image signal that performs color conversion using a multidimensional lookup table. When the color image signal input to the multidimensional lookup table is created using a display device such as a CRT, the color gamut is a color range that can be displayed on the display device. When the image is read by an image reading device such as a scanner, the color range is unique to the reading device. In either case, for example, a color image signal in a color space depending on a device such as an RGB color space is created, and the color image signal uses the entire color space. When converting from such a device-dependent color space to a device-independent color space such as the CIELab color space, the color gamut of the entire device-dependent color space is a part of the color in the device-independent color space. It becomes an area. The color gamut generation unit 1 generates a color gamut in a device-independent color space after converting the color gamut of the entire color space depending on the device into a color space independent of the device. In the following description, the CIELab color space is assumed as a device-independent color space, but the present invention is not limited to this.

  When a color image signal is generated, one specific device is not always used. For this reason, for example, even if the same type of device is used, a machine difference occurs. In addition, the color gamut changes slightly due to various factors such as adjustment of the apparatus, changes with time, and replacement parts. In order to cope with such a change in the color gamut, when the color gamut generation unit 1 generates the color gamut, it may be generated based on color image signals obtained by a plurality of devices. When color image signals in a plurality of apparatuses are obtained, a color gamut can be generated from the color image signals based on, for example, the color of maximum saturation in each hue. The color gamut generated in this way is a color gamut including the color gamut of color image signals obtained from a plurality of devices.

  The color gamut correction unit 2 divides the color gamut generated by the color gamut generation unit 1 into a plurality of regions, and corrects the color gamut by a method suitable for each region. Details will be described later.

  The multidimensional lookup table generation unit 3 generates output data corresponding to the lattice points of the multidimensional lookup table. At this time, the color gamut corrected by the color gamut correction unit 2 is used, and the color image signal corresponding to each grid point is input so that desired color conversion is performed on the grid points in the color gamut. Set the color image signal after color conversion. In addition, color image signals are rarely input for colors outside the color gamut, but colors that can be taken by the output color image signal according to a predetermined rule for grid points outside the color gamut so as not to cause an error when input. The converted value is set so that the color is in the range. The multi-dimensional lookup table generated in this way is used at the time of actual color conversion. In the color gamut creation apparatus and color gamut creation method of the present invention, the multidimensional lookup table generation unit 3 is not necessarily required, and a means for generating a multidimensional lookup table may be provided separately.

  Next, an example of the operation in the embodiment of the present invention will be described.

  FIG. 2 is a flowchart illustrating an example of the operation of the color gamut generation unit. This program can be read and executed by a computer, and is stored in advance in a memory (not shown).

  First, in S11, color image signals created using a plurality of input devices are prepared. When the color space when creating the color image signal is a color space other than the CIELab color space for obtaining the color gamut, the color value in the color space for obtaining the color gamut is obtained. As an example, it is assumed here that color image signals are created in the RGB color space, and color values in the CIELab color space are obtained for color image signals of various colors in the RGB color space. Such a set of color image signals and color values is obtained in each device on the input side and stored as, for example, a look-up table (LUT). A conversion matrix for converting from the RGB color space to the CIELab color space may be obtained and stored for each of a plurality of input-side devices. In this embodiment, it is assumed that a conversion matrix is stored in advance for each input device.

In S12, values of L ^* , a ^* , and b ^* in the CIELab color space when a specific color is input as a color image signal are obtained. For example, even if the color has only the R component in the RGB color space, the hue is not always red in the CIELab color space, and there may be some deviation. Further, by obtaining from a plurality of input side devices, even if the same color is expressed by each device, there may be different color image signals. Here, the L of the specific color in each input side device may be obtained. ^* , A ^* , b ^* values are obtained. The specific color desirably includes at least a saturated color of red, yellow, green, cyan, blue, and magenta. These colors are a primary color in which only one component has a maximum value in the RGB color space, and a secondary color in which both components have a maximum value. Of course, various colors other than these may be specified colors.

