JP5106436B2 - Color conversion apparatus and color conversion processing method - Google Patents

Description

  The present invention relates to a color conversion apparatus and a color conversion processing method for performing color conversion of a color image.

  In an apparatus for copying and printing a document image read by an image reading apparatus, generally, a read image is expressed in RGB format, and data at the time of printing is expressed in CMY format. Therefore, color conversion from RGB format image data to CMY format image data is performed. Color conversion processing from RGB format image data to CMY format image data is usually performed by using a lookup table and combining interpolation operations as necessary.

  The look-up table is a table in which RGB axes are divided at predetermined intervals, lattice points are formed in a three-dimensional space, and CMY or CMYK converted values are associated with the respective lattice points. For intermediate RGB values, interpolation calculation is performed from surrounding grid points to calculate a converted CMY value or CMYK value.

  By the way, when an image of a document read by the image reading apparatus is copied and printed, the density of the printed result may be light even though the black density of the document is a sufficiently high portion. In order to solve this problem, Patent Document 1 describes that color conversion is performed using a look-up table in which all areas close to black are converted to black when the read image is printed. Yes. Thereby, the blackish portion of the document is printed in clear black, and the characters can be easily read.

JP 2002-252781 A

  According to the technique described in Patent Document 1, it is possible to prevent a decrease in black density due to copy printing, but on the other hand, all dark portions having color are also converted to black. For this reason, a delicate color tone is lost, and for example, gradation expression such as a dark green part or a dark blue part becomes insufficient. Accordingly, an object of the present invention is to prevent a decrease in black density due to copy printing while maintaining the color of a dark portion having a color.

  In order to solve the above problems, a color conversion device according to a first aspect of the present invention is a color conversion device that converts color data in RGB format into color data in a different color representation format, and includes RGB representative values. A look-up table that defines the correspondence between the combination and the value after conversion, and color data other than the combination of the RGB representative values are calculated by performing an interpolation operation using the look-up table. Interpolating means, and the look-up table forms a grid point by a combination of the representative values with a coordinate indicating black as an origin on a three-dimensional space with RGB as an axis, and each grid point When the converted values are assigned, the RGB values are the same for the grid points (excluding the origin) that are the vertices of the cube formed by the representative values closest to the origins of RGB. The converted value assigned to the grid point determined by the representative value is equal to the converted value assigned to the origin, and the converted value assigned to another grid point is the transformation assigned to the origin. It is different from the later values.

  According to the present invention, since the converted value assigned to the grid point determined by the representative value closest to the origin having the same RGB value is equal to the converted value assigned to the origin, A decrease in concentration can be prevented. Furthermore, since the value after conversion assigned to the other grid points is different from the value after conversion assigned to the origin, the color of the dark portion having the color can be maintained.

  Here, the different color expression format may be a CMY format or a CMYK format. The look-up table can determine the RGB representative values at predetermined intervals. Further, the interpolation calculation means divides a cube formed by eight lattice points surrounding the RGB values to be converted into six triangular pyramids, and sets the vertexes of the triangular pyramids including the RGB values to be converted. Interpolation can be performed using the converted values assigned to the corresponding grid points. At this time, each of the six triangular pyramids is preferably divided so as to include two lattice points defined by representative values having the same RGB value as vertices.

  In order to solve the above-described problem, the color conversion processing method according to the second aspect of the present invention uses a lookup table that defines a correspondence relationship between a combination of RGB representative values and a value after conversion. A color conversion processing method for converting data into color data of a different color representation format, wherein the lookup table is a combination of the representative values with a coordinate indicating black as an origin on a three-dimensional space with RGB as an axis. When the grid points are formed and the converted values are assigned to the respective grid points, the RGB values of the grid points that are the vertices excluding the origin of the cube formed by the representative values closest to the respective RGB origins are The transformed value assigned to a grid point defined by an equal representative value is equal to the transformed value assigned to the origin, and the transformed value assigned to another grid point is Characterized in that different from the value of the converted assigned to the point.

  According to the present invention, it is possible to prevent a decrease in black density due to copy printing while maintaining the color of a dark portion having a color.

FIG. 2 is a block diagram illustrating a functional configuration of a color conversion apparatus according to the present invention and a printing apparatus connected to the color conversion apparatus. It is a block diagram which shows the specific example of a system configuration. It is a figure which shows the concept of a look-up table. It is a figure which shows an example of a lookup table. It is a figure explaining an interpolation calculation. It is a figure which shows the concept of the look-up table of this embodiment. It is a figure which shows an example of the lookup table of this embodiment. It is a figure which shows the production | generation procedure of the lookup table of this embodiment. It is a figure which shows the color conversion result of this embodiment.

  Embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a block diagram illustrating a functional configuration of a color conversion apparatus 10 according to the present invention and a printing apparatus 20 connected to the color conversion apparatus 10. As shown in the figure, the color conversion apparatus 10 includes an RGB dot image input unit 110 for inputting RGB dot image data, a color conversion processing unit 120 for converting RGB image data into CMYK format image data, and the like. And a CMYK dot image output unit 140 that outputs dot image data in the CMYK format.

  The RGB dot image input unit 110 inputs, for example, RGB dot image data represented by 8 bits for each color from an external device or another function unit in the device. The color conversion processing unit 120 includes a lookup table (LUT) 122 and converts RGB dot image data into CMYK dot image data with reference to the lookup table 122. The look-up table 122 is a table in which the correspondence between RGB representative values and CMYK values is recorded. Since the lookup table 122 records the correspondence between the representative values of RGB and the CMYK values, it is necessary to calculate the CMYK values by performing an interpolation operation on the intermediate values. In order to perform this interpolation calculation processing, the color conversion processing unit 120 includes an interpolation calculation unit 124. Note that a plurality of types of lookup tables 122 may be prepared in accordance with the type of printing paper.

  The CMYK dot image output unit 140 performs halftone processing or the like on the dot image data converted into the CMYK format and outputs the result to the printing apparatus 20. Here, the halftone process is, for example, a process for expressing a CMYK dot image represented by 8 bits for each color with gradation that can be printed by the printing apparatus 20.

  As shown in the figure, the printing apparatus 20 includes a CMYK dot image input unit 210 that inputs dot image data in CMYK format, a printing mechanism 220, and a paper supply / discharge mechanism 230 that performs a paper supply / discharge operation for printing paper. I have.

  In the present embodiment, the printing mechanism 220 includes a plurality of print heads that extend in a direction perpendicular to the paper conveyance direction and in which a large number of nozzles are formed, and performs printing by discharging black or color ink from each print head. Inkjet method is adopted. However, other methods may be adopted.

  The color conversion apparatus 10 and the printing apparatus 20 as shown in FIG. 1 can be specifically realized as a system configuration as shown in FIG. FIG. 2A shows an example in which the system is configured by the image reading device 40, the dot image development unit 510, the color conversion unit 520, and the inkjet printer 50 including the inkjet printing mechanism 530. In this system, the RGB image data read by the image reading device 40 is color-converted into the CMYK format by the color conversion unit 520 of the inkjet printer 50 and printed by the inkjet printing mechanism 530. In this case, the inkjet printer 50 functions as the color conversion device 10 and the printing device 20.

  FIG. 2B shows an example in which the system is configured by a PC 60 having a printer driver 610 and an ink jet printer 70 having an ink jet printing mechanism 710. In this system, the printer driver 610 includes a color conversion unit 612. The RGB image data created by the application of the PC 60, the image data read from a digital camera, or the like is developed into a dot image on the PC 60 side, and further CMYK. A color conversion process is performed on the format and output to the inkjet printer 70. In this case, the PC 60 functions as the color conversion device 10, and the ink jet printer 70 functions as the printing device 20.

  Next, the lookup table will be described. FIG. 3 is a diagram showing a general concept of the lookup table. As shown in this figure, the lookup table forms grid points on a three-dimensional space by dividing the mutually orthogonal RGB axes at a predetermined interval, and values obtained by converting CMY or CMYK to the respective grid points. Is a table in which In the present embodiment, 256 gray levels (0 to 255) of 8 bits for each RGB are divided into 4 bits (16) to generate grid points, and CMYK values in the range of 0 to 100 are associated with each grid point. I am letting. This is because if the grid points are generated by dividing each gradation or bit by bit and the CMYK values in the range of 0 to 100 are associated with each point, the capacity of the lookup table becomes large. However, the interval between the lattice points is not limited to 4 bits, and can be changed according to the memory capacity, the calculation processing speed, and the like. The lattice points can be expressed in the range of (0, 0, 0) to (16, 16, 16).

  The lookup table has a three-dimensional concept, but is actually stored in the color conversion apparatus 10 in a two-dimensional table format as shown in FIG. That is, a combination of RGB values corresponding to each grid point is described as an input value, and a CMYK value corresponding to each combination is set as an output value. FIG. 4 also shows a grid point correspondence table for reference. Further, since the input values are arranged according to a predetermined rule, the lookup table may be configured with only the output values.

