JP2007158928A - Apparatus, method and program for color space conversion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an apparatus, a method and a program for color space conversion which can properly interpolate a three-dimensional lookup table in consideration of reproducibility of gradation while giving priority to reproducibility of a gray color for each input color space. <P>SOLUTION: The color space conversion apparatus converts a color space of a digital image expressed by an input color space into an output color space, using a three-dimensional lookup table corresponding to the input color space and an arbitrary output color space. The color space conversion apparatus is provided with a determining means for determining whether points corresponding to pixel values representing a gray color are a color space arranged on a diagonal of a cube, when the input color space is expressed as a cube on a three-dimensional space; and a conversion means for interpolating the three-dimensional lookup table by tetrahedral interpolation if the determining means gives positive determination, and interpolating the three-dimensional lookup table by hexahedral interpolation or triangular prism interpolation if the determining means gives negative determination, thereby converting the color space of the digital image. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a color space conversion apparatus, a color space conversion method, and a color space conversion program that perform color space conversion using a three-dimensional lookup table.

Conventionally, hexahedral interpolation, triangular prism interpolation, tetrahedral interpolation and the like are known as interpolation algorithms for interpolating a three-dimensional lookup table used for color space conversion of a digital image. Each of these interpolation algorithms has the following advantages and disadvantages.
For example, hexahedral interpolation has an advantage in that the reproducibility of gradation is superior to tetrahedral interpolation regardless of the color space because there are more reference points than tetrahedral interpolation. On the other hand, hexahedral interpolation corresponds to a pixel value representing a gray color (for example, R = G = B) when expressed as a cube in a three-dimensional space with each pixel component as a coordinate axis, such as an RGB color space. When the input color space is aligned on the diagonal line of the cube, the gray color is less reproducible than tetrahedral interpolation because it refers to the grid points representing non-gray colors for gray interpolation. is there. Triangular prism interpolation has the same properties as hexahedral interpolation.

  On the other hand, tetrahedral interpolation has an advantage that gray color reproducibility is superior to hexahedral interpolation because the gray color interpolation refers only to the grid points representing the gray color when the input color space is the RGB color space. On the other hand, tetrahedron interpolation has a drawback in that the reproducibility of gradation is inferior to hexahedron interpolation regardless of the color space because the number of grid points to be referenced is smaller than hexahedron interpolation.

By the way, in general, the reproducibility of gray color is higher than the reproducibility of gradation in terms of the degree of influence on the impression of image quality that a user receives when viewing a digital image. However, although the degree of influence is lower than the reproducibility of the gray color, the reproducibility of the gradation naturally affects the user's impression of the image quality.
However, in the conventional color space conversion, any one interpolation algorithm is used. For this reason, for example, when tetrahedral interpolation is used with priority given to gray color reproducibility, there is a problem that the impression that the reproducibility of gradation is always low is given. Conversely, when hexahedral interpolation or triangular prism interpolation is used with priority given to gradation reproducibility, there is a problem that gray color reproducibility is poor when the input color space is the RGB color space.

  The present invention was created in order to solve the above-described problem, and appropriately takes into account the reproducibility of the gray color while giving priority to the reproducibility of the gray color for each input color space, and appropriately three-dimensional lookup table. An object of the present invention is to provide a color space conversion device, a color space conversion method, and a color space conversion program.

