JP4019692B2 - Color conversion device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a color conversion device, and in particular, performs image processing on image data indicated by density values of each color component corresponding to each of a plurality of color materials to an image output device that outputs an image using the plurality of color materials. The present invention relates to a color conversion apparatus that inputs image data of a color image and performs arbitrary color coordinate conversion and color conversion in real time when outputting.
[0002]
[Prior art]
Conventionally, a copying machine, a FAX, and a printer that form a color image using a plurality of color materials such as Y (yellow), M (magenta), C (cyan), and K (black), and a plurality of these functions are provided. Image output devices such as multifunction peripherals have become widespread. In order to use such an image output device, it is necessary to convert the image data from the color system when the image is generated to the color system of the image output device.
[0003]
That is, for example, in order to output image data generated in the RGB color system from the image output device, the RGB image data must be converted into YMCK image data. In this case, L suitable for correction of image data ^* a ^* b ^* It is common to convert the color system once.
[0004]
RGB image data L ^* a ^* b ^* It is well known to convert to color system image data such as ^* a ^* b ^* When converting color image data such as image data to YMCK image data, it is highly device-dependent, so as a simple and fast method of color signal conversion, conversion is performed using a look-up table method based on three-dimensional interpolation. It is known to do. In the three-dimensional interpolation method, a color space is divided into a plurality of unit interpolation solid groups, a unit interpolation solid including an input color is selected, and an output value at a plurality of vertices of the unit interpolation solid is arbitrarily used over the entire color space. The color conversion is interpolated while ensuring continuity. Using a lookup table such as a three-dimensional lookup table (DLUT), RGB and L ^* a ^* b ^* The color conversion processing from the color represented by the above to the YMCK color system corresponding to the color material of the printer is performed. In an image output apparatus such as a printer, colors are reproduced by overlapping each color material in accordance with the image data converted into the YMCK color system.
[0005]
By the way, when interpolation is performed using DLUT, if the MIN operation is used to output the minimum value of the RGB input colors as black, the interpolation result sags in a concave shape while maintaining continuity within the interpolation interval. Interpolated to shape.
[0006]
In order to solve this problem, the technique described in Japanese Patent Application Laid-Open No. Hei 7-99587 divides a three-dimensional space created by three primary color input color signals into a plurality of unit cube regions, and along the diagonal direction of the space. Assuming an oblique triangular prism or parallelepiped composed of the vertices forming the upper and lower bases of each unit cube adjacent to each other, the entire input color space is set to be included in the oblique solid group, The output value corresponding to an arbitrary input color is interpolated using the output value at each vertex of the oblique solid. As a result, it is possible to avoid a problem caused by the MIN operation frequently used in color conversion, that is, interpolation in a undulating shape while maintaining continuity in any direction in the input color space, not just the achromatic color direction. it can.
[0007]
[Problems to be solved by the invention]
However, in the conventional technique, a rapid density fluctuation occurs in the highlight portion, and the rising characteristics become steep.
[0008]
SUMMARY An advantage of some aspects of the invention is that it provides a color conversion apparatus that can supply conversion characteristics with a gradual density fluctuation.
[0009]
[Means for Solving the Problems]
In order to achieve the above object, the present invention divides a three-dimensional space related to the color of an input image into a plurality of rectangular parallelepipeds such as a unit cube and a unit rectangular parallelepiped, and then makes the corresponding rectangular parallelepiped an interpolation solid. In this interpolation solid, three-dimensional interpolation is performed by an interpolation means. At this time, when a plurality of predetermined grid points are included in the color space, an output value corresponding to the corresponding grid point is set to a predetermined output value, thereby obtaining a moderate density fluctuation conversion characteristic.
[0010]
In particular, in interpolation, a three-dimensional space related to the color of an input image is divided into a plurality of unit cubes and unit cubes, and then the corresponding cube is divided into six triangular pyramids to form an interpolation solid. Can do. Then, it is also possible to determine which triangular pyramid the input color of the image data is included in and perform triangular pyramid interpolation using the output value at each vertex of the triangular pyramid by the interpolation means. At this time, when a plurality of predetermined lattice points are included in the color space, an output value corresponding to the corresponding lattice point can be set to a predetermined output value.