  The look-up table 122a shown in FIG. 4 is a conventional look-up table. For example, when the RGB state shows the blackest state where all the RGB values are 0, CMY is 20 and K100 is represented as black on printing. Is done. This can be determined, for example, so as to be in the blackest state and to minimize the sheet offset. Further, when all the RGB values indicate the whitest state of 255, each of CMYK becomes 0 and white on printing is expressed by not discharging ink. Also, the grid point (1, 1, 1) having all the RGB values of 16 indicates a color close to black in the achromatic gray. In this case, as the output values, CMY is 15 and K90 is printed as gray close to black. Therefore, the RGB values on the gray axis from the grid point (0, 0, 0) to the grid point (1, 1, 1) are 20 for CMY and 20 for K100, and 15 for CMY and black for K90. Tones that are close to gray are drawn.

FIG. 5 is a diagram for explaining the interpolation calculation performed by the interpolation calculation unit 124. As shown in FIG. 4, the look-up table is discrete because values after conversion are set for each lattice point. For this reason, when the RGB values do not coincide with the grid points, the CMYK values after conversion are obtained by interpolation calculation. In the present embodiment, the interpolation calculation can be performed as follows. That is, a cube composed of eight lattice points surrounding the RGB values to be converted is divided into six triangular pyramids determined by four lattice points as shown in FIGS. 5 (a) to 5 (f). Then, using the CMYK values assigned to the lattice points corresponding to the vertices of the triangular pyramid including the RGB values to be converted, an interpolation operation is performed by calculating a weighted average using a weighting coefficient obtained from the positional relationship. As shown in FIGS. 5A to 5F, the six triangular pyramids include (n, n, n) lattice points and (n + 1, n + 1, n + 1) lattice points. For this reason, the CMYK values assigned to the (n, n, n) grid points and the (n + 1, n + 1, n + 1) grid points greatly affect the result of the interpolation calculation.

  Next, the lookup table 122 in this embodiment will be described. FIG. 6 is a diagram in which a cube (shaded portion in FIG. 3) defined by the lattice point closest to the black side is extracted from the three-dimensional lookup table shown in FIG. That is, lattice point (0, 0, 0), lattice point (0, 1, 0), lattice point (0, 1, 1), lattice point (0, 0, 1), lattice point (1, 0, 0) ), A lattice point (1, 1, 0), a lattice point (1, 0, 1), and a cube defined by the lattice point (1, 1, 1).

  Conventionally, with respect to the lattice points constituting this cube, the lattice point (0, 0, 0) is converted to the blackest color, and all other lattice points are set so that the black density decreases. Further, in the lookup table described in Patent Document 1, all eight lattice points constituting this cube are converted to the blackest color.

  For this reason, when the former lookup table is used, there is a problem that the density of the black portion is lowered, and when the latter lookup table is used, all the RGB values contained in the cube are converted to the blackest color. Therefore, although the density reduction of the black portion can be prevented, there is a problem that the color of the dark portion having the color is lost.

  Therefore, in the present embodiment, among the eight lattice points constituting the cube closest to the black side, the lattice point (0, 0, 0) and the lattice point (1, 1, 1) are converted to the blackest color. The CMYK values are set as described above, and the CMYK values are set so that the black density of the other lattice points is lowered. Further, other lattice points constituting the look-up table are set to the same CMYK values as in the past. As a result, the look-up table 122 used in the present embodiment, as shown in FIG. 7, except that the lattice point (1, 1, 1) is converted to the same value as the lattice point (0, 0, 0), The same value as before can be used.

  Therefore, the look-up table 122 of this embodiment, for example, as shown in FIG. 8, outputs the output value of the lattice point (1, 1, 1) from the conventional look-up table 128 to the lattice point (0, It can be created by performing the processing (S101) to make it the same as the output value of (0, 0). Specifically, the conversion source lookup table is read, and the output value of the grid point (0, 0, 0) is acquired. Next, the output value of the grid point (1, 1, 1) may be changed to the output value of the acquired grid point (0, 0, 0).

  In the present embodiment, by using such a lookup table 122, a grid point closest to the origin among the grid points on the gray axis having the same RGB value is converted to the blackest color same as the origin, and other grid points are obtained. Is converted so that the density of black decreases. Therefore, the output when gray gradations having the same RGB value are input has characteristics as shown in FIG.

  As shown in this figure, the RGB values from the grid point (0, 0, 0) to the grid point (1, 1, 1) are the blackest as the grid point (0, 0, 0) as a result of the interpolation calculation. The RGB values on the gray axis from the grid point (1, 1, 1) to the grid point (16, 16, 16) are converted so that the gray gradation is expressed as a result of the interpolation operation. Will be. Further, colors close to the gray axis having substantially the same RGB values are also strongly influenced by the grid point (0, 0, 0) and the grid point (1, 1, 1), and can prevent the density from being lowered. .