  In S13, the average brightness and average hue angle are calculated for each specific color from the Lab value of the specific color calculated for each device in S12. Further, in S14, adjacent hues are equally divided based on the average brightness and average hue angle for each specific color obtained in S13, and the hue angle and the brightness at that time are set. FIG. 3 is an explanatory diagram of an example of the hue angle and brightness to be set. Here, the number of divisions is set to 3. The circles in the figure are the average hue angle and average brightness obtained in S13. Also, the hue angle and lightness points obtained by equally dividing adjacent hues are indicated by ●. The color gamut generated by the color gamut generation unit 1 is two-dimensional on the lightness-saturation plane at the average hue angle of the specific color obtained in S13 and the hue angle obtained by equally dividing the hues in S14. The outline of the color gamut is set. Therefore, if the number of divisions is increased, the outline of the color gamut can be obtained more precisely.

  In S15, in each hue plane (lightness-saturation plane) of the average hue angle of the specific color obtained in S13 and the hue angle obtained in S14, the maximum saturation point Cusp averaged by a plurality of devices is obtained, and white ( An intermediate point on the outline of the color gamut is set between 0,0,0) and Cusp, and black (100,0,0) and Cusp. FIG. 4 is an explanatory diagram of an example of Cusp and an intermediate point set on the hue plane. Here, only one hue plane in the Lab color space is shown. In this hue plane, the average value of the maximum saturation of each device (Cusp) and the lightness at that time are obtained, and between white (0,0,0) and Cusp, black (100,0,0) and Cusp, In this example, two intermediate points on the outline of the color gamut (shown by ●) are set. Such Cusp and the intermediate point are obtained for each of the average hue angle of the specific color obtained in S13 and the hue angle obtained in S14. For example, in the example shown in FIG. 3, the average hue angle of six specific colors and two hue angles obtained by dividing the hue angle of the adjacent specific color into three, ie, 18 hue angles in total, are the same as in FIG. And a plurality of intermediate points. Thereby, an average color gamut can be obtained.

  Further, in S16, the average color gamut obtained in S15 is corrected to obtain the outermost contour of the color gamut of each device. That is, for the average Cusp and intermediate point obtained in S15, the color image signal having the maximum saturation is acquired from the color image signals in the respective apparatuses, and the values of Cusp and intermediate point are corrected. FIG. 5 is an explanatory diagram of an example of processing for obtaining the outermost contour of the color gamut. For example, when the outline of the color gamut in each apparatus varies in the hatched area shown in FIG. 5, the values of Cusp and the midpoint are corrected to the values on the high saturation side of the variation range. As a result, the outline of the color gamut set in S15 indicated by the bold line in FIG. 5 is corrected to the outermost outline of the variation range of each device.

  In S17, the Lab value is calculated from the values of Cusp, hue at the midpoint, brightness, and saturation for each hue corrected in S16. In S18, a color gamut (color gamut formation data) is generated from the Lab value calculated in S17 and the black (minimum brightness) and white (maximum brightness) points. The color gamut formation data of the color gamut generated in this way (hereinafter referred to as basic Gamut) is output to the color gamut correction unit 2.

  Not based on the basic Gamut generated as described above, but based on a color conversion matrix or color conversion look-up table as color gamut formation data of the color gamut (first color gamut) of a plurality of input devices. The basic Gamut (second color gamut) may be output to the color gamut correction unit 2.

  FIG. 6 is a flowchart illustrating an example of the operation of the color gamut correction unit. This program can be read and executed by a computer, and is stored in advance in a memory (not shown). Here, correction processing of color gamut formation data is performed based on each specific color and intermediate point used when the color gamut generation unit 1 generates the color gamut.

Hereinafter, six specific colors of red, yellow, green, cyan, blue, and magenta are referred to as first specific colors, and two hue angles obtained by dividing the hue angle of the adjacent first specific color into three. Is called the second specific color. In addition, the first specific color and the second specific color are points of maximum saturation on the hue plane including the specific color. That is, it is assumed that the point is the maximum chroma Cusp obtained in S16 of FIG. Further, hereinafter, the first specific color, the second specific color, and the intermediate point are referred to as setting points, and the values of L ^* , a ^* , and b ^* are referred to as point data.