[0011]
Specifically, in order to input image data to an image output apparatus that outputs an image using a plurality of color materials, the density of each color component corresponding to each of the plurality of color materials from the image data of the first color system A color conversion device that converts image data of a second color system represented by values into image data at lattice point positions of a rectangular parallelepiped obtained by dividing the color space of the first color system into a plurality of colors. A color conversion table storage means corresponding to the output value of the image data of the color system, the image data of the first color system is inputted, and the image data Upper bit The upper data and the image data Low order bit Based on the higher order data, the input means for separating the lower order data and the color conversion table for converting the first color system image data into the second color system image data. The position of the rectangular parallelepiped to which the image data of the first color system belongs is determined, designation means for designating a grid point of the defined rectangular parallelepiped, and grid points of the rectangular parallelepiped defined by the designation means are within the color space Grid points without density The corresponding grid point The grid points around the corresponding grid point on the color plane including The output value of the color conversion table storage means Predetermined negative number Compulsory setting means for setting an output value; and interpolation means for performing three-dimensional interpolation on the basis of output data from the lower order data of the image data and the color conversion table storage means and outputting an interpolation result. Features.
[0012]
The color conversion device further includes a selection unit that divides the rectangular parallelepiped into six triangular pyramids and selects a triangular pyramid to which the image data belongs based on the lower data, and the designation unit is selected by the selection unit. A grid point including the apex of the triangular pyramid is further specified, and the interpolation means is based on the lower data of the image data, the selection signal from the triangular pyramid selection means, and the output value from the color conversion table storage means Triangular pyramid interpolation is performed and an interpolation result is output.
[0013]
In order to input image data to an image output device that outputs an image using a plurality of color materials, the color conversion device of the present invention corresponds to each of the plurality of color materials from the image data of the first color system. This is for conversion into image data of the second color system indicated by the density value of each color component, and is provided with a color conversion table storage means.
[0014]
The first color system is L ^* a ^* b ^* A color system can be adopted. The second color system can be a YMC color system or a YMCK color system.
[0015]
The color conversion table storage means associates the output value of the second color system image data with the image data at the grid point positions of the rectangular parallelepiped obtained by dividing the color space of the first color system. In this conversion, the image data of the first color system is input to the input unit, and the input unit stores the input image data. Upper bit The upper data and the image data Low order bit Separated into lower data.
[0016]
In order to specify the position in the color space where the input image data exists, the position of the rectangular parallelepiped to which the first color system image data belongs is determined based on the higher order data in the specifying means.
[0017]
In order to designate a color conversion table for converting the image data of the first color system to the image data of the second color system by the defined rectangular parallelepiped, the designation means sets the grid points of the defined rectangular parallelepiped specify. Here, when the grid point is a position where there is no density, that is, a position where the color is not expressed, the conversion characteristic becomes steep when the output value of density exists.
[0018]
Therefore, the compulsory setting means includes a color conversion table storage means corresponding to a corresponding grid point when the rectangular grid point determined by the specifying means includes at least one of a plurality of grid points predetermined in the color space. Is set to a predetermined output value. The interpolation means performs three-dimensional interpolation based on the lower order data of the image data and the output value from the color conversion table storage means, and outputs the interpolation result.
[0019]
In the three-dimensional interpolation, the interpolation result is obtained by calculating the output value of the color conversion table storage means corresponding to the lattice point. At least one of a plurality of lattice points determined in advance in the color space is a rectangular parallelepiped lattice point. If it is included, that is, if it is a grid point in the color region corresponding to white, it is not preferable to obtain an interpolation result having a density. For this reason, the output value corresponding to the lattice point is set to a predetermined output value. As this output value, a predetermined negative value can be adopted as the predetermined output value of the color conversion table storage means. This predetermined output value acts on the interpolation calculation, and an output value of conversion characteristics that results in a smooth density change can be obtained.
[0020]
Triangular pyramid interpolation can be employed for the interpolation. In this case, the selection unit divides the rectangular parallelepiped into six triangular pyramids, and selects the triangular pyramid to which the image data belongs based on the lower data. In order to designate a color conversion table for converting the image data of the first color system to the image data of the second color system by using these rectangular parallelepipeds or triangular pyramids, And a grid point including the apex of the triangular pyramid selected by the selection means is designated. Here, when the grid point is a position where there is no density, that is, a position where no color is expressed, the conversion characteristic becomes steep when there is an output value of density.