  On the other hand, for RGB data having a color away from the gray axis, the black density of the grid points other than the grid point (0, 0, 0) and the grid point (1, 1, 1) decreases. Since it is set, gradation expression such as a dark green portion or a dark blue portion is maintained. As a result, as shown in FIG. 9B, it is possible to prevent the chromatic color portion from being lost in color and being converted to the blackest color, and a good printing result can be obtained.

  As described above, the color conversion apparatus 10 according to the present embodiment performs the color conversion process using the lookup table 122 described above, so that the black color by copy printing is maintained while maintaining the color of the dark portion having the color. It is possible to prevent a decrease in concentration.

  In the above embodiment, the output value of the grid point (1, 1, 1) is made the same as the output value of the grid point (0, 0, 0) which is the blackest color. The above does not necessarily have to match. That is, it is sufficient if the output value of the grid point (1, 1, 1) is a value that can be regarded as black having a sufficiently high density. Further, the output value may be set to black having a sufficiently high density by extending to the grid point (2, 2, 2) on the gray axis or higher grid points (n, n, n). Good. Furthermore, if the black density is the same at the grid points (0, 0, 0) and (1, 1, 1), the value of CMY is slightly reduced so that the paper back-off is minimized. You may make it increase a value a little.

  In the above-described embodiment, the case of converting RGB format color data into CMYK format color data has been described. However, the present invention can also be applied to conversion between other color representation formats. For example, there is a case where RGB format color data is once converted into Lab format color data and then converted into CMYK format color data. In this case as well, a dark portion having a color tone can be obtained by applying the present invention. It is possible to prevent a decrease in black density while maintaining the color tone.

DESCRIPTION OF SYMBOLS 10 ... Color conversion apparatus 20 ... Printing apparatus 40 ... Image reading apparatus 50 ... Inkjet printer 60 ... PC
70: inkjet printer 110 ... RGB dot image input unit 120 ... color conversion processing unit 122 ... lookup table 124 ... interpolation calculation unit 128 ... lookup table 140 ... CMYK dot image output unit 210 ... CMYK dot image input unit 220 ... printing mechanism 230: paper feeding / discharging mechanism 510 ... dot image developing unit 520 ... color converting unit 530 ... inkjet printing mechanism 610 ... printer driver 612 ... color converting unit 710 ... inkjet printing mechanism

Claims (5)

A color conversion device that converts color data in RGB format into color data in a different color representation format,
A lookup table that defines the correspondence between the combination of RGB representative values and the converted value;
For color data other than the combination of RGB representative values, interpolation calculation means for calculating a converted value by performing an interpolation calculation using the lookup table, and
The lookup table is
In a three-dimensional space with RGB as the axis, grid points are formed by combining the representative values with the coordinates indicating black as the origin, and when converted values are assigned to the respective grid points, the RGB points are the most at the origin. For the lattice points (excluding the origin) that are the vertices of the cube formed by the close representative values, the converted values assigned to the lattice points defined by the representative values with the same RGB values are the converted values assigned to the origin A color conversion apparatus characterized in that a converted value assigned to another grid point is different from a converted value assigned to the origin. The color conversion device according to claim 1,
The color conversion apparatus according to claim 1, wherein the lookup table has representative values of RGB at predetermined intervals. The color conversion device according to claim 2,
The interpolation calculation means divides a cube formed by eight lattice points surrounding the RGB values to be converted into six triangular pyramids determined by the four lattice points, and a triangular pyramid including the RGB values to be converted A color conversion apparatus that performs an interpolation operation using a value after conversion assigned to a lattice point corresponding to a vertex of the color. The color conversion device according to claim 3,
Each of the six triangular pyramids is divided so as to include two lattice points defined by representative values having the same RGB value as vertices. A color conversion processing method for converting color data in RGB format into color data in a different color representation format using a look-up table that defines the correspondence between combinations of RGB representative values and converted values,
The lookup table forms a grid point by a combination of the representative values with a black coordinate as an origin on a three-dimensional space with RGB as an axis, and assigns a converted value to each grid point. For the grid points (excluding the origin) which are the vertices of the cube formed by the representative values closest to the origins of RGB, the converted values assigned to the grid points defined by the representative values having the same RGB values are A color conversion processing method characterized in that a converted value assigned to another grid point is different from a converted value assigned to the origin, which is equal to the converted value assigned to.