JP2006215808A - Color conversion method, color conversion device, program and recording medium - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To reliably reproduce a high saturation color without impairing a device color gamut in CMS for real time gamut processing. <P>SOLUTION: Color gamut information about an output device is generated with the use of an ICC profile (Step 201) and a color gamut conformance determination is made (Step 202). If an input color signal is outside the color gamut, it is mapped to a point within the color gamut (Step 203) and an output signal for the output device is calculated by interpolation using a polygon (Step 204). As for a color signal within the color gamut, an output signal is calculated by interpolation by 3D-LUT (Step 205). <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image processing device, an image processing method, a color conversion table creation method, and a recording medium. More specifically, when the color reproduction range of the input device is different from the color reproduction range of the output device, the input color image information is obtained. The present invention relates to a color reproduction processing apparatus that converts color image information within a color reproduction range of an output system, for example, a color image output apparatus such as a color facsimile, a color printer, and a color copying machine, and software that generates color conversion parameters used in the apparatus. It is about.

  Currently, a color management system (CMS) that realizes color matching between different devices such as a scanner, a display, and a printer is realized.

  In this CMS, as shown in FIG. 8, a color signal (for example, RGB signal) of an input device is mapped to a device-independent color space (for example, Lab space) with reference to the profile of the input device, and the profile of the output device The color signal of the device-independent space is converted into the color signal (for example, CMY signal) of the output device with reference to FIG. As a device profile, an ICC profile standardized by ICC (International Color Consortium) is often used. In addition, a device-independent color space is hereinafter referred to as a PCS space. As the PCS space, in addition to the Lab space, an XYZ space, a CIECAM02 space, or the like can be used.

  In the ICC profile, the correspondence between the PCS space and the output color space of the output device is described by a 3D-LUT or the like. In the color conversion process, the output value is obtained by performing a memory map interpolation operation using this 3D-LUT. calculate. Here, the 3D-LUT is a table in which a color space is divided on a grid as shown in FIG. 9 and an output value is associated with each grid point. For example, in the case of a CMY device, in order to convert a Lab signal, which is a PCS signal, into a CMY signal, the Lab space is divided into a grid and a CMY output value is associated with each grid point (for example, an ICC profile). (BtoA0Tag) and a CMY space are partitioned into a grid and a table in which Lab values are associated with each grid point (for example, AtoB0Tag of an ICC profile) is described. Here, the AtoBX tag is a tag for converting a device signal into a standard signal, and the BtoAX tag is a tag for reverse conversion of AtoBX.

  Incidentally, devices such as a display, a printer, and a scanner that process a color image have a color range that can be input or output, that is, a color gamut unique to the device. Since this color gamut is different for each device, a gamut process for converting the input color signal into a color within the color gamut of the output device is required. Until now, this gamut processing has been implemented by embedding it in the above-mentioned device profile because of its high calculation load. That is, gamut processing is performed in advance on each grid point in the PCS space to convert it into a PCS signal that can be reproduced by the output device, and an output signal CMY that reproduces the PCS signal after gamut processing is described as an output value of the grid point. I have to.

  The above-described method of embedding gamut processing in a profile is suitable for high-speed processing because it is not necessary to perform gamut processing during color matching. However, on the other hand, since the device manufacturer performs gamut processing independently, there is a problem that even if the same image data is output, the color reproduction of the output device varies greatly depending on the manufacturer.

  In order to improve such a situation, there has recently been a movement to execute gamut processing on the PC side. The possibility of realization of this is increasing due to the dramatic increase in CPU computing power and the development of gamut processing algorithms suitable for high-speed processing. When the gamut processing is executed on the PC side, the processing flow is as shown in FIG. That is, gamut processing is performed with reference to the input profile and output profile, and then converted into an output color signal. However, there is a case where the gamut processing is performed with reference to only the output profile without referring to the input profile.

  In such a gamut processing method that can be executed in real time, the color gamut of a target device is generally obtained in the PCS space, and the color gamut inside / outside determination and mapping processing are performed with reference to the color gamut. As such conventional techniques, there are methods described in Patent Documents 1 and 2, for example.

