JP5093133B2 - Color conversion apparatus, color conversion method, and color conversion program - Google Patents

Description

  The present invention relates to a color conversion device that converts input color data to output color data, and in particular, a good color that can convert a black-only color into a black-only color and that ensures continuity of colors in the vicinity thereof. The present invention relates to a color conversion device that enables conversion.

  In an apparatus that outputs a color image such as a color printer, an image is output based on color data such as bitmap data in which each pixel has a density gradation value of each color. In this case, normally, color data generated by another image input device or application is input, and the input color data is expressed in a color space depending on the input device or a commonly used color space. The Therefore, the output device converts the input color data into color data that matches the characteristics of the output device, and executes output processing according to the converted output color data. As described above, when a color image is output, a color conversion process for changing the color space of the color data is required, and this process is performed in the output device or in a device that issues an output instruction to the output device. Executed in

  In such color conversion processing, one problem is that an image expressed only in black in the input color data may not be only black in the output color data. In particular, in the case of printing characters and the like, the sharpness and sharpness are deteriorated by mixing other colors. Therefore, it is desired to reproduce only black originally from only black.

  In order to solve this problem, Patent Document 1 below proposes updating the color conversion look-up table (LUT) so that black-only color data is converted into black-only color data. .

JP 2003-324623 A

  However, the above-mentioned Patent Document 1 is a countermeasure for the case of only black, and does not consider a color located in the vicinity in the color space. Normally, if the conversion of black only is changed to black only as described above, there is a concern that discontinuity with the neighboring colors may occur in the color space, and the color tone in the output result of this part of the color May deteriorate.

  Accordingly, an object of the present invention is a color conversion device that converts input color data into output color data, which can convert a black-only color into a black-only color, and secures continuity of colors in the vicinity of the color. The object is to provide a color conversion device or the like that enables good color conversion.

  In order to achieve the above object, according to one aspect of the present invention, a color conversion device that converts color data expressed by chromatic and achromatic colors converts input color data to output color data. In the first conversion information and the first conversion information, when the input color data indicates an achromatic color and no chromatic color, the converted output color data indicates an achromatic color. Having second conversion information configured to indicate a color having no chromatic color and having the achromatic color and having a chromatic color when performing the conversion process using the second conversion information. For the neighboring colors that are not present, after performing the conversion using the second conversion information to obtain the output color data, the value of each color of the chromatic color in the output color data is set to the value of each color. The output color data after changing the value of each color without changing the relative ratio Lightness of the color indicated by the first conversion information to be the same as the output color data indicates color brightness in the case of executing the conversion using the, change, it performs the conversion processing is that.

  Further, in the above-described invention, a preferable aspect thereof is that the first conversion information and the second conversion information are respectively the colors in the color space of the output color data with respect to the colors in the color space of the input color data. Each color associated with the second conversion information is the same as each color associated in the first conversion information, except for a color associated with the achromatic color and not having a chromatic color. It is characterized by that.

  In order to achieve the above object, another aspect of the present invention outputs input color data that is input in a color conversion method in a color conversion apparatus that performs conversion processing of color data expressed in chromatic and achromatic colors. In the first conversion information for converting to color data, and in the first conversion information, when the input color data indicates a color having an achromatic color and no chromatic color, the output color data after the conversion Is provided with second conversion information that indicates an achromatic color and a color that does not have a chromatic color, and when performing the conversion process using the second conversion information, For neighboring colors that have no chromatic color, after performing the conversion using the second conversion information to obtain the output color data, the value of each color of the chromatic color in the output color data is Change the value of each color without changing the relative proportions of the values of each color. After that, the brightness of the color indicated by the output color data is changed to be the same as the brightness of the color indicated by the output color data when the conversion is executed using the first conversion information, and the conversion process is performed. To do.