[0021]
Therefore, the compulsory setting means is a color conversion table storage means corresponding to a corresponding grid point when the grid point of the rectangular parallelepiped determined by the specifying means includes at least one of a plurality of grid points predetermined in the color space. Is set to a predetermined output value. The triangular pyramid interpolation means performs triangular pyramid interpolation and outputs the interpolation result based on the lower order data of the image data, the selection signal from the triangular pyramid selection means, and the output value from the color conversion table storage means.
[0022]
In the triangular pyramid interpolation, the interpolation result is obtained by calculating the output value of the color conversion table storage means corresponding to the apex of the triangular pyramid, but the rectangular grid point is at least one of a plurality of grid points determined in advance in the color space. In other words, it is not preferable that an interpolation result is obtained so as to have a density when it includes two points, that is, when it is a grid point in a color region corresponding to white. For this reason, the output value corresponding to the lattice point is set to a predetermined output value. As this output value, a predetermined negative value can be adopted as the predetermined output value of the color conversion table storage means. This predetermined output value acts on the interpolation calculation, and an output value of conversion characteristics that results in a smooth density change can be obtained.
[0023]
The color conversion table storage means further includes a table of predetermined output values different from the correspondence, and the forcible setting means can set the table. In addition, multiple color conversion tables are provided for each different color conversion process, and the color conversion table to be used can be switched for each pixel, and the color conversion table can be switched for each pixel by a change signal from the outside. It can also be set as the structure.
[0025]
The color conversion table storage means includes an independent table for each of the rectangular parallelepiped lattice points, and can simultaneously access a table of lattice points corresponding to at least the apexes of the triangular pyramid.
[0026]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in detail with reference to the drawings. Hereinafter, a case where the present invention is applied to a copying machine will be described as an example.
[0027]
As shown in FIG. 1, the copying machine 10 includes an image input unit (IIT) 12 that reads an image from a document and acquires image data, and various types of image data acquired by the image input unit 12. An image processing unit 14 that performs image processing and a printer unit 16 that records and outputs an image on an image recording medium such as recording paper using YMCK color material based on the image data.
[0028]
The image input unit 12 includes a CCD sensor 18 that optically reads an original with a plurality of CCDs (Charge-Coupled Devices), an A / D converter 20 that converts image data read by the CCD sensor into a digital signal, and a digital And a correction unit 22 that performs shading correction on the converted image data. In general, the CCD sensor 18 reads a document for each of R (red), G (green), and B (blue) color components, so that the color of each pixel is represented by RGB color from the image input unit 12 to the image processing unit 14. Image data expressed in a system, that is, a signal indicating the density value of each of the RGB color components is output.
[0029]
The image processing unit 14 includes a color conversion circuit 24 that performs color conversion on the image data input from the image input unit 12, an image processing circuit 26 that performs various image processing on the color-converted image data, and a printer unit 16. A color conversion circuit 28 is provided as a color conversion device that converts image data after image processing into a YMCK color system for output to the printer unit 16 for output.
[0030]
In the color conversion circuit 24, data indicating the device characteristics of the RGB sensor color system depending on the device is converted from the data indicating the device characteristics of the CCD sensor 18 in advance. In the form, as an example, L ^* a ^* b ^* Color conversion to the color system. Therefore, the color conversion circuit 24 changes the color of each pixel to L. ^* a ^* b ^* Image data expressed in a color system, that is, L ^* a ^* b ^* A signal indicating each stimulation value is output.
[0031]
The image processing circuit 26 is L ^* a ^* b ^* For image data converted to the color system, enlargement / reduction processing according to the print instruction, sharpening processing such as MTF (Modulation Transfer Function / Factor) correction, and the type of original (semi-half paper, recycled paper, colored paper, etc.) ), And performs various image processing such as background removal processing for removing the background component, and outputs it to the color conversion circuit 28 at the subsequent stage.
[0032]
As shown in FIG. 2, the color conversion circuit 28 includes a DLUT circuit 30, and the L after image processing input from the image processing circuit 26. ^* a ^* b ^* The color system image data is input to the DLUT circuit 30.
[0033]
Although the detailed configuration of the DLUT circuit 30 will be described later, L ^* a ^* b ^* As a conversion table showing the correspondence between the color system image data and the YMCK color system image data, ^* a ^* b ^* A three-dimensional look-up table (DLUT) indicating a combination of density values of each color component of YMCK for expressing a color substantially equivalent to the color is stored in the internal memory in advance for each color represented by the combination of each stimulus value. Has been. The DLUT circuit 30 uses the three-dimensional look-up table to perform the image processing L input from the image processing circuit 26. ^* a ^* b ^* L color image data ^* a ^* b ^* Color conversion from the color system to the YMCK color system is performed, and image data in which the color of each pixel is expressed in the YMCK color system, that is, a signal indicating the density value of each color component of YMCK is output.