In S21, it is determined whether or not there is an unprocessed intermediate point having a hue angle within a predetermined first specific hue region among the set points. The first specific hue region is a region having a hue angle around yellow, which is the first specific color, for example. Specifically, in the hue direction, that is, on the a ^* axis-b ^* axis plane, for example, the hue angle to the set point on both sides of yellow that is the first specific color is determined to be within the first specific hue region. This is possible, but is not limited to this.

If there is an unprocessed intermediate point in the first specific hue area, the process proceeds to S22 to correct the point data of the set point. Specifically, in S22, only the lightness (L ^* ) is corrected to the maximum lightness while maintaining the saturation (a ^* , b ^* ).

On the other hand, if there is no unprocessed intermediate point in the first specific hue area, the process proceeds to S23, and is there an unprocessed intermediate point having a hue angle in the predetermined second specific hue area? Judge whether or not. The second specific hue area is an area having hue angles around red and magenta, which are the first specific colors, for example. Specifically, on the a ^* axis-b ^* axis plane, for example, in addition to the hue angle in the region including the second specific colors red and magenta, the hue angle to the set point on both sides of this region is the first. Although it can be determined that it is within the specific hue area of 2, the present invention is not limited to this.

  If there is an unprocessed intermediate point in the second specific hue area, the process proceeds to S24, and the point data of the set point is corrected. Specifically, in S24, the color image signals of a plurality of input-side devices with the same brightness as the selected set point are acquired with the same brightness as the selected set point, while maintaining the brightness. And

  On the other hand, when there is no unprocessed intermediate point in the second specific hue area, the process proceeds to S25, and it is determined whether or not there is an unprocessed intermediate point belonging to a predetermined high brightness area. Specifically, when the lightness of the intermediate point to be processed is L, the maximum lightness is Lmax, and the lightness of the point of maximum saturation at the hue angle of the intermediate point to be processed is Lcusp, for example, (L-Lcusp) / It is determined whether or not (Lmax−Lcusp)> 0.7 is satisfied. Note that the determination of the high brightness area is not limited to this.

  If there is an unprocessed intermediate point belonging to the high brightness area, the process proceeds to S26. In S26, among the color image signals of the plurality of input-side devices, the color image signal having the same brightness as that of the intermediate point to be processed is acquired, and this is set as a new saturation.

In S27, position information of the lattice points (eight) of the multidimensional lookup table arranged around the intermediate point to be processed (intermediate point after the saturation is corrected in S26) is acquired. That is, the position information of the lattice point of the lattice to which the intermediate point to be processed belongs is acquired. The grid points of the multi-dimensional lookup table are vertices obtained by dividing the color space into rectangular parallelepipeds or cubes as shown in FIG. 10, and the isochromatic surface is a cylindrical surface centered on the lightness (L ^* ) axis, that is, L ^* The distance from the axis is the saturation. Therefore, the lattice point with the maximum saturation among the eight lattice points is two lattice points on one side parallel to the L ^* axis.

  In S28, it is determined whether or not the saturation-corrected intermediate point is located in the vicinity of a lattice point in the color gamut (hereinafter referred to as a specific lattice point) among the surrounding lattice points. That is, it is determined whether or not the intermediate point after saturation correction is in a position biased in the lattice. Specifically, for example, when the length of one side of the grid is D and the distance between the set point and the specific grid point is E, it is determined whether or not there is a specific grid point that satisfies E ≦ D × r. . If r is equal to or smaller than this value, r is set to a value of 1 or less (for example, 0.2) that can be determined that the set point is in a biased position in the grid, that is, in the vicinity of a specific grid point.

  If it is determined that the intermediate point after saturation correction is located in the vicinity of the specific lattice point, the process proceeds to S29, and if not, the process proceeds to S40.