(1) A color space conversion apparatus for achieving the above object uses a three-dimensional lookup table corresponding to an input color space representing a color by a pixel value composed of three pixel components and an arbitrary output color space. A color space conversion device for converting a color space of a digital image represented in an input color space into the output color space, wherein the input color space is expressed as a cube in a three-dimensional space with the pixel components as coordinate axes. If it is determined that the point corresponding to the pixel value representing the gray color is a color space lined up on the diagonal of the cube, and if the determination is positive, the 3 A conversion that interpolates a three-dimensional lookup table and interpolates the three-dimensional lookup table by hexahedral interpolation or triangular prism interpolation when a negative determination is made, and converts the color space of the digital image It includes a stage, a.
According to the present invention, in the case of an input color space in which points corresponding to pixel values representing a gray color are arranged on a diagonal line of a cube, interpolation with excellent gray color reproducibility can be performed by performing tetrahedral interpolation. On the other hand, in the case of an input color space that is not a color space in which the gray colors are arranged on the diagonal line, the gray color is reproduced for the input color space in which the gray colors are arranged on the coordinate axes of the three-dimensional space by performing hexahedral interpolation or triangular prism interpolation. Interpolation with excellent gradation and reproducibility is possible, and the input color space that is not a color space in which the gray colors are arranged on the coordinate axes of the three-dimensional space is inferior in gray color reproducibility. Interpolation with excellent tone reproducibility.
That is, according to the present invention, hexahedral interpolation or triangular prism interpolation is applied to an input color space that can be interpolated with excellent gray color and gradation reproducibility by hexahedral interpolation or triangular prism interpolation. In addition, even if hexagonal interpolation or triangular prism interpolation is performed, both gray color and gradation reproducibility cannot be interpolated, but input color space that can be interpolated with excellent gray color reproducibility by tetrahedral interpolation is a gradation. The tetrahedral interpolation is applied in favor of the gray color reproducibility over the color reproducibility. For input color spaces that cannot be interpolated with excellent gray color and gradation reproducibility even with hexahedral interpolation or triangular prism interpolation, and cannot be interpolated with excellent gray color reproducibility even with tetrahedral interpolation Interpolation with excellent gradation reproducibility is performed by applying hexahedral interpolation or triangular prism interpolation. Therefore, according to the present invention, in color space conversion using a three-dimensional lookup table, a three-dimensional lookup table is appropriately set in consideration of gradation reproducibility while giving priority to gray color reproducibility for each input color space. Can be interpolated.

(2) The determination unit may determine the type of the input color space without checking whether or not the points corresponding to the pixel values representing the gray color are aligned on the diagonal of the cube. .
According to the present invention, since the type of the input color space is determined and determined, there is no need to actually check whether or not the points corresponding to the pixel values representing the gray color are aligned on the diagonal of the cube. Therefore, it can be determined by simple processing.

(3) The input color space may be an RGB color space, and the output color space may be an L * a * b * color space.
Since the RGB color space is a color space in which points corresponding to pixel values representing a gray color are arranged on a diagonal of a cube, according to the present invention, the color space of a digital image is converted from the RGB color space to the L * a * b * color space. In this case, although the gradation reproducibility is inferior, interpolation with excellent gray color reproducibility can be performed.

(4) The input color space may be an L * a * b * color space, and the output color space may be an RGB color space.
The L * a * b * color space is a color space in which points corresponding to pixel values representing a gray color are not arranged on the diagonal of the cube, but are arranged on the coordinate axis of the three-dimensional space. When converting from the L * a * b * color space to the RGB color space, interpolation with excellent gray color and gradation reproducibility can be performed.

(5) A color space conversion method for achieving the above object uses the three-dimensional lookup table corresponding to an input color space representing a color by a pixel value composed of three pixel components and an arbitrary output color space. A color space conversion method for converting a color space of a digital image represented in an input color space into the output color space, wherein the input color space is expressed as a cube in a three-dimensional space with the pixel components as coordinate axes. A determination step for determining whether a point corresponding to a pixel value representing a gray color is a color space arranged on a diagonal line of the cube, and, if affirmative determination is made in the determination step, the step 3 A conversion that interpolates a three-dimensional lookup table and interpolates the three-dimensional lookup table by hexahedral interpolation or triangular prism interpolation when a negative determination is made, and converts the color space of the digital image It includes a floor, a.
According to the present invention, in color space conversion using a three-dimensional lookup table, a three-dimensional lookup table is appropriately interpolated in consideration of gradation reproducibility while giving priority to gray color reproducibility for each input color space. it can.