  In order to achieve the above object, still another aspect of the present invention provides a color conversion program for causing a control device to perform conversion processing of color data expressed in chromatic and achromatic colors. In the first conversion information for converting to output color data, and the first conversion information, when the input color data indicates an achromatic color and no chromatic color, the output color after the conversion Second conversion information is provided in which the data indicates an achromatic color and a color that does not have a chromatic color, and when the control means performs the conversion process using the second conversion information, For neighboring colors that have an achromatic color and have no chromatic color, after performing the conversion using the second conversion information to obtain the output color data, the chromatic color of the output color data is obtained. Without changing the relative proportions of each color value After changing the value of each color, change the brightness of the color indicated by the output color data to be the same as the brightness of the color indicated by the output color data when conversion is performed using the first conversion information. Then, the conversion process is executed so that the conversion process is performed.

  Further objects and features of the present invention will become apparent from the embodiments of the invention described below.

1 is a configuration diagram according to an embodiment of a printer 2 including a color conversion device to which the present invention is applied. FIG. It is the figure which illustrated lookup table LUT (A) and LUT (B). It is the figure which illustrated lookup table LUT (AB1). It is the flowchart which illustrated the procedure of the color conversion process. It is a graph which shows L value after color conversion about the color of the K-axis vicinity.

  Embodiments of the present invention will be described below with reference to the drawings. However, such an embodiment does not limit the technical scope of the present invention. In the drawings, the same or similar elements are denoted by the same reference numerals or reference symbols.

  FIG. 1 is a configuration diagram according to an embodiment of a printer 2 including a color conversion apparatus to which the present invention is applied. The printer 2 shown in FIG. 1 executes printing of the image in accordance with print data including color data of the print target image transmitted from the host computer 1. The received input color data is used as output color data for the printer 2. In the color conversion processing for conversion, when a lookup table (LUT (AB2)) that converts only achromatic colors to only achromatic colors is used, the brightness and color balance of the converted colors of the neighboring colors are changed. The change is made in consideration, and the continuity of the converted color is to be ensured.

  A host computer 1 shown in FIG. 1 is a host device that makes a print request to the printer 2, and is configured by a personal computer or the like. The host computer 1 is provided with a printer driver 11. When a print request is made, the printer driver 11 generates print data including image data and a control command based on a user operation or the like and transmits it to the printer 2. Here, it is assumed that data transmitted from the printer driver 11 to the printer 2 is described in PDL. The printer driver 11 can be configured by a program that executes the above-described processing, a control device (not shown) of the host computer 1 that executes processing according to the program, and the like. The program for the printer driver 11 can be installed in the host computer 1 by downloading from a predetermined site via a network such as the Internet, or from a storage medium such as a CD.

  As shown in FIG. 1, the printer 2 is a laser printer as an example, which includes a controller 21, an engine 22, and the like.

  The controller 21 receives a print request from the host computer 1 and controls the print mechanism unit 22 to issue a print instruction. As shown in FIG. 1, the controller 21 has an I / F 23, a CPU 24, a ROM 25, a RAM 26, and an engine I / F. F27 is provided.

  The I / F 23 is a part that receives the print data transmitted from the host computer 1.

  The CPU 24 is a part that controls various processes performed in the controller 21. When a print request is received from the host computer 1, the CPU 24 performs predetermined image processing on the image data included in the received print data and performs a printing mechanism. Processing for generating bitmap data (plane data for each color) to be output to the unit 22 side, processing for interpreting a control command included in the print data and instructing the printing mechanism unit 22 to perform appropriate printing processing, etc. To manage. The CPU 24 executes color conversion processing regarding the processing for generating the bitmap data. The printer 2 is characterized by the color conversion processing, and the specific contents thereof will be described later. Note that the processing executed by the CPU 24 is mainly performed according to a program stored in the ROM 26.

  The RAM 26 is a memory that stores received print data, image data after each process, and the like, and stores the bitmap data (plane data) of each color described above. Regarding the color conversion process, input color data which is input data of the process and output color data which is processed data are held here. The contents of these data will be described later.