  In S29, the saturation of the peripheral intermediate points (for example, both adjacent intermediate points) is corrected to the low saturation side so as to be close to the saturation of the intermediate point after the saturation correction. For example, the saturation is corrected to the lowest saturation value in the grid to which the peripheral intermediate points belong. Note that the correction amount is not limited to this, and may be corrected toward the low saturation side by a fixed value, or may be determined according to the degree of correction of the peripheral intermediate points.

  If it is determined in S25 that there is no unprocessed intermediate point belonging to the high brightness area, the process proceeds to S30.

  In S30, it is determined whether there is a set point belonging to the high saturation region, that is, whether there is an unprocessed first specific color or second specific color.

  If there is an unprocessed first specific color or second specific color, the process proceeds to S31. In S31, the position information of the lattice points of the multidimensional lookup table arranged around the specific color to be processed is acquired. In the three-dimensional color space, eight grid points are acquired. Among these, the maximum saturation (two points) is selected in S32.

  In S33, it is determined whether each of the two grid points obtained in S32 is an upper part or a lower part of the specific color to be processed. In S34, for each grid point, it is determined whether the color gamut is inside (including the outline) or outside. If the grid point is out of the color gamut, a line segment passing through the white point and the grid point is calculated in S35 if the grid point is above the specific color to be processed. If the grid point is below the specific color to be processed, a line segment passing through the black point and the grid point is calculated. If the grid point is within the color gamut, a line segment passing through the white point and the specific color to be processed is calculated in S36 if the grid point is above the specific color to be processed. If the grid point is below the specific color to be processed, a line segment passing through the black point and the specific color to be processed is calculated.

  In S37, the intersection of the upper and lower line segments of the specific color obtained in S35 or S36 is obtained. The intersection obtained in this way is set as the maximum saturation point (Cusp) of the corrected color gamut.

  FIG. 7 is an explanatory diagram of an example of the color gamut correction process. It is assumed that the point P in FIG. 7 is a specific color point selected. When the lattice points of the multi-dimensional lookup table in the vicinity of the point P are acquired, eight lattice points are actually acquired. However, since a two-dimensional plane is shown here, four lattice points (t, u, v, w) are obtained. ) Only. Among these, the points of maximum saturation are points u and v. In this example, a lattice point u exists above the specific color P, and a lattice point v exists below the specific color P. Both are out-of-gamut grid points. At this time, a line segment passing through the white point and the grid point u and a line segment passing through the black point and the grid point v are considered and the intersection R is obtained by the process of S35. This point is set as the maximum saturation point (Cusp) of the corrected color gamut.

  Returning to FIG. 6, in S38, it is determined whether or not correction processing has been performed for all the specific colors (first specific color and second specific color). If unprocessed specific colors remain, S40 is determined. Return to. As a result, the processes of S31 to S37 are repeated for unprocessed specific colors. In this way, correction processing is performed for all specific colors.

  When correction processing is performed for all the specific colors and the corrected maximum saturation point (Cusp) is obtained, a corrected color gamut is created from the corrected maximum saturation point (Cusp) and the intermediate point in S39. . In this way, a hatched portion different from the original color gamut in FIG. 7 is newly added to the color gamut.

  Thus, since the color gamut of the high saturation region is corrected as shown in FIG. 7, even when a color image signal of point P is input, for example, points t, u, Since v and w are all processed as grid points in the color gamut, color conversion processing in the color gamut is correctly performed. In addition, since the color gamut of the color image signal that is actually input is the color gamut before correction, even if the grid points in the color gamut are used in the color gamut newly added by the correction processing as in the past, There is no problem because it is originally a process for a color image signal outside the color gamut.

  In S40, it is determined whether or not the processing for all the setting points has been completed. If the processing has not been completed, the process returns to S21 and the same processing as described above is repeated. To S41.

  In S41, when there is a set point having a large local change in the created color gamut, the point data of the set point is reduced based on the point data of the surrounding set points. to correct.

  In S42, a color gamut (third color gamut) is generated based on the point data of the set point corrected as described above.