  Patent Document 1 describes a method of calculating a polygon describing a color gamut boundary surface at high speed by defining a polygon aggregate in a device color space and then converting the vertex coordinates of the polygon to a device-independent space. ing. Patent Document 2 proposes a method for calculating an arbitrary cross-sectional area in a device-independent space while using a polygon obtained from the device color space. In this method, a simple polygon is first defined, and a vector perpendicular to each side of the polygon and the intersection of the polygon are obtained. For example, a two-dimensional color gamut boundary in a predetermined hue can be obtained. . Such a cross-sectional shape in a predetermined hue has an advantage that it is easy to handle when performing a flexible mapping process.

  In Patent Document 3, by using different gamut processing algorithms in the vicinity of the color gamut boundary and other areas, the problem of accuracy reduction in the vicinity of the color gamut due to the interpolation calculation, and the color separated from the color gamut Trying to achieve both reproducibility.

JP 2003-8912 A JP 2003-244461 A JP 2000-134490 A

  In the gamut processing method using color gamut calculation described in Patent Documents 1 and 2, an input PCS signal can be converted into a PCS signal in the color gamut of the output device at a relatively high speed. However, to generate an output image, the mapped PCS signal must be converted into a device signal such as CMY. As a method for converting the PCS signal into a device signal, it is necessary to perform memory map interpolation with reference to the 3D-LUT described in the BtoA tag of the ICC profile described above.

  In the interpolation calculation using the 3D-LUT, since it is difficult to ensure the interpolation accuracy near the color gamut boundary, the gamut processing result in the PCS space cannot be reflected well in many cases. This phenomenon will be described with reference to the example of FIG. FIG. 11 shows an enlarged part of the color gamut of the output device on a certain hue plane. The dotted line is a line obtained by dividing the LC space in a lattice shape, and intersecting points (T0, T1, T2, T3, etc.) correspond to the lattice points. When generating a 3D-LUT of PCS → output signal, an output value must be assigned to each grid point. However, since T0, T1, T2, and T3 are out of the color gamut, the grid point output values are determined by compressing in the directions of the arrows. When an interpolation operation is performed using such grid point output values, even the point Pi on the color gamut boundary is shifted into the color gamut. Therefore, when the gamut processing is performed to compress the color gamut boundary and the interpolation calculation by the 3D-LUT is performed, it seems that the color gamut is apparently reduced.

  This problem is particularly problematic for image data that emphasizes vividness such as characters and graphics, and causes phenomena such as magenta color mixing and cloudiness even for characters originally intended to be reproduced in yellow. End up.

  To solve the above problem, Patent Document 3 attempts to solve the problem by generating a 3D-LUT using a plurality of gamut processes. However, the point that the interpolation calculation is performed using the 3D-LUT is the same as the conventional one, and the color gamut cannot be completely guaranteed.

The present invention has been made in view of the above problems,
An object of the present invention is to provide a color conversion method and apparatus capable of reliably reproducing high-saturation colors without impairing the device color gamut in a CMS that performs gamut processing in real time.

  That is, an object of the present invention is to provide a color conversion method for reproducing a color by using the color gamut of the output device to the maximum when converting a device independent color signal to a color signal of the output device. It is an object.

  The object of claim 2 is a color conversion that can reliably reproduce a color signal outside the color gamut of the output device with the color gamut boundary color of the output device when the device independent color signal is converted into the color signal of the output device. It aims to provide a method.

  It is an object of the present invention to provide a color conversion method for reproducing text and graphics with a turbid color.

  The purpose of claim 4 is to provide a color conversion method for execution in an open system.

  An object of the present invention is to enable color gamut compression and color conversion to an output signal at high speed.

  An object of the present invention is to provide a color conversion method for reproducing text and graphics in a color with no turbidity.

  An object of the present invention is to provide a color conversion method for achieving both gradation and vividness for a gradation image.

  An object of the present invention is to perform color conversion with an emphasis on gradation on a bitmap image.

  An object of the present invention is to convert a color signal that is out of the color gamut of the output device with the color gamut boundary color of the output device when converting the device independent color signal into the color signal of the output device. The object is to provide a device.

  The object of the tenth and eleventh aspects is to reliably reproduce a color signal outside the color gamut of the output device with the color gamut boundary color of the output device when the device independent color signal is converted into the color signal of the output device. It is an object of the present invention to provide a program for causing a computer to execute a color conversion method and a storage medium storing the program.

  According to a first aspect of the present invention, in the color conversion method for converting an input color signal into a color signal of an output device, a step of reading polygon data representing a color gamut boundary surface of the output device in a device-independent color space, and the input color A step of mapping a signal onto the read polygon surface, a calculation step of performing an interpolation calculation with reference to the color coordinate value of the mapped point and the vertex data of the polygon, and converting it into a color signal of an output device; It is characterized by having.