  The ROM 25 is a memory that stores a program for each process executed by the CPU 24. Further, here, four lookup tables LUT (A), LUT (B), LUT (AB1), and LUT (AB2) are stored for the color conversion process. The contents of these LUTs will be described later.

  The color conversion device in the printer 2 is constituted by the CPU 24, the ROM 25, the RAM 26, and the like in the controller 21.

  Next, when printing is performed by the printing mechanism unit 22, the engine I / F 27 reads the image data (bitmap data of each color) stored in the RAM 26 at a predetermined timing, and performs predetermined processing on the data. This is the part that controls the interface between the controller 21 and the printing mechanism unit 22 side, which is handed over to the printing mechanism unit 22 side after the above. Although not shown, the engine I / F 27 includes a memory for temporarily storing data, a decompression unit, a screen processing unit, and the like. The image data read from the RAM 26 is compressed data. Decompression and screen processing to convert to dot data. Further, the engine I / F 27 is specifically composed of an ASIC.

  Although not shown, the printing mechanism unit 22 includes a photosensitive drum, a charging unit, an exposure unit, a developing device, a transfer unit, and the like. When printing is performed, the photosensitive drum is charged by the charging unit, and the charged photosensitive drum is irradiated with a beam from a light source such as a laser or an LED array built in the exposure unit to form a latent image by static electricity. Thereafter, the latent image is developed into an image by the developing material by a developing device including a toner cartridge that accommodates the developing material (toner), and the toner image developed by the transfer unit is transferred to a printing medium such as paper, and is then It is fixed and discharged out of the printer 2.

  In the printer 2 having the configuration as described above, when a print request is received from the host computer 1, the print data is interpreted, and each pixel has a gradation value for each color from the image data in the PDL format. Bitmap data is generated. Here, as an example, the color of each pixel is represented by CMYK (cyan, magenta, yellow, black), and the color is represented according to the color space of the image data received from the host computer 1. Thereafter, a color conversion process is performed on the bitmap data to obtain data expressed in the color space of the printer 2. Note that the color material used in the printer 2 is toner of each color of CMYK. Therefore, here, bitmap data is expressed in CMYK even after conversion.

  The converted bitmap data is composed of CMYK color plane data for one print medium, and is stored in the image buffer of the RAM 26. Thereafter, bitmap data is read from the engine I / F 27, and after the above-described processing, printing processing in the printing mechanism unit 22 is executed.

  As described above, the printer 2 is characterized by the above-described color conversion processing, and the specific contents thereof will be described below.

First, a lookup table LUT used for color conversion processing will be described. As described above, the color space of the image data received from the host computer 1 is CMYK. This is a color space that reflects the characteristics of the device in which the data is generated, or a standard CMYK color space. Yes, it is different from the CMYK color space in which the device characteristics of the printer 2 are reflected. Thus, in this color conversion processing, the image data received from the host computer 1 and the input color data, assumed to represent the color space _{C A M A Y A K A} , was subjected to color conversion processing to the input color data The later data, that is, the data in the color space reflecting the device characteristics of the printer 2 is set as output color data, and the color space is expressed by C _B M _B Y _B K _B.

Therefore, in the generated bitmap data before color conversion, each pixel is expressed in C _A M _A Y _A K _A and data expressed in C _B M _B Y _B K _B by the color conversion process. It becomes. In other words, the density gradation value of each color of C _A M _A Y _A K _A is converted into the density gradation value of each color of C _B M _B Y _B K _B.

  As described above, the printer 2 stores four look-up tables, and the look-up tables LUT (A) and LUT (B) among them change the color space of the input color data to the Lab color space. A lookup table for conversion and a lookup table for converting the Lab color space to the color space of the output color data. FIG. 2 is a diagram illustrating these look-up tables LUT (A) and LUT (B).

  FIGS. 2A and 2B show the look-up tables LUT (A) and LUT (B), respectively. These look-up tables LUT are used to convert each component value (coordinate value) in each color point after conversion into each component value (coordinate value) of each grid point that is created when each axis of each color component in each color space before conversion is divided into a plurality of parts. Value).