[0034]
That is, the DLUT circuit 30 has an L ^* a ^* b ^* Are input (three inputs), and four signals indicating the density values of each color component of YMCK are output (four outputs).
[0035]
In the present embodiment, the printer unit 16 using four colorants of YMCK has been described as an example, so that the DLUT circuit 30 can obtain four outputs corresponding to each color material used in the printer unit 16. However, the present invention is not limited to this. When the printer unit uses N color materials, it is needless to say that the N output can be obtained from the DLUT circuit 30 correspondingly. It is also easy to configure so that an output of a color system such as a display (for example, RGB color system) can be obtained.
[0036]
As shown in FIG. 2, the printer unit 16 includes a gradation correction LUT circuit 34 that performs gradation correction on image data input from an image processing unit 14, that is, a color conversion circuit 28 including a DLUT circuit 30. And an image output terminal (IOT) 38 for forming and outputting an image based on the image data after gradation correction on an image output medium such as recording paper using color materials of YMCK and YMCK. Has been.
[0037]
In the gradation correction LUT circuit 34, for each density value of each color component of YMCK, a lookup table indicating density values for actually reproducing a predetermined density corresponding to the density value in the printer unit is stored in advance in an internal memory (illustrated). (Omitted). Note that the look-up table is set in advance based on the output characteristics of the printer unit 16 obtained by, for example, outputting a test print of a predetermined pattern from the printer unit 16.
[0038]
The tone correction LUT circuit 34 actually uses the lookup table to obtain a predetermined density corresponding to each of the density values Y, M, C, and K of the YMCK color components for each pixel of the input image data. Each density value is corrected (tone correction) to be reproduced. Hereinafter, the density values after the gradation correction of the gradation correction LUT circuit 34 are indicated as Y ′, M ′, C ′, and K ′.
[0039]
The image output unit 38 has a function of printing image data using color materials such as YMCK toner and ink, that is, recording and outputting an image based on the image data on a recording sheet as an image recording medium. That's fine. In this embodiment, as an example, the image output unit 38 includes a photoconductor for each color of YMCK, uses toner as a color material, and converts the image data from the total amount regulation condition circuit 36 into the density for each color component of YMCK. Each of the color components is divided into image data indicating values, and electrostatic latent images are recorded on the respective photoconductors based on the image data of the corresponding colors, and the electrostatic latent images are developed using the corresponding color toners. A toner image for each color component is formed, and the formed toner image for each color component is transferred to a recording paper as an image recording medium, directly or via an intermediate transfer member, and output.
[0040]
In the above description, the case where the color conversion circuit 28 is provided outside the printer unit 16 and the gradation correction LUT circuit 34 is provided inside the printer unit 16 has been described as an example, but the color conversion circuit is sequentially arranged from the image input side. 28 and the gradation correction LUT circuit 34 may be provided in front of the image output unit 38, and may be provided inside or outside the printer unit 16.
[0041]
Next, the DLUT circuit 30 included in the color conversion circuit 28 will be described.
[0042]
As shown in FIG. 3, the DLUT circuit 30 includes an LUT ^* a ^* b ^* Color data of the color system image data is converted, and image data expressed in the YMCK color system (a signal indicating the density value of each color component of YMCK) is output.
[0043]
The DLUT circuit 30 includes an input unit 40, a memory designation unit 42, a triangular pyramid selection unit 44, an input value forced setting unit 46, a color conversion table memory 48, and a triangular pyramid interpolation calculation unit 50. The input unit 40 is L ^* a ^* b ^* The color system image data is input, and the output of the input unit 40 is a predetermined bit out of predetermined bits (8 bits will be described as an example in the present embodiment) input as image data. The high-order bits (in this embodiment, the high-order 4 bits will be described as an example) are configured to be input to the memory designating unit 42. Further, at the output of the input unit 40, the lower bits (in the present embodiment, the lower 4 bits will be described as an example) are input to the triangular pyramid selection unit 44.
[0044]
The memory designation unit 42 is connected to the triangular pyramid interpolation calculation unit 50 through the input value forced setting unit 46 and the color conversion table memory 48. This triangular pyramid interpolation calculation unit 50 is configured to output image data (signal indicating the density value of each color component of YMCK) expressed in the YMCK color system. Further, the triangular pyramid selection unit 44 is connected to the memory designation unit 42 and the triangular pyramid interpolation calculation unit 50.