  According to a second aspect of the present invention, in the color conversion method for converting an input color signal into a signal of an output device, a step of reading polygon data representing a color gamut boundary surface of the output device in a device-independent color space; Determining whether or not the output color gamut is within the color gamut of the output device, and according to the determination result, an interpolation operation is performed with reference to the color coordinate value of the point where the input color signal is mapped onto the polygon surface and the vertex data of the polygon. A step of selecting a first calculation process to be performed and a second calculation process to perform an interpolation calculation with reference to a three-dimensional lookup table having a device-independent space as an input space.

  According to a third aspect of the present invention, in the color conversion method for converting input image data into a signal of an output device, the step of determining the gradation of the input image by analyzing the attribute or color distribution of the input image, and the determination As a result, when the input image is a gradation image, an operation step of performing an interpolation operation with reference to a three-dimensional lookup table having a device-independent space as an input space, and the input image is a non-gradation image and an input When the color signal included in the image is out of the color gamut, polygon data representing the color gamut boundary surface of the output device in the device-independent color space is read, mapped onto the polygon surface, and the color coordinates of the mapped point And a calculation step of performing an interpolation calculation with reference to the value and the vertex data of the polygon.

  According to a fourth aspect of the invention, the polygon data is generated by reading characteristic data in which a device-independent signal is associated with a device signal from a color profile of an output device.

  According to a fifth aspect of the present invention, the vertex data of the polygon includes a color coordinate value in a device-independent space of each vertex and an output value in an output device color space.

  The invention according to claim 6 is characterized in that the determination means is performed in units of objects.

  According to a seventh aspect of the present invention, when the object is a gradation object, a step of creating a list of colors used by the gradation, a step of determining the inside / outside of a color gamut for each of the color lists, and the determination As a result, when all the colors are out of the color gamut, the polygon data representing the color gamut boundary surface of the output device in the device-independent color space is read, mapped onto the polygon surface, and the color of the mapped point And interpolating calculation with reference to coordinate values and vertex data of the polygon.

  The invention according to claim 8 is characterized in that, in the determination step, when the input object is a bitmap image, a gradation image is obtained.

  According to a ninth aspect of the present invention, in the color conversion device for converting an input color signal into a signal of an output device, means for reading polygon data representing a color gamut boundary surface of the output device in a device-independent color space, and the input color signal A first calculation for performing an interpolation calculation with reference to a color coordinate value of a point obtained by mapping the input color signal on the polygon surface and vertex data of the polygon Means, second calculation means for performing an interpolation calculation with reference to a three-dimensional lookup table having a device-independent space as an input space, and output using the first or second calculation means according to the determination result And a means for calculating a signal.

  According to the tenth and eleventh aspects of the present invention, a computer is caused to execute the color conversion method according to any one of the first to eighth aspects.

  In the color conversion method according to claim 1, a step of reading polygon data representing a color gamut boundary surface of an output device in a device-independent color space, a step of mapping the input color signal onto the read polygon surface, An output process using the color gamut to the maximum, because it has an operation step of performing an interpolation operation with reference to the color coordinate value of the mapped point and the vertex data of the polygon and converting it to the color signal of the output device can do.

  3. The color conversion method according to claim 2, further comprising: reading polygon data representing a color gamut boundary surface of the output device in a device-independent color space; and determining whether the input color signal is within the color gamut of the output device. In accordance with the determination result, a first calculation process for performing an interpolation calculation with reference to a color coordinate value of a point obtained by mapping the input color signal on the polygon surface and vertex data of the polygon, and a device-independent space are input. A second calculation process for performing an interpolation calculation with reference to a three-dimensional lookup table as a space, and a step of selecting a color signal that is out of the color gamut of the output device. Can be output in color.

  In the color conversion method according to claim 3, the step of determining the gradation property of the input image by analyzing the attribute or color distribution of the input image, and as a result of the determination, when the input image is a gradation image, A calculation step of performing an interpolation calculation with reference to a three-dimensional lookup table having a device-independent space as an input space, and a device in which the input image is a non-tone image and the color signal included in the input image is out of the color gamut Polygon data representing the color gamut boundary surface of the output device in an independent color space is read, mapped onto the polygon surface, and an interpolation operation is performed with reference to the color coordinate value of the mapped point and the vertex data of the polygon. And a calculation process to be performed, so that high-saturated text and graphics can be output in a non-turbid color.