(6) A color space conversion program for achieving the above object uses the three-dimensional lookup table corresponding to an input color space representing a color by a pixel value composed of three pixel components and an arbitrary output color space. A color space conversion program for converting a color space of a digital image represented in an input color space into the output color space, wherein the input color space is expressed as a cube in a three-dimensional space with the pixel components as coordinate axes. If it is determined that the point corresponding to the pixel value representing the gray color is a color space lined up on the diagonal of the cube, and if the determination is positive, the 3 Interpolate a three-dimensional lookup table, and if a negative determination is made, interpolate the three-dimensional lookup table by hexahedral interpolation or triangular prism interpolation to obtain the color space of the digital image. Causing a computer to function as converting means for conversion.
According to the present invention, in color space conversion using a three-dimensional lookup table, a three-dimensional lookup table is appropriately interpolated in consideration of gradation reproducibility while giving priority to gray color reproducibility for each input color space. it can.

  Each function of the plurality of means provided in the present invention is realized by hardware resources whose functions are specified by the configuration itself, hardware resources whose functions are specified by a program, or a combination thereof. Further, the functions of the plurality of means are not limited to those realized by hardware resources that are physically independent of each other.

Hereinafter, embodiments of the present invention will be described based on examples.
FIG. 2 is a block diagram showing a schematic configuration of the printer 10 as a color space conversion apparatus according to an embodiment of the present invention. The printer 10 is a so-called stand-alone printer that can read a digital image from the removable memory 11 and perform printing based on the digital image. The printer 10 can input a digital image from an external system such as a digital camera 30, a PC (Personal Computer) 31, and a camera-equipped mobile phone terminal 32, and can print based on the digital image.

The external IF (Inter Face) 12 includes a USB controller, a USB connector, and the like for communicating with an external system such as the digital camera 30, the PC 31, and the camera-equipped mobile phone terminal 32. The communication standard is not limited to USB, but may be any standard such as IEEE 1394, infrared, Ethernet (registered trademark).
A removable memory controller (RMC) 13 is connected to the removable memory 11 via a connector (not shown), and controls data transfer between the removable memory 11 and the RAM 14. The removable memory 11 may be a card-type flash memory (so-called memory card) or any other nonvolatile storage medium that can be repeatedly written.

  The image processing unit 15 performs decompression processing of compressed digital images, γ correction, color matching, image correction processing such as sharpness correction processing / gradation correction processing, color separation processing, halftoning, interlace processing, and the like with the CPU. An image processing LSI that executes in cooperation and generates print data. The image processing unit 15 may be configured by a DSP, for example. These processes may be executed by a program process by the CPU 16. In this embodiment, a case where color matching is executed by the CPU 16 will be described as an example.

  The printing unit 17 includes a recording head for forming an image on a sheet by an inkjet method based on the print data generated by the image processing unit 15, a reciprocating mechanism of the recording head, a paper supply / discharge mechanism, and the like. As the printing method, any printing method such as an ink jet method, a laser method, a thermal method, or a dot impact method can be adopted.

  The display unit 20 includes an FPD (Flat Panel Display) such as an LCD, a graphic controller, and the like. A frame memory area necessary for displaying characters and images on the FPD may be secured in a dedicated memory of the display unit 20 or may be assigned to a partial area of the RAM 14. Characters and images displayed on the FPD are updated by periodically reading data written in the frame memory area by the graphic controller.

The operation unit 19 includes operation buttons for accepting user menu operations and print requests, a jog dial, and other various buttons. When a specific button is pressed in a specific mode, various requests corresponding to the mode are input to the printer 10.
The RAM 14 is a volatile memory that temporarily stores various programs and data.

  The CPU 16 controls each part of the printer 10 by executing a control program stored in the flash memory 18 to control execution of printing. The CPU 16 also functions as a determination unit and a conversion unit by executing a color space conversion program stored in the flash memory 18. The flash memory 18 is a nonvolatile memory that stores a control program, a color space conversion program, various ICC profiles, and the like. These various programs and data may be transferred from a computer-readable storage medium to the flash memory 18 or may be transferred from a remote server to the flash memory 18 via a communication network.