[0045]
The input value forced setting unit 46 is configured to receive a setting instruction signal from the mode setting 52.
[0046]
In the present embodiment, the color conversion table memory 48 is configured to be able to simultaneously access a table corresponding to the number of eight vertices of a unit cube, more specifically, the number of four vertices of a triangular pyramid described later. The color conversion table memory 48 corresponds to a configuration in which one set of four tables selected from eight tables is used to obtain one type of color conversion output value. That is, the color conversion table memory 48 outputs each density value of the YMCK color component corresponding to the indication value of the apex of the triangular pyramid described later.
[0047]
In this DLUT circuit 30, the image data L ^* a ^* b ^* Is input to the input unit 40 as an 8-bit digital signal representing a color image. These signals are separated into upper 4 bits and lower 4 bits before being input to the memory designation unit 42 and the triangular pyramid selection unit 44.
[0048]
The DLUT circuit 30 considers that the color space is divided into unit cubes. L ^* a ^* b ^* The color conversion from the color system image data to the image data represented in the YMCK color system uses the upper 4 bits of image data to determine which unit cube in the color space belongs, and the grid of the unit cube to which it belongs This is performed by interpolating the data of the color conversion table memory corresponding to the point with the lower 4 bits of image data.
[0049]
The input unit 40 corresponds to the input unit of the present invention, and the memory designation unit 42 corresponds to the designation unit of the present invention. The triangular pyramid selection unit 44 corresponds to the selection unit of the present invention, and the input value forced setting unit 46 corresponds to the forced setting unit of the present invention. Further, the color conversion table memory 48 corresponds to the color conversion table storage means of the present invention, and the triangular pyramid-type interpolation calculation unit 50 corresponds to the interpolation means of the present invention. In particular, the interpolation means of the present invention performs triangular pyramid interpolation. It corresponds to the applied configuration.
[0050]
As shown in FIG. ^* a ^* b ^* L in color space ^* , A ^* , B ^* Are divided into divided areas designated by 4, 4 and 4 bits, that is, 16, 16, and 16 areas, respectively, so that the entire color space is divided into 4096 cubic areas. In terms of the number of lattice points corresponding to the vertices of the divided cubes, there are a total of 4913 points of 17, 17 and 17 axes. The coordinate value of the grid point is an 8-bit input image, and each axis (0, 16, 32, 48, 64, 80, 96, 112, 128, 144, 160, 176, 192, 208, 224, 240, 256) ). All the divided areas are defined as a section from a grid point to the next (grid point coordinate value −1).
[0051]
Therefore, the maximum grid point such as the coordinate value 256 is virtually present, and the value of 255 is the maximum as the actual input signal that enters the maximum divided area. Output values on the grid points are stored in the color conversion table. When the image data is 8 bits, the divided area from 0 to 15 is area number 0, 16 to 31 is area number 1, and finally 240 to 256 is area number 15. That is, there are 8 areas from 0 to 7. Therefore, in the three-dimensional space, there are 4096 areas number (0, 0, 0) to (15, 15, 15). The color conversion color conversion table memory is read by this address. In other words, the memory designating unit 42 designates an area number in the three-dimensional space and outputs data for reading the color conversion table memory 48. At this time, although details will be described later, the image data is L ^* = A ^* = B ^* In the vicinity of = 0, the input value forced setting unit 46 forcibly changes the setting of the color conversion table memory 48.
[0052]
The upper bits of the image data are input to the memory designation unit 42 and the lower bits are input to the triangular pyramid selection unit 44 at the same time. Further, all three types of lower bits are input to the triangular pyramid interpolation calculation unit 50. A selection signal that is an output from the triangular pyramid selection unit 44 is input to the memory designation unit 42 and the triangular pyramid interpolation calculation unit 50.
[0053]
With the above configuration, one cube (hereinafter referred to as a unit cube) is selected from the cube group of FIG.
[0054]
As shown in FIG. 5, the unit cube 54 has eight lattice points 56A, 56B, 56C, 56D, 56E, 56F, 56G, and 56H. The unit cube 54 is composed of six triangular pyramids 54A, 54B, 54C, 54D, 54E, and 54F. The triangular pyramid selection unit 44 assumes triangular pyramids 54A to 54F obtained by dividing the unit cube shown in FIG. 5 into six, and whether the lower bits of the input image data are included in any of the six triangular pyramids 54A to 54F. And data representing it are output.