  The color gamut after the correction processing is passed to the multidimensional lookup table generation unit 3 as shown in FIG. 1, and the value after color conversion is set at each grid point. Note that the value set for each grid point in the multi-dimensional lookup table is the same as the color space of the color image signal on the output side, converted to a different color space, or converted to a color space. A color image signal in the same color space as that on the input side may be set without performing this.

  An example of the color gamut generated as described above is shown in FIG. In the figure, a color gamut 50 that can be reproduced by an output device (for example, a printer), a color gamut 52 generated by a conventional Gamut process, a color gamut 54 generated by the method described in Patent Document 1, and the present embodiment. A gamut 56 generated by such a method is shown. In this embodiment, the color space is divided into a first specific hue area, a second specific hue area, a high brightness area, a high saturation area, and others, and the color gamut is corrected and generated by a method suitable for each area. As shown in the figure, the color gamut 56 according to the present embodiment has a shape in which the portion of the high brightness area is compressed to the low saturation side as compared with the color gamut 54 according to the method described in Patent Document 1. Yes. For this reason, even when an image signal having a wide color gamut is input, turbidity in the high brightness region can be suppressed, and in the high saturation region, optimum color reproduction can be performed without losing saturation.

  In the above description of the operation, it has been described that the color gamut is set in a device-independent color space such as the CIELab color space. However, the present invention is not limited to this, and the color of a color image signal input in an arbitrary color space. The same process can be performed when the area is limited.

  FIG. 9 is a block diagram showing an embodiment of the color conversion apparatus and color conversion method of the present invention. In the figure, reference numeral 41 denotes an address generation unit, 42 denotes a multidimensional lookup table, and 43 denotes an interpolation calculation unit. The address generator 41 generates an address of the multidimensional lookup table 42 according to the value of the color image signal on the input side. At this time, the interpolation operation unit 43 can also generate addresses corresponding to a plurality of grid points used for the interpolation operation. If the color signal to which the multi-dimensional lookup table 42 is input can be directly used as an address, the address generation unit 41 can be omitted.

  The multidimensional lookup table 42 is the multidimensional lookup table itself generated as described above. The converted data corresponding to each grid point is held at an address corresponding to each grid point, receives the address input from the address generation unit 41, and outputs the grid point data held at the address. .

  The interpolation calculation unit 43 performs an interpolation calculation based on a plurality of grid point data read from the multidimensional lookup table 42, and obtains a converted output color image signal corresponding to the input color image signal. The interpolation method is arbitrary. Depending on the interpolation method to be used, the address of the grid point required for interpolation is generated by the address generation unit 41 and input to the multidimensional lookup table 42, and the required grid point data is read from the multidimensional lookup table 42. Just put it out.

  In the case of this interpolation calculation, when the outline of the color gamut of the color image signal on the input side, especially the maximum saturation color, is input, it has not been possible to convert accurately by the grid points outside the color gamut. However, as in this embodiment, the color gamut is corrected in accordance with the grid points of the multidimensional lookup table, and the grid point data of the multidimensional lookup table is set using the corrected color gamut. Even if a saturation color is input, the extracted grid point data is data processed within the color gamut, so that the color image signal after color conversion can be accurately obtained by performing interpolation processing in the interpolation calculation unit 43. it can.

  Conventionally, turbidity may occur in the high brightness area. However, in this embodiment, the color space is divided into a plurality of areas, and the color gamut is corrected and generated by a method suitable for each area. It is possible to perform optimal color conversion while suppressing the turbidity of the chromaticity region and suppressing the saturation reduction in the high chromaticity region.

  In addition, when generating the color gamut of the color image signal on the input side, since the color gamut including them is generated from the color gamuts of a plurality of devices, it was used when generating the color image signal on the input side. Even if the apparatuses are different, color conversion can be performed satisfactorily.