  5. An open color management system such as an ICC profile for generating polygon data by reading characteristic data in which a device-independent signal is associated with a device signal from a color profile of an output device. Can be easily installed.

  6. The color conversion method according to claim 5, wherein the vertex data of the polygon includes a color coordinate value in the device-independent space of each vertex and an output value in the output device color space, so that an interpolation operation using the polygon is executed at high speed. I can do it.

  In the color conversion method according to the sixth aspect, since the determination is performed in units of objects in the method according to the third aspect, it is possible to execute an appropriate interpolation calculation for each object.

  In the color conversion method according to claim 7, when the object is a gradation object, a step of creating a list of colors used by the gradation, a step of determining inside / outside of a color gamut for each of the color lists, In the determination result, when all the colors are out of the color gamut, the polygon data representing the color gamut boundary surface of the output device in the device-independent color space is read, mapped onto the polygon surface, and the mapped point And a step of performing an interpolation calculation with reference to the color coordinate values and the vertex data of the polygon, an appropriate interpolation calculation can be executed according to the color characteristics of the gradation.

  In the color conversion method according to the eighth aspect, when the attribute of the input object is a bitmap image, since it is a gradation image, it is not necessary to analyze the color distribution and the processing can be speeded up.

  10. The color conversion apparatus according to claim 9, wherein means for reading polygon data representing a color gamut boundary surface of an output device in a device-independent color space; means for determining whether the input color signal is within the color gamut of the output device; A first calculation means for performing an interpolation calculation with reference to a color coordinate value of a point obtained by mapping the input color signal on the polygon surface and vertex data of the polygon, and a three-dimensional system having a device-independent space as an input space Since there is a second calculation means for performing an interpolation calculation with reference to a lookup table and a means for calculating an output signal using the first or second calculation means according to the determination result, the color of the output device It is possible to provide a color conversion apparatus that can reliably output a color signal that is out of the gamut with the color gamut boundary color of the output device.

  In the program according to claim 10 and the storage medium according to claim 11, in order to cause the computer to execute the color conversion method according to any one of claims 1 to 8, a color signal outside the color gamut of the output device is received. It is possible to provide software for reliably outputting in the color gamut boundary color of the output device.

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

Example 1:
1. Configuration of Image Processing Device FIG. 1 is a diagram showing a configuration of a color image processing device according to the first embodiment of the present invention. In the figure, reference numeral 30 denotes a color image processing apparatus main body, which is a computer including a calculation means 31 such as a CPU and MPU, an internal storage device 32 such as a magnetic disk and a semiconductor memory, an input / output interface (I / O) 33 and the like. The color image processing apparatus main body 30 includes an external storage device 35 composed of a magnetic disk, a recording medium interface 34 for accommodating a removable recording medium such as a CD-ROM, DVD, memory card, magneto-optical disk, and the like. Are connected to a communication device 36 for communicating with a device such as a server, an input device 41 such as a keyboard and a mouse, and a display device 42 such as a CRT and a liquid crystal display.

  In this embodiment, the color image processing apparatus performs color management so that optimum color reproduction can be performed even when an image forming apparatus having a different color reproducibility such as a color reproduction range is used. Device. For example, when the same color image is reproduced between devices having different color reproduction ranges such as a color display such as a CRT and a color image forming apparatus such as a color printer or a color copier, the color image processing is performed. The device can be used for color management.

  Here, the internal storage device 32 stores a computer program for causing the calculation means 31 to execute a series of operations of the color image processing. The calculation means 31 executes a color conversion process described later by sequentially reading and executing the computer programs stored in the internal storage device 32. The internal storage device 32 or the external storage device 35 also stores a color profile corresponding to a device such as a display or a printer.

  The recording medium interface 34 accommodates a removable recording medium storing the computer program and various data. The computer program and various data stored in the recording medium are read out, transferred to the external storage device 35, and stored. Accordingly, the computer program can be transferred from the external storage device 35 to the internal storage device 32 when necessary, such as when the color image processing apparatus main body 30 is started up. The computer program can be directly transferred from the recording medium to the internal storage device 32. The result of the color image processing executed by the calculation means 31 can also be stored in the internal storage device 32, the external storage device 35, or a recording medium accommodated in the recording medium interface 34.