[0055]
In the memory designating unit 42, as described above, the apex of the triangular pyramid selected by the triangular pyramid selecting unit 44 in any of the region numbers (0, 0, 0) to (15, 15, 15) in the three-dimensional space. The data to represent is output.
[0056]
The input value forcible setting unit 46 is a setting unit for forcibly setting the input value in order to output an output value output as a color conversion table in accordance with an input setting signal with a predetermined setting value. This is for adjusting the DLUT so that the density conversion is smoothly performed in the highlight area of the image.
[0057]
That is, in the present embodiment, in order to suppress abrupt density fluctuations in the highlight portion of the image and to perform density conversion smoothly in the highlight area, the color conversion table memory 48 is used. The output value is forcibly set as a negative number and output.
[0058]
In the input value forced setting unit 46, the apex of the triangular pyramid output from the memory designating unit 42 is L ^* = A ^* = B ^* If the vertex of a unit cube including a grid point of = 0, data is output to the color conversion table memory 48 so that an output value output as a color conversion table is output with a predetermined setting value.
[0059]
In FIG. ^* a ^* b ^* L in color space ^* = 0 color area. In FIG. 4, the color space is set to L for handling image data. ^* Axis, a ^* Axis, b ^* It exists in one quadrant divided by the axis, but handles color a ^* b ^* Considering the range, its center is only brightness (a ^* = B ^* = 0). That is, L ^* = 0 and a ^* = B ^* = 0 (hereinafter referred to as origin 62) is white, L ^* = 255 and a ^* = B ^* = Around 0 is black. L that handles this white ^* In the color plane of = 0 (hereinafter referred to as the target color region 60), the degree of reproduction of the obtained image depends on the density variation in the highlight region depending on the position where the density appears.
[0060]
Therefore, in the present embodiment, in the target color region 60, the density value when color conversion is performed is forcibly set to a negative number. That is, since the unit cube 54 is spread in the color space, eight lattice points P2, P3, P4, P5, P6, P7, P8, P9 are located around the lattice point P1 of the origin 62. . In the target color region 60, the color region Area_A is a white region, and in this color region Area_A, the density value should be “0”.
[0061]
Originally, the density should be “0” at the origin 62, but as shown in FIG. 7A, the density may appear due to an interpolation process or the like, resulting in a sudden density change.
[0062]
Therefore, conventionally, the color conversion table memory 48 is set so that the density of the grid point P1 is “0”, or the 9 vertices (hereinafter referred to as 9 vertices) of the grid points P2 to P9 around the grid point P1 are the density “ The color conversion table memory 48 is set to “0” as much as possible.
[0063]
However, when the color conversion table memory 48 is set so that the density becomes “0” at the grid point P1, as shown in FIG. 7C, the density appears when the color conversion table memory 48 slightly deviates from the grid point P1, that is, the origin 62. The change in color varies depending on the image. When the color conversion table memory 48 is set so that the density is “0” at 9 vertices, as shown in FIG. 7B, the density is “0” in the color area Area_B within the 9 vertices, but this range is exceeded. The density changes suddenly in the range moving to the color area Area_C.
[0064]
The present inventor recalled setting a negative number at 9 vertices in consideration of performing an interpolation operation instead of forcibly setting to “0”. This is because even if a negative number is set for the density, it does not exist, so it is “0”, and the rise from “0”, that is, the appearance of the density appears to appear by the calculation. In the present embodiment, “−1.5” is set for all nine vertices.
[0065]
For example, when the lattice point P1 is set to “−1.5”, the density appears on the way to the next lattice point as shown in FIG. Therefore, by setting “−1.5” to all nine vertices, the density “0” is maintained in the color area Area_B, the density appears in the color area Area_C, and the density gradually increases as the distance from the origin increases. Can be obtained.
[0066]
Therefore, in the memory designating unit 42, for the target image data, lattice points (any four of 56A to 56G) belonging to the triangular pyramids 54A to 54F of the unit cube 54 selected by the triangular pyramid selecting unit 44 are unit. It is output together with data representing the cube 54. The input value forced setting unit 46 determines whether or not the input grid point corresponds to the above nine vertices, and if there is a corresponding grid point, the output of the density is “−1.5”. Next, the color conversion table memory 48 is set.