Next, a three-dimensional lookup table (3DLUT) will be described.
FIG. 3A is a schematic diagram showing a 3DLUT for converting a color space of a digital image from an RGB color space to a CIE L * a * b * color space (hereinafter simply referred to as “Lab color space”). If the RGB color space is represented on a three-dimensional space with the R component, G component and B component as coordinate axes, it can be represented as a cube as shown in the figure. When this cube is equally divided into nine in the R-axis, G-axis, and B-axis directions, it is divided into 729 cubes (the cube of 9). Note that the number of divisions can be selected as appropriate, and may be, for example, 16 divisions. In the following description, a simple cube means an individual divided cube. The 3DLUT is a table in which a pixel value (output pixel value) of the conversion destination of the Lab color space is associated with each vertex (hereinafter referred to as “grid point”) of the cube. When the point corresponding to the input pixel value (R, G, B) is a grid point, the output pixel value (L, a, b) corresponding to the input pixel value can be directly obtained by referring to the 3DLUT. it can. However, if the point corresponding to the input pixel value is not a grid point, the output pixel value cannot be obtained directly. In this case, the output pixel value can be obtained by interpolating the 3DLUT using the output pixel value associated with each grid point constituting the cube to which the point corresponding to the input pixel value belongs. As the interpolation method, hexahedral interpolation, triangular prism interpolation, tetrahedral interpolation and the like are known.

  FIG. 3B is a schematic diagram showing a 3DLUT for converting the color space of the digital image from the Lab color space to the RGB color space. In the 3DLUT shown in FIG. 3B, a point whose coordinate axes are the L * axis, a * axis, and b * axis, and a pixel value (output pixel value) of the RGB color space are associated with each grid point. This is the same as the 3DLUT shown in FIG.

Next, tetrahedral interpolation will be described.
4 and 5 are schematic diagrams for explaining tetrahedral interpolation. In tetrahedral interpolation, the cube (see FIG. 4) to which the point Qx corresponding to the input pixel value to be interpolated belongs is divided into six triangular pyramids (tetrahedron) as shown in FIG. In this division, the cube is divided so that the diagonal line 31 starting from the lattice point Q5 closest to the origin of the three-dimensional coordinate and starting from the lattice point Q2 farthest from the origin of the three-dimensional coordinate is used as one side in any tetrahedron. Is done. In the tetrahedron trap, the output pixel value associated with each grid point of the tetrahedron to which the point Qx belongs is weighted and averaged with a volume ratio corresponding to the position of the point Qx, thereby outputting the output corresponding to the point Qx. Interpolate pixel values. Hereinafter, a case where the point Qx belongs to the tetrahedron 32 having the lattice points Q5, Q0, Q1, and Q2 as vertices will be described as an example.

  Expressions 1 to 4 shown below are expressions for calculating the volume of the triangular pyramid corresponding to each lattice point of the tetrahedron 32. Here, it is assumed that the length of each side of the cube is 1, and the position of the point Qx when the lattice point Q5 of the cube is the origin is (DL, Da, Db). In the following equation, V2 is the volume of the triangular pyramid (Q0, Q1, Q5, Qx) corresponding to the lattice point Q2, V5 is the volume of the triangular pyramid (Q0, Q1, Q2, Qx) corresponding to the lattice point Q5, V1 Is the volume of the triangular pyramid (Q0, Q2, Q5, Qx) corresponding to the lattice point Q1, and V0 is the volume of the triangular pyramid (Q1, Q2, Q5, Qx) corresponding to the lattice point Q0.

  Expression 5 shown below is an expression that weights and averages the output pixel values associated with the lattice points Q5, Q0, Q1, and Q2 with the volume ratio obtained by Expressions 1 to 4. P2 is an output pixel value associated with the lattice point Q2, P5 is an output pixel value associated with the lattice point Q5, P1 is an output pixel value associated with the lattice point Q1, and P0 is a lattice point Q0. Assuming that the output pixel value is associated with, the output pixel value Px corresponding to the point Qx is obtained by Equation 5. Here, 1/6 of the denominator is the volume of the tetrahedron 32.