It is a block diagram which shows one Embodiment of the color gamut creation apparatus of this invention. It is a flowchart which shows an example of operation | movement of a color gamut production | generation part. It is explanatory drawing of an example of the hue angle and lightness to set. It is explanatory drawing of an example of Cusp set to a hue plane, and an intermediate point. It is explanatory drawing of an example of the process which calculates | requires the outermost contour of a color gamut. It is a flowchart which shows an example of operation | movement of a color gamut correction part. It is explanatory drawing of an example of a color gamut correction process. It is explanatory drawing of the produced | generated color gamut. 1 is a block diagram showing an embodiment of a color conversion device and a color conversion method of the present invention. It is explanatory drawing of an example of the lattice point in a multidimensional lookup table. It is a conceptual diagram which shows an example of the color gamut of the color image signal input. It is explanatory drawing of an example of the relationship between the color gamut of the color image signal input in a certain hue surface, and the lattice point of a multidimensional lookup table.

Explanation of symbols

1 Color Gamut Generation Unit 2 Color Gamut Correction Unit 3 Multidimensional Lookup Table Generation Unit 41 Address Generation Unit 42 Multidimensional Lookup Table 43 Interpolation Operation Unit.

Claims (11)

The second color gamut generated based on the first color gamut in the predetermined color space of the input color image signal is divided into a plurality of areas, and the color of the second color gamut is divided for each divided area. Correction means for correcting the area formation data;
Generating means for generating a third color gamut based on the corrected color gamut formation data;
Color gamut creation device with   The predetermined color space is a color space defined by lightness, saturation, and hue, and the correction unit includes a plurality of predetermined specific colors on the outline for the saturation of the second color gamut. The color gamut formation data of the second color gamut is corrected based on each of the plurality of specific colors and a color at an intermediate point between the maximum brightness and the minimum brightness. Color gamut creation device.   The specific color includes a first specific color including at least a pure saturated color of red, yellow, green, cyan, blue, and magenta, and a second specific color that is an intermediate color of the first specific color. The color gamut creation device according to claim 2.   4. The color gamut creation device according to claim 2, wherein the plurality of areas include a first specific hue area, a second specific hue area, a high brightness area, and a high saturation area.   The color gamut creation device according to claim 1, wherein the input color image signal is a color image signal depending on a plurality of devices.   6. The correction unit according to claim 2, wherein the correction unit corrects the lightness to the maximum lightness while maintaining the saturation of the color gamut formation data of the first specific hue region. The described color gamut creation device.   The correction means has a saturation of color gamut formation data of the second specific hue region of the color image signals of the plurality of devices having the same hue angle and the same brightness as the color of the gamut formation data. The color gamut creation device according to claim 5 or 6, wherein the color gamut creation device corrects to a minimum saturation. The correction means, for the color gamut formation data of the high brightness area, the saturation is the minimum saturation of the color image signals of the plurality of devices having the same hue angle and the same brightness as the color of the color gamut formation data. First correction means for correcting to
When the color space is divided into a plurality of grids corresponding to the multi-dimensional lookup table, the color gamut after correction near a specific grid point of the grid to which the color of the color gamut formation data corrected by the first correction unit belongs When the color of the formation data is located, a second correction unit that corrects the saturation of the color gamut formation data around the color to the low saturation side;
The color gamut creation device according to claim 5, comprising: When the color space is divided into a plurality of grids corresponding to a multi-dimensional lookup table for the color gamut formation data of the high saturation area, the correction means includes the grid points of the grid to which the color of the color gamut formation data belongs. A selection means for selecting a grid point of maximum saturation;
A saturation correction means for correcting the saturation of the color gamut formation data based on the position of the selected grid point;
The color gamut creation device according to claim 2, comprising: Dividing the second color gamut generated based on the first color gamut in a predetermined color space of the input color image signal into a plurality of regions;
For each divided area, correct the color gamut formation data of the second color gamut,
Generating a third color gamut based on the corrected color gamut formation data;
Color gamut creation method. Dividing the second color gamut generated based on the first color gamut in a predetermined color space of the input color image signal into a plurality of regions;
Correcting the color gamut formation data of the second color gamut for each of the divided areas;
Generating a third color gamut based on the corrected color gamut formation data;
A gamut creation program for causing a computer to execute processing including

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