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

2. Color Conversion Processing Hereinafter, the color conversion processing according to the first embodiment will be described with reference to the flowchart of FIG. In the description of the first embodiment, the target output device color space is a CMY color space, and the device-independent color space (PCS) is a CIELAB (L * a * b *) color space. However, the present invention is not limited to this, and can be applied to other output device color spaces and other PCS color spaces such as CIECAM02 (J, C, H). In the following description, it is assumed that the range of CMY color signals handled in the first embodiment is [0, 255] for both C, M, and Y.

  FIG. 2 is a flowchart showing a color conversion process executed by the image processing apparatus according to the first embodiment. A program for executing this process is stored in the internal storage device 32 and executed by the computing means 31. .

[2.1 Generation of color gamut information]
First, in step S201, color gamut information is generated using color profile data stored in the external storage device 35 or the internal storage device 32. As shown in FIG. 3, the color gamut information includes an identification ID of a polyhedron (polygon) that covers the surface of the color gamut at a predetermined hue and brightness, a vertex list that constitutes the polygon, coordinate values in the output color space of each vertex, and PCS. It is represented by coordinate values in space. In the example of FIG. 3, in the device-independent color space, the lightness L is divided into 10 parts and the hue H is divided into 12 parts every 30 °, and IDs of polygons belonging to the divided lightness values and hues are described. The ID numbers of vertices constituting each polygon are registered in another list. Furthermore, C, M, Y, L, a, and b data are also associated with the vertex ID number.

  By creating such color gamut information in advance, it is possible to quickly access a polyhedron to be searched when performing inside / outside determination or mapping processing on the input signal.

  As a method for creating such color gamut information, a known method can be used. As an example, a method for generating color gamut information of a CMY device using an ICC profile will be described.

(Create vertex list)
From the ICC profile, the lattice points on the cube surface in the CMY space and the PCS coordinate values thereof are read from the 3D-LUT table described in the AtoB tag, and a list of vertex IDs, CMY coordinate values, and PCS coordinate values is created.

  For example, if the AtoB tag divides C, M, and Y into 5 parts, Lab values for 6 × 6 × 6 = 216 lattice points are described in the AtoB tag. As described above, if the range of the CMY color signal is [0, 255] for both C, M, and Y, the points (0, 0, 0), (255, 0, 0), ( 0, 255, 0), (0, 0, 255), (255, 255, 0), (255, 0, 255), (0, 255, 255), (255, 255, 255) Since the surface of the cube to be configured is the color gamut of the target device, when only the lattice points on the surface are extracted, the number of vertices registered in the vertex list is 216−4 × 4 × 4 = 152 points.

  The distribution of the 152 vertexes is shown in FIG. As shown in FIG. 4, the vertices are regularly arranged in the CMY space, but are irregularly arranged in the PCS space as shown in FIG.

(Create polygon list)
After the vertex list is created, a plurality of polygons (here, triangles) that cover the cubic color gamut in the device color space without excess or deficiency are determined. This triangle is defined using three vertices registered in the vertex list. Here, the triangle is determined by dividing all the quadrilaterals, which are the smallest units on the surface of the color gamut generated by dividing the cubic color gamut in the CMY color space into a grid, by diagonal lines. Then, a polygon list is created by registering a triple of vertex list indices corresponding to the vertices of each triangle. Accordingly, when C, M, and Y are divided into five as described above, the number of polygons is 5 × 5 × 6 × 2 = 300 (pieces).

  With the above configuration, the polygon list covering the color gamut of the target CMY device represented by the triple list of the vertex list on the color gamut surface of the CMY device represented by the coordinates in the PCS color space and the vertex list index is obtained. A polyhedron representing the color gamut of the RGB device in the target L * a * b * color space can be constructed. However, in the above data, since a large amount of calculation is required when specifying a polygon at an arbitrary lightness and hue, the polygon ID is further associated with the lightness and hue.

For example, it is assumed that the Lab coordinate values of the three vertices of the triangle constituting the polygon with ID = 1 are (90, 60, 50), (85, 66, 55), and (80, 72, 45).
Hue angle H = tan ⁻¹ (b / a)
Therefore, the hue H at the three vertices is 39 °, 39 °, and 32 °. Therefore, since this polygon is located at a lightness of 80-90 and a hue of 32 ° -39 °, the polygon ID = 1 is registered in the memory corresponding to the hue of 30 ° and the lightness of 80 in FIG. The above table is executed for all polygons to complete the table of FIG. By performing such association, mapping processing described later can be performed at higher speed.