[0067]
In this case, the color conversion table memory 48 may have a table memory as independent as possible so that the density output is “−1.5”, and the density output is “−1.5” in the table. May be included. In the present embodiment, since the triangular pyramid interpolation calculation unit 50 reads data from the color conversion table memory 48, the color conversion table memory 48 includes an independent color conversion table memory, and the input value forced setting unit 46 includes The independent color conversion table memory 48 is forcibly set. That is, the input value compulsory setting unit 46 outputs a color conversion table corresponding to the lattice points of the unit cube 54 selected by the triangular pyramid selection unit 44 and corresponding to the lattice points corresponding to the above nine vertices. “−1.5” is set so that an independent table memory is read as much as possible.
[0068]
The triangular pyramid interpolation calculation unit 50 includes four lattice points (56A to 56G) belonging to any of the triangular pyramids 54A to 54F of the unit cube 54 selected for the target image data input from the triangular pyramid selection unit 44. The output value corresponding to any four) is read from the color conversion table memory 48. The read output value is multiplied by a coefficient determined from the lower bits of the image data, and added to obtain a final density value. This process is performed for each of the YMCKs to obtain color-converted image data.
[0069]
Therefore, in the triangular pyramid interpolation calculation unit 50, the output value from the color conversion table memory 48, the selection signal from the triangular pyramid selection unit 44, that is, the type of the triangular pyramid, and the low-order bit signal of the image data are input, and the triangular pyramid. Interpolate to generate and output an interpolation output value. In this embodiment, the color conversion output is composed of three inputs and one output in a single color conversion interpolation operation. In the case of three inputs and four outputs such as when converting from a Lab signal to a YMCK signal, the converted YMCK is converted. Only one type of signal is generated. Of course, by installing three or four types of color conversion table memory 48 and triangular pyramid interpolation calculation unit 50, conversion to various color systems can be supported.
[0070]
Note that the interpolation calculation in the triangular pyramid interpolation calculation unit 50 is a technique described in JP-A-2-187374, JP-A-5-110840, JP-A-7-212583, etc. previously proposed by the present applicant. Can be used.
[0071]
Here, the input value forced setting unit 46 is configured to receive a mode setting signal from the mode setting 52. The signal from the mode setting 52 is a signal for instructing to operate the input value forced setting unit 46. That is, by adjusting the DLUT as described above, the density conversion is smoothly performed in the highlight area of the image. However, depending on the image, such adjustment of the color conversion process may not be necessary. Therefore, it is necessary to have a configuration in which the setting of the forcible color conversion table for the nine vertices described above can be selectively applied.
[0072]
In the present embodiment, whether or not the input value forced setting unit 46 can be operated can be selected according to the mode set in the mode setting 52. This mode setting 52 may be set by a CPU (not shown) inside the copying machine, or may be a setting input by a user's instruction operation. For example, in the case of an image including a photographic image that requires smooth density conversion in the highlight area, the above-described operation is performed by setting the input value forced setting unit 46 to operate, and smooth density conversion is performed. Is done.
[0073]
Next, the overall flow of the copying machine 10 will be described. When a copy job is input, such as when a start button provided on a user interface (not shown) is pressed, the copying machine 10 reads a document for each RGB color component by the CCD sensor 18 in the image input unit 12. Image data is acquired, the acquired image data is converted into digital data by the A / D converter 20, shading correction is performed by the correction unit 22, and then output to the image processing unit 14. As a result, the image processing unit 14 receives image data representing the color of each pixel in the RGB color system, that is, a signal indicating the density value of each RGB color component.
[0074]
The image data input to the image processing unit 14 is first converted from the RGB color system by the color conversion circuit 24 to L. ^* a ^* b ^* Convert the color to the color system and change the color of each pixel to L ^* a ^* b ^* After being converted into image data expressed in a color system and subjected to various types of image processing by the image processing circuit 26, the color conversion circuit 28 outputs L ^* a ^* b ^* The color system is converted to the YMCK color system corresponding to the color material of the printer unit 16.
[0075]
In the color conversion circuit 28, the input value forced setting unit 46 forcibly sets the color conversion table memory 48 in order to perform density conversion smoothly in the highlight region. With this setting, the data of nine vertices corresponding to white is set to a negative number, and the output value obtained by the interpolation processing has a characteristic that has a smooth gradient. This suppresses a steep density change and gradually changes the density. Color conversion.