In FIG. 2A, the component values of each grid point in the color space of the input color data are, for example, in Lab space associated with C _Ai M _Ai Y _Ai K _Ai and C _Aj M _Aj Y _Aj K _Aj. , For example, L _i a _i b _i and L _{j aj} b _j are associated with each other. Similarly, in FIG. 2B, the component values in the color space of the output color data associated with the component values in the Lab space, for example, L _m a _m b _m and L _n a _n b _n are used. The value is associated with C _Bm M _Bm Y _Bm K _Bm or C _Bn M _Bn Y _Bn K _Bn . These look-up tables LUT (A) and LUT (B) are used for color conversion from the C _A M _A Y _A K _A color space to the Lab color space, which is an absolute color space independent of device characteristics, and the Lab color. It is one that enables the color conversion from the space to C _B M _B Y _B K _B color space, is stored in the printer 2 is generated in a known manner.

  These look-up tables LUT (A) and LUT (B) associate the three-dimensional space with the four-dimensional space, and various tables can be generated according to other conditions. Are designed so as to produce an image with excellent color reproducibility and appearance when output by the printer 2.

For colors located between the grid points of these tables, the corresponding colors are obtained by interpolation calculation from the values of the grid points by a known method. Also, color conversion in the reverse direction is possible using each table. For example, using the lookup table LUT (B), the corresponding Lab value can also be obtained from the component value of C _B M _B Y _B K _B. That is, so-called reverse table lookup is also possible.

Next, the third look-up table LUT (AB1) is the same as associating the color space C _A M _A Y _A K _A of the input color data with the color space C _B M _B Y _B K _B of the output color data. It is a table. FIG. 3 is a diagram illustrating the lookup table LUT (AB1). The example shown in FIG. 3 is a case where the density gradation of each color component is expressed by 256 gradations of 0 to 255, and the axis of each color is divided every 16 gradations. With this table, input color data can be directly converted into output color data. Also for the table, the color reproducibility and appearance at the time of output by the printer 2 are determined by the designer of the printer 2 based on the lookup tables LUT (A) and LUT (B) using a known method. Designed to be an excellent image.

The fourth look-up table LUT (AB2) is obtained by changing values associated with lattice points on the K axis of the look-up table LUT (AB1). Specifically, the associated value is obtained by a K _B value only. That is, the value (0, 0, 0, K _B ) of the output color data is associated with the color represented by (0, 0, 0, K _A ) in the color space C _A M _A Y _A K _A of the input color data. By using this table, pixels expressed only in black (achromatic color) are expressed only in black (achromatic color). The value is not changed for other grid points.

The value to be changed is obtained as follows. First, the value of (C _B M _B Y _B K _B ) associated with (0, 0, 0, K _A ) in the lookup table LUT (AB1) is obtained. That is, color conversion using the lookup table LUT (AB1) is performed. Thereafter, a Lab value corresponding to the value of (C _B M _B Y _B K _B ) is obtained using the ICC profile of the printer 2 (not shown). The ICC profile of the printer 2 performs color conversion from the color space CMYK reflecting the device characteristics of the printer 2 to the Lab color space, which is a device-independent color space, and is stored in the ROM 25 of the printer 2. .

Next, (C _B M _B Y _B K _B ) value associated with the Lab value of the same L value as the L value of the calculated Lab value, which is (0, 0, 0, K _B ) Is obtained using the ICC profile of the printer 2. This is obtained by reversely looking up the ICC profile of the printer 2. Then, the value of the C _B M _B Y _B K _B space associated with (0, 0, 0, K _A ) is changed to the obtained (0, 0, 0, K _B ). The processing is executed for each (0, 0, 0, K _A ) to generate a lookup table LUT (AB2).

  The lookup table LUT (AB2) is also generated and stored in advance by the designer of the printer 2.

  In the lookup tables LUT (AB1) and LUT (AB2), as described above, the values between the lattice points are converted by interpolation calculation, and reverse lookup is possible.