  In the above description, the vertex list and the polygon list are used as the data structure representing the polyhedron. However, the present invention is not limited to this, and the data structure can uniquely represent the polyhedron, that is, the target color gamut. Any data structure may be used. For example, each polygon may be directly represented by the coordinates of three vertices without dividing the vertex list and the polygon list.

[2.2 Color Gamut Inside / Outside Judgment]
When the color gamut information is created, next, in step 202, the color gamut inside / outside determination is performed. For example, assume that the input signal is LCH = (50, 90, 0). At this time, polygons belonging to lightness = 50 and hue = 0 are extracted using the above-described color gamut information. Then, it is possible to determine whether the color gamut is inside or outside by performing geometric determination on each polygon. As a geometric determination method using this polygon, a tetrahedron connecting the three vertexes of the polygon and the point LCH = (50, 0, 0) on the achromatic color axis is assumed, and the input signal exists inside the tetrahedron. It can be determined by examining the sign of the plane equation.

  In addition, the inside / outside determination may be performed using a gamut tag that is standardly used in the ICC profile, and in the present invention, the method is not particularly limited.

[2.3 Mapping processing]
Next, when the input color signal is out of the color gamut, mapping is performed to a point inside the color gamut using a known method such as a mapping method using a projection point (step 203).

  A specific procedure will be described with reference to FIGS. First, in step 301, a compression projection point is determined. In general, as the compression projection point, a point on the achromatic axis and in the color gamut of the output device is often set (T in FIG. 5). Next, in step 302, polygons used to calculate mapping points are extracted. For example, if a mapping process with a constant hue is performed, a polygon assigned on the same hue as the input signal is extracted from the above-described color gamut information.

  In step 303, mapping points are obtained in order for the extracted polygons. The calculation of the mapping point uses a method for obtaining an intersection of a polygon representing a plane and a straight line connecting the input signal and the compression projection point.

Now, the input signal (l, a, b), the compression target point is (lc, ac, bc), and the polygonal plane equation is ax + by + cz + d = 0.
And At this time, the vector representing the straight line is
(F, g, h) = (lc−1, ac−a, bc−b)
It is represented by At this time, the intersection of the plane and the straight line can be calculated by the following equation.
(X, y, z) = (x0 + ft, y0 + gt, z0 + ht)
However, t = − (ax0 + by0 + cz0 + d) / (af + bg + ch)

  Although the intersection is obtained by the above calculation, the intersection is not necessarily a point inside the polygon. Therefore, in step 304, the value of the intersection is used to check whether there is an intersection in the triangle. In general, when the point P is inside or on the circumference of the triangle ABC, the condition is

と表されるため、上式を満たすか否かを判定すればよい。もし、三角形内に交点がない場合には、交点が見つかるまで次のポリゴンについて調べる。

Therefore, it may be determined whether or not the above formula is satisfied. If there is no intersection in the triangle, the next polygon is examined until an intersection is found.

  If an intersection point is found by the above processing, the Lab value of the intersection point is set as a color signal after color gamut mapping.

  In the above description, the mapping is performed on the same hue plane as the input color signal. However, the present invention is not limited to this. For example, the mapping process is performed after shifting the hue of the input color signal. It may be a method.

[2.4 Interpolation calculation]
When the coordinate values in the perceived color space after color gamut mapping are determined by the above processing, output signals C, M, and Y of the output device are calculated in step 204. The present invention is characterized in that the device output value is calculated not by interpolation using 3D-LUT but by interpolation using polygons. In this interpolation operation, the triangle vector addition formula described above is used.

を使って計算できる。

Can be calculated using.

For example, the color signal subjected to color gamut mapping is P (Lx, ax, bx), and the vertices of the polygon including the point are A (L0, a0, b0), B (L1, a1, b1), C Assume that (L2, a2, b2). For A, B, and C, the output values of the CMY device read from the vertex list data are A (c0, m0, y0), (c1, m1, y1) (c2, m2, y2).
At this time,