  Here, the case where the point Qx belongs to the tetrahedron 32 has been described as an example, but the same applies to the case where the point Qx belongs to another tetrahedron. Since tetrahedral interpolation is a known interpolation method, a detailed description of the case of belonging to another tetrahedron is omitted.

Next, advantages and disadvantages of tetrahedral interpolation will be described.
(1) Gray color reproducibility when converting from the RGB color space to another color space.
When the RGB color space is represented by three-dimensional coordinates, a point corresponding to a pixel value representing a gray color (R = G = B) is located on the diagonal line 31. According to Equation 3 and Equation 4 above, DL = Da = Db holds when the point Qx is on the diagonal line 31.
Da-Db = 0
DL-Da = 0
Therefore, V1 and V0 become zero. That is, in the tetrahedral interpolation, the input pixel value representing the gray color in the RGB color space is not referred to the non-gray grid points Q1 and Q0, but only the gray grid points Q2 and Q5 are referred to. To do. For this reason, in the tetrahedral interpolation, the gray color reproducibility is excellent in the color space conversion from the RGB color space to another color space.

(2) Reproducibility of gray color when converting from the Lab color space to another color space.
When the Lab color space is represented by three-dimensional coordinates, a point corresponding to a pixel value representing a gray color (L, 0, 0) is located on the L axis. When point Qx is on the L axis,
Da = Db = 0
Therefore, V2 and V1 become zero. That is, in tetrahedral interpolation, the input pixel value representing the gray color in the Lab color space is not referred to the non-gray grid points Q1 and Q2, but only the gray grid points Q0 and Q5 are interpolated. To do. For this reason, in tetrahedral interpolation, gray color reproducibility is excellent even in color space conversion from the Lab color space to another color space.

(3) Tone reproducibility.
Tetrahedral interpolation is inferior in gradation reproducibility compared with hexahedral interpolation because it has a smaller number of reference points than hexahedral interpolation.

Next, hexahedral interpolation will be described.
FIG. 6 is a schematic diagram for explaining hexahedral interpolation. In hexahedral interpolation, a cube (hexahedron) to which a point Qx corresponding to an input pixel value to be interpolated belongs is divided into eight rectangular parallelepipeds by three planes passing through the point Qx and perpendicular to the coordinate axes. In the hexahedron trap, the output pixel value Px corresponding to the input pixel value is weighted and averaged with the output pixel values P0 to P7 associated with the lattice points Q0 to Q7 of the hexahedron at a volume ratio of eight cuboids. Interpolate.

  Expression 6 shown below is an expression that weights and averages the output pixel values P0 to P7 associated with the hexahedral lattice points Q0 to Q7 by the volume ratio of the eight rectangular parallelepipeds. Here, it is assumed that the length of each side of the cube before division is 1, and the position of the point Qx when the lattice point Q5 of the cube is the origin is (DL, Da, Db).

Next, advantages and disadvantages of hexahedral interpolation will be described.
(1) Gray color reproducibility when converting from the RGB color space to another color space.
When the RGB color space is represented by three-dimensional coordinates, a point corresponding to a pixel value representing a gray color (R = G = B) is located on the diagonal line 31. According to Equation 6 described above, when the point Qx is on the diagonal line 31, the output pixel value is interpolated with reference to all the lattice points Q0 to Q7. That is, in the hexahedral interpolation, even an input pixel value representing a gray color in the RGB color space is interpolated with reference to grid points Q0, Q1, Q3, Q4, Q6, and Q7 that are not gray. For this reason, in the hexahedral interpolation, the gray color reproducibility is inferior to the tetrahedral interpolation in the color space conversion from the RGB color space to another color space.