[0076]
The image data converted into the YMCK color system is subjected to tone correction by the tone correction LUT circuit 34, and the tone-corrected image data is output to the image output unit 38 and printed.
[0077]
Thus, in the present embodiment, a negative number (for example, a predetermined number) is set for all nine lattice points (9 vertices) of the unit cube 54 belonging to the color area Area_A that should have a density “0” in the target color area 60. , −1.5) is set, so that the obtained density characteristic is a gentle rising characteristic (see FIG. 7D). This improves the reproducibility of color reproduction and density reproduction of an image including a highlight area such as a map document.
[0078]
The present invention can be applied to any apparatus that outputs an image using a plurality of color materials in addition to a copying machine. For example, the present invention may be applied to a FAX, a printer used by being connected to a PC, a copier, a FAX, and a multifunction machine having at least two functions of a printer.
[0079]
【The invention's effect】
As described above, according to the present invention, when the rectangular grid point includes at least one of a plurality of predetermined grid points in the color space, for example, the grid point in the color region corresponding to white By setting the output value corresponding to the lattice point to a predetermined output value, there is an excellent effect that the output value of the conversion characteristic that causes a smooth density change can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a copying machine according to an embodiment of the present invention.
FIG. 2 is a detailed configuration diagram of a color conversion circuit and a printer unit.
FIG. 3 is a block diagram showing a conceptual configuration of a DLUT circuit.
FIG. 4 is a perspective view showing a color space composed of unit cubes.
FIG. 5 is a diagram showing a unit cube and a triangular pyramid.
FIG. 6 is an explanatory diagram for explaining a target color region for obtaining a gentle rise characteristic;
FIG. 7 shows the conversion characteristics of the color conversion circuit. (A) is no adjustment, (B) is when all nine vertices are set to “0”, and (C) is set to “0” for only one vertex. (D) shows the case where nine vertices according to the present embodiment are set to negative numbers.
[Explanation of symbols]
Area_A Color area
P1 to P9 ... Lattice points
10 ... Copier
28: Color conversion circuit
30 ... DLUT circuit
40 ... Input section
42 ... Memory specification section
44 ... Triangular pyramid selector
46: Input value forced setting section
48 ... Color conversion table memory
50 ... Triangular pyramid interpolation calculation unit
52 ... Mode setting
54 ... Unit cube
54A ... Triangular pyramid
56A ... Lattice points
60 ... Target color area
62 ... Origin

Claims (5)

In order to input image data to an image output apparatus that outputs an image using a plurality of color materials, the density value of each color component corresponding to each of the plurality of color materials is indicated from the first color system image data. A color conversion device that converts image data of a second color system,
Color conversion table storage means for associating an output value of the image data of the second color system with image data of lattice point positions of a rectangular parallelepiped obtained by dividing the color space of the first color system
Input means for inputting the image data of the first color system and separating the upper data as the upper bits of the image data and the lower data as the lower bits of the image data;
In order to designate a color conversion table for converting from the first color system image data to the second color system image data, the first color system image data based on the higher order data Specifying the position of the rectangular parallelepiped to which the
When the rectangular grid points determined by the specifying means include grid points having no density in the color space, the color of the grid points on the color plane including the corresponding grid points and around the corresponding grid points. a force setting means for setting the negative of the output value determined in advance an output value of the conversion table storing means,
Interpolation means for performing three-dimensional interpolation based on the lower order data of the image data and the output value from the color conversion table storage means and outputting the interpolation result;
A color conversion device comprising: The cuboid is further divided into six triangular pyramids, further comprising selection means for selecting a triangular pyramid to which the image data belongs based on the lower data,
The designation means further designates a lattice point including the apex of the triangular pyramid selected by the selection means,
The interpolation means performs triangular pyramid interpolation and outputs an interpolation result based on lower order data of the image data, a selection signal from the triangular pyramid selection means, and an output value from the color conversion table storage means. The color conversion apparatus according to claim 1. The color conversion apparatus according to claim 1, wherein the first color system is L ^* a ^* b ^* .   4. The color conversion table storage unit further includes a table of predetermined output values different from the correspondence, and the forcible setting unit sets the table. The color conversion device described in 1. The said color conversion table memory | storage means is provided with the table independent about each of the grid point of the said rectangular parallelepiped, and accesses simultaneously the table of the grid point corresponding to the vertex of the said triangular pyramid at least simultaneously. Color conversion device .

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