が成り立つとする。上式は、Ｌａｂ空間上での式であるが、Ｌａｂ信号からＣＭＹ信号への変換は線形変換で行なうため、ＣＭＹ空間においても上式が成り立つものとみなせる。
従って、ＣＭＹ信号値は
ｃｐ＝０．２（ｃ１−ｃ０）＋０．６（ｃ２−ｃ０）＋ｃ０
＝０．２・ｃ０＋０．２・ｃ１＋０．６・ｃ２
ｍｐ＝０．２（ｍ１−ｍ０）＋０．６（ｍ２−ｍ０）＋ｍ０
＝０．２・ｍ０＋０．２・ｍ１＋０．６・ｍ２
ｙｐ＝０．２（ｙ１−ｙ０）＋０．６（ｙ２−ｙ０）＋ｙ０
＝０．２・ｙ０＋０．２・ｙ１＋０．６・ｙ２
として簡単な補間演算で求めることができる。

Suppose that The above expression is an expression on the Lab space, but since the conversion from the Lab signal to the CMY signal is performed by linear conversion, it can be considered that the above expression also holds in the CMY space.
Therefore, the CMY signal value is cp = 0.2 (c1-c0) +0.6 (c2-c0) + c0
= 0.2 · c0 + 0.2 · c1 + 0.6 · c2
mp = 0.2 (m1-m0) +0.6 (m2-m0) + m0
= 0.2 · m0 + 0.2 · m1 + 0.6 · m2
yp = 0.2 (y1-y0) +0.6 (y2-y0) + y0
= 0.2 · y0 + 0.2 · y1 + 0.6 · y2
Can be obtained by a simple interpolation calculation.

  With the above calculation, an output signal can be obtained by performing an interpolation operation using a polygon surface. Each vertex of the polygon is a point on the color gamut surface of the CMY device. As described above, if the range of the CMY color signal is [0, 255] for both C, M, and Y, the CMY device color space Points (0,0,0), (255,0,0), (0,255,0), (0,0,255), (255,255,0), (255,0,255), ( 0, 255, 255) and (255, 255, 255), which correspond to points on the surface of the cube. Therefore, at the vertex of the polygon, any of the CMY values takes either 0 or 255, and the value obtained by the linear interpolation also has either 0 or 255. Therefore, it is possible to reliably reproduce the color on the color gamut boundary that cannot be obtained by the interpolation calculation by the conventional 3D-LUT.

  On the other hand, since the above interpolation calculation cannot be used for a color signal that is determined to be within the color gamut after performing the color gamut determination in step 202, the interpolation calculation based on the conventional 3D-LUT is performed.

  In the configuration example shown in FIG. 1, the color conversion process is performed by the calculation means 31, but of course, the calculation may be performed by dedicated hardware. In the above description, the CMY device is described. However, a printer apparatus that can be decomposed into CMYK by BG / UCR processing or the like can also be regarded as a CMY output device. Further, Patent Document 1 described above describes a method for generating color gamut information of a CMYK device. If color gamut information is generated by this method, an output value is calculated by polygon interpolation for any CMYK device. Can also be performed.

(Example 2)
In the above-described color conversion method, the color signal of the output device is obtained by performing an interpolation operation using a polygon on the color signal outside the color gamut. However, since colors in the color gamut cannot be obtained by the above method, it is necessary to use the same interpolation calculation using a 3D-LUT as in the prior art. However, when color conversion is performed on a drawing object that continuously changes from the color gamut to the color outside the color gamut, a gradation jump occurs and becomes a problem. On the other hand, many drawing objects such as characters and graphics are filled with a single color. Therefore, in the present embodiment, the problem of discontinuity at the color gamut boundary is avoided by switching the color conversion according to the characteristics of the object.

  Hereinafter, the color conversion method according to the second embodiment will be described with reference to the flowchart of FIG. When printing is instructed, first, in step 401, color gamut information of the output device is generated. The color gamut information generation method is the same as that in the first embodiment. Next, when image data is input, the attribute of the object is analyzed. In step 402, it is determined whether the object is image data. In the case of image data, since it is a gradation image including data in the color gamut in most cases, the process advances to step 407 to perform an interpolation operation (second interpolation operation) using a 3D-LUT. Next, in step 403, it is determined whether or not the object is gradation data. In the case of gradation data, some gradations are used only outside the color gamut, while others include data within the color gamut. Therefore, it is desirable to analyze the color distribution used in gradation and determine whether to perform the first interpolation calculation or the second interpolation calculation. However, if color gamut inside / outside determination is performed for all colors used in gradation, color conversion takes time. Therefore, interpolation calculation may be performed using a 3D-LUT as in the case of a bitmap.

  If the CPU's computing power is high, a list of colors constituting the gradation is first created, and the color gamut determination in step 404 is performed for all the colors. If all the colors are out of the color gamut, An interpolation calculation using a polygon (first interpolation calculation) may be performed.