(2) Reproducibility of gray color when converting from the Lab color space to another color space.
When the Lab color space is represented by three-dimensional coordinates, a point corresponding to a pixel value representing a gray color (L, 0, 0) is located on the L axis. According to the above equation, when the point Qx is on the L axis,
Px = (1-DL) × P5 + DL × P0
become. That is, in the hexahedral interpolation, for the input pixel value representing the gray color in the Lab color space, the grid points Q1 to Q4, Q6, and Q7 that are not gray are not referred to, and only the grid points Q0 and Q5 that are the gray color are referred to. And interpolate. For this reason, in the hexahedral interpolation, in the color space conversion from the Lab color space to another color space, the reproducibility of the gray color is superior to the case of referring to other grid points.

(3) Tone reproducibility.
Since hexahedral interpolation has a larger number of reference points than tetrahedral interpolation, gradation reproducibility is superior to tetrahedral interpolation.

  To summarize the above, only tetrahedral interpolation can accurately interpolate the output pixel value corresponding to the gray color in the conversion from the RGB color space to another color space. In this embodiment, the gray color is prioritized over the gradation reproducibility, so it can be said that tetrahedral interpolation is appropriate when converting from the RGB color space to another color space.

  In the conversion from the Lab color space to another color space, the output pixel value corresponding to the gray color can be accurately interpolated by either tetrahedral interpolation or hexahedral interpolation. Gray interpolation can be performed accurately by either interpolation method, but hexagonal interpolation is superior in terms of gradation reproducibility. In this embodiment, grayscale is prioritized and gradation reproducibility is taken into account, so that hexahedral interpolation is appropriate when converting from the Lab color space to another color space.

  However, there is a color space in which the point corresponding to the gray color is neither on the coordinate axis nor on the diagonal line 31 like the XYZ color space. If the point corresponding to the gray color is neither on the coordinate axis nor on the diagonal line 31, the output pixel value corresponding to the gray color cannot be accurately interpolated by either tetrahedral interpolation or hexahedral interpolation. In this embodiment, the gray color is prioritized and the reproducibility of the gradation is taken into account. Therefore, if the gray color cannot be accurately interpolated by either interpolation method, it can be said that the hexahedral interpolation which is excellent in the gradation reproducibility is appropriate. .

Next, color space conversion executed in color matching will be described.
In color matching, for example, color space conversion for converting a digital image generated by a digital still camera (DSC) from the RGB color space to the Lab color space using the DSC profile, and the digital image converted to the Lab color space are printed Color space conversion is performed using the 17 profiles to convert to the RGB color space unique to the printing unit 17. A profile is data that stores a 3DLUT for converting from a PCS (Profile Connection Space) such as Lab color space or XYZ color space to a color space depending on a certain device, or from a color space depending on a certain device to PCS. is there. As the profile, for example, an ICC profile, which is an international standard for color management technology, is known. The flow of color space conversion processing performed in color matching will be described below.

FIG. 1 is a flowchart showing a flow of color space conversion processing using a 3DLUT corresponding to an input color space and an output color space.
In S105, the CPU 16 determines whether or not the type of the color space (input color space) of the digital image is the Lab color space. This determination may be made, for example, by determining which color space the 3DLUT handles as the input color space, or by determining the color space of the digital image to be converted. .

For example, when the 3DLUT handles the Lab color space as the input color space, the CPU 16 determines that the type of the input color space is the Lab color space. In other words, the CPU 16 determines that the input color space is not a color space where the points corresponding to the pixel values representing the gray color are arranged on the diagonal line 31 of the cube. In this determination, since the CPU 16 determines based on the type of the input color space as described above, it is not necessary to actually check whether or not the points corresponding to the pixel values representing the gray color are aligned on the diagonal line of the cube. Can be determined by processing.
If the input color space is the Lab color space, the CPU 16 proceeds to hexahedral interpolation processing (S120), and otherwise proceeds to S110.

In S110, the CPU 16 determines whether or not the type of the input color space is the RGB color space.
For example, when the 3DLUT handles the RGB color space as the input color space, the CPU 16 determines that the type of the input color space is the RGB color space. In other words, the CPU 16 determines that the input color space is a color space in which points corresponding to pixel values representing a gray color are arranged on the diagonal line 31 of the cube.
The CPU 16 proceeds to tetrahedral interpolation processing (S115) when the input color space is RGB color space, and proceeds to hexahedral interpolation processing (S120) otherwise.

  According to the above-described processing, tetrahedral interpolation is executed in the RGB color space, and hexahedral interpolation is executed in all other color spaces. That is, whether tetrahedron interpolation or hexahedron interpolation is to be performed can be determined only by the determination in S110, so that only the processing of S110 may be performed.

FIG. 7 is a flowchart showing the flow of hexahedral interpolation processing.
In S205, the CPU 16 selects one target pixel from the digital image.
In S210, the CPU 16 acquires the output pixel values of the eight lattice points of the hexahedron to which the point corresponding to the pixel value (input pixel value) of the target pixel belongs from the 3DLUT.
In S215, the CPU 16 interpolates the output pixel value corresponding to the input pixel value by hexahedral interpolation.

  In S220, the CPU 16 determines whether or not all pixels have been selected. If not, the process returns to S205 and repeats the process until all pixels have been selected. If all pixels have been selected, the process ends.

FIG. 8 is a flowchart showing the flow of tetrahedral interpolation processing.
In S305, the CPU 16 selects one input pixel value from the digital image.
In S310, the CPU 16 refers to the determination table shown in FIG. 9 to determine which tetrahedron the point corresponding to the input pixel value belongs to. In FIG. 9, “◯” means true and “x” means false. For example, if DB ≧ DR is true, DR ≧ DG is true, and DG ≧ DB is false, it is determined to belong to the tetrahedron (Q5, Q0, Q1, Q2).

In S315, the CPU 16 acquires the output pixel values of the four lattice points of the determined tetrahedron from the 3DLUT.
In S320, the CPU 16 interpolates the output pixel value corresponding to the input pixel value by tetrahedral interpolation.
In S325, the CPU 16 determines whether or not all pixels have been selected. If not, the process returns to S305 and repeats the process until all pixels have been selected. If all pixels have been selected, the process ends.

  According to the above-described flow, for example, hexahedral interpolation is performed in the conversion from the Lab color space to the RGB color space, and tetrahedral interpolation is performed in the conversion from the RGB color space to the Lab color space. Further, for example, hexahedral interpolation is performed in the conversion from the XYZ color space to the RGB color space, and tetrahedral interpolation is performed in the conversion from the RGB color space to the XYZ color space.

  According to the color space conversion method according to the embodiment of the present invention described above, hexahedral interpolation or triangular prism interpolation is applied to the Lab color space, and the gray color reproducibility is higher than the gradation reproducibility for the RGB color space. Preferentially apply tetrahedral interpolation. For the XYZ color space, by applying hexahedral interpolation or triangular prism interpolation, interpolation with excellent grayscale reproducibility is performed, although gray color reproducibility is poor. Therefore, according to the present invention, in color space conversion using a three-dimensional lookup table, a three-dimensional lookup table is appropriately set in consideration of gradation reproducibility while giving priority to gray color reproducibility for each input color space. Can be interpolated.

  In the present embodiment, the case where color matching is performed has been described as an example. However, the present invention can also be applied to a case where color space conversion is performed for purposes other than color matching. For example, when a digital image expressed in the RGB color space unique to the scan unit is converted into a YCbCr color space using 3DLUT, or a digital image expressed in the YCbCr color space is converted into a unique property of the printing unit 17 using 3DLUT. It may be applied when converting to the CMYK color space. In the YCbCr color space, points corresponding to pixel values representing a gray color are arranged on the coordinate axis (Y axis) in the same manner as the Lab color space. Therefore, in the conversion from the YCbCr color space to the RGB color space, gray color and gradation are obtained by hexahedral interpolation. Can be interpolated with excellent reproducibility. Conversely, in the conversion from the RGB color space to the YCbCr color space, the gray color can be preferentially interpolated by tetrahedral interpolation.