  An object other than a bitmap or gradation is generally an image such as a character object such as a single color or a shaded pattern. Therefore, when the object does not have such gradation, interpolation calculation using a polygon is performed. In particular, there are many users who prefer vivid output for text and single color painting, and the color reproducible impression is improved even in this embodiment.

The structure of Example 1 of this invention is shown. 2 is a flowchart of color conversion processing according to the first exemplary embodiment. It is a figure explaining color gamut information. An example of vertex data and polygons is shown. It is a figure explaining a mapping process. 2 is a flowchart of mapping processing according to the first embodiment. 7 is a flowchart of color conversion processing according to the second embodiment. 1 shows a schematic configuration of a color management system. An example of a grid point is shown. 1 shows a schematic configuration of a color management system when gamut processing is executed on a PC side. It is a figure explaining the problem of the conventional interpolation calculation.

Explanation of symbols

30 Color Image Processing Device 31 Computing Means 32 Internal Storage Device 33 I / O
34 Storage media I / F
35 External storage device 36 Communication device 41 Input device 42 Display device

Claims (11)

  In a color conversion method for converting an input color signal to a color signal of an output device, a step of reading polygon data representing a color gamut boundary surface of the output device in a device-independent color space, and the input color signal on the read polygon surface And a calculation step of performing an interpolation calculation with reference to the color coordinate value of the mapped point and the vertex data of the polygon to convert it into a color signal of an output device. Conversion method.   In a color conversion method for converting an input color signal into a color signal of an output device, a step of reading polygon data representing a color gamut boundary surface of the output device in a device-independent color space, and whether the input color signal is within the color gamut of the output device Determining whether or not, and according to the determination result, a first calculation process for performing an interpolation calculation with reference to a color coordinate value of a point obtained by mapping the input color signal on the polygon surface and vertex data of the polygon; A color conversion method comprising: a step of selecting a second calculation process for performing an interpolation calculation with reference to a three-dimensional lookup table using a device-independent space as an input space.   In a color conversion method for converting input image data into a color signal of an output device, the step of analyzing the attribute or color distribution of the input image to determine the gradation of the input image, and as a result of the determination, the input image In the case of a gradation image, an operation step of performing an interpolation operation with reference to a three-dimensional lookup table using a device-independent space as an input space, and a color signal included in the input image when the input image is a non-gradation image. When out of the color gamut, polygon data representing the color gamut boundary surface of the output device in the device-independent color space is read, mapped onto the polygon surface, the color coordinate value of the mapped point and the vertex data of the polygon And a calculation step of performing an interpolation calculation with reference to the color conversion method.   4. The color conversion method according to claim 1, wherein the polygon data is generated by reading characteristic data in which a device-independent signal is associated with a device signal from a color profile of an output device.   4. The color conversion method according to claim 1, wherein the vertex data of the polygon includes a color coordinate value in a device-independent space of each vertex and an output value in an output device color space.   4. The color conversion method according to claim 3, wherein the determination step is performed for each object.   When the object is a gradation object, a step of creating a list of colors used by the gradation, a step of determining the inside / outside of the color gamut for each of the color lists, and all the colors in the determination result are colors. In the case of out-of-gamut, polygon data representing the color gamut boundary surface of the output device in the device-independent color space is read, mapped onto the polygon surface, and the color coordinate value of the mapped point and the vertex data of the polygon The color conversion method according to claim 6, further comprising a step of performing an interpolation operation with reference to FIG.   7. The color conversion method according to claim 6, wherein in the determination step, when the attribute of the input object is a bitmap image, a gradation image is obtained.   In a color conversion apparatus for converting an input color signal into a color signal of an output device, means for reading polygon data representing a color gamut boundary surface of the output device in a device-independent color space, and whether the input color signal is within the color gamut of the output device Means for determining whether or not, first calculation means for performing interpolation calculation with reference to the color coordinate value of the point obtained by mapping the input color signal on the polygon surface and vertex data of the polygon, and device-independent space Second calculation means for performing an interpolation calculation with reference to a three-dimensional lookup table using as input space, and a means for calculating an output signal using the first or second calculation means according to the determination result. A color conversion device comprising:   The program for making a computer perform the color conversion method of any one of Claim 1 thru | or 8.   A computer-readable recording medium having recorded thereon a program for causing a computer to execute the color conversion method according to claim 1.

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