In the present invention, hexahedral interpolation has been described as an example, but triangular prism interpolation can be used instead of hexahedral interpolation. Triangular prism interpolation is a well-known method, and the properties relating to gray color reproducibility and gradation reproducibility are substantially the same as hexahedral interpolation, and therefore detailed description of triangular prism interpolation is omitted.
In the present embodiment, the stand-alone printer 10 is described as an example of the color space conversion device. However, the present invention can be applied to any device that performs color space conversion. For example, the color space conversion device may be configured by a personal computer (PC) and a printer that performs printing under the control of the PC. The color space conversion device may be a personal computer that displays a digital image, an image viewer, a digital still camera that generates a digital image, or the like.

The flowchart which concerns on one Example of this invention. 1 is a block diagram showing a schematic configuration of a color space conversion apparatus according to an embodiment of the present invention. (A) And (B) is a schematic diagram of the three-dimensional lookup table which concerns on one Example of this invention. The schematic diagram for demonstrating tetrahedral interpolation which concerns on one Example of this invention. The schematic diagram for demonstrating tetrahedral interpolation which concerns on one Example of this invention. The schematic diagram for demonstrating the hexahedral interpolation which concerns on one Example of this invention. The flowchart which concerns on one Example of this invention. The flowchart which concerns on one Example of this invention. The schematic diagram of the determination table which concerns on one Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Printer (color space conversion device), 11 Removable memory, 12 External IF, 13 Removable memory controller, 14 RAM, 15 Image processing part, 16 CPU (determination means, conversion means), 17 Printing part, 18 Flash memory, 19 Operation Part, 20 display part

Claims (6)

A color space of a digital image represented by the input color space is output using a three-dimensional lookup table corresponding to an input color space representing a color by a pixel value composed of three pixel components and an arbitrary output color space. A color space conversion device for converting to a color space,
When the input color space is expressed as a cube in a three-dimensional space with the pixel components as coordinate axes, it is determined whether the point corresponding to the pixel value representing the gray color is a color space arranged on the diagonal of the cube. Determination means to perform,
If the determination means makes an affirmative determination, the three-dimensional lookup table is interpolated by tetrahedral interpolation. If the determination is negative, the three-dimensional lookup table is interpolated by hexahedral interpolation or triangular prism interpolation. Conversion means for converting the color space;
A color space conversion device comprising:   The determination means determines the type of the input color space without checking whether or not the points corresponding to the pixel values representing the gray color are aligned on the diagonal of the cube. Item 2. The color space conversion device according to Item 1.   The color space conversion apparatus according to claim 1 or 2, wherein the input color space is an RGB color space, and the output color space is an L * a * b * color space.   3. The color space conversion device according to claim 1, wherein the input color space is an L * a * b * color space, and the output color space is an RGB color space. A digital image color space represented by the input color space is output using a three-dimensional lookup table corresponding to an input color space representing a color by a pixel value composed of three pixel components and an arbitrary output color space. A color space conversion method for converting to a color space,
When the input color space is expressed as a cube in a three-dimensional space with the pixel components as coordinate axes, it is determined whether the point corresponding to the pixel value representing the gray color is a color space aligned on the diagonal of the cube. A determination stage to perform,
If the determination is affirmative, the three-dimensional lookup table is interpolated by tetrahedral interpolation. If the determination is negative, the three-dimensional lookup table is interpolated by hexahedral interpolation or triangular prism interpolation. A conversion stage to convert the color space;
A color space conversion method comprising: A digital image color space represented by the input color space is output using a three-dimensional lookup table corresponding to an input color space representing a color by a pixel value composed of three pixel components and an arbitrary output color space. A color space conversion program for converting to a color space,
When the input color space is expressed as a cube in a three-dimensional space with the pixel components as coordinate axes, it is determined whether the point corresponding to the pixel value representing the gray color is a color space aligned on the diagonal of the cube. Determination means to perform,
If the determination means makes an affirmative determination, the three-dimensional lookup table is interpolated by tetrahedral interpolation. If the determination is negative, the three-dimensional lookup table is interpolated by hexahedral interpolation or triangular prism interpolation. A color space conversion program which causes a computer to function as conversion means for converting a color space.

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