  Next, the contents of color conversion processing using these lookup tables will be described. FIG. 4 is a flowchart illustrating the procedure of the color conversion process. Here, a case where color conversion processing using the lookup table LUT (AB2) is executed will be described. That is, a case will be described in which a lookup table for converting only black color to only black is used.

  First, the CPU 24 reads out the bitmap data that is the input color data described above and held in the RAM 26. Although data for one pixel will be described here, the same processing is performed for other pixels.

Then, it is determined whether or not the color of the read pixel is located in the vicinity of the K axis in the C _A M _A Y _A K _A space (step S1). Specifically, from the position of each (0, 0, 0, K _A ) which is a grid point of the lookup table LUT (AB2), to the area from each position of C _A M _A Y _A to the next grid point Check if the color is located. In this check, all the values of C _A M _A Y _A of the read input color data (C _A M _A Y _A K _A ) are all lattice point intervals of the look-up table LUT (AB2), in this example, less than 16. If it is, it is determined that it is located near the K axis. On the other hand, if any of the values of C _A M _A Y _A is 16 or more, it is determined that the position is not near the K axis.

In the above determination, if it is determined that the position is not near the K axis (No in step S1), color conversion processing using the lookup table LUT (AB2) is executed (step S2). In this processing, output color data (C _B M _B Y _B K _B ) corresponding to the input color data is obtained by a general conversion method using a lookup table. In this case, the conversion process is terminated as it is.

  On the other hand, if it is determined in the above determination that the position is near the K-axis (Yes in step S1), color conversion using the same lookup table LUT (AB2) as described above, and subsequent chromatic colors (CMY) ) Is changed (step S3).

Specifically, first, similarly to step S2, the corresponding (C _B M _B Y _B K _B ) is obtained using the lookup table LUT (AB2).

Thereafter, color conversion using the lookup table LUT (AB1) is similarly performed on the input color data (C _A M _A Y _A K _A ), and the corresponding (C _B M _B Y _B K _B ) is executed. Ask for. Here, the obtained output color data is represented by (C _BF M _BF Y _BF K _BF ) for identification. Then, the lightness (L value) of the color represented by (C _BF M _BF Y _BF K _BF ) is obtained by a reverse lookup method using the look-up table LUT (B) described above. This calculated brightness is taken as the L _OFF value.

Next, (C _B M _B Y _B K _B ) obtained above is used to change the lightness L of the color without changing the relative relationship (relative ratio) between each value of K _B and C _B M _B Y _B. Is changed to be the same as the lightness L _OFF . Specifically, the L value in the color of (XC _B XM _B XY _B K _B ) obtained by multiplying each value of C _B M _B Y _B of (C _B M _B Y _B K _B ) by X is used as the lookup table LUT. If the value obtained by the reverse lookup method using (B) is the same as the lightness L _OFF , the value of X is appropriately changed and obtained. Then, (XC _B XM _B XY _B K _B ) when it is the same as the lightness L _OFF is set as a value after color conversion. That ends the color conversion by changing the values of C _B M _B Y _B to XC _B XM _B XY _B.

For example, when (C _B M _B Y _B K _B ) is (4, 6, 2, K _B ) and L _OFF is 5, each value of C _B M _B Y _B is multiplied by 1.5. If the L value at this time becomes 5, the converted value is changed to (6, 9, 3, K _B ).

  In this way, the color conversion process is also completed when the input color is near the K axis.

  As described above, the same processing is performed for each pixel of the corresponding image.

Next, the effect of the color conversion process described above will be described. FIG. 5 is a graph showing L values after color conversion for colors near the K axis. In the graph of FIG. 5, the horizontal axis indicates the distance from the lattice point on the K axis to any adjacent lattice point, and the vertical axis indicates the lightness. Then, the position of the horizontal axis (near 140) at the right end of each graph corresponds to the position of the adjacent grid point. Thus, each plot point in each graph, the color of the position indicated by the value of the _{(C A M A Y A K} A) from a lattice point on the K axis in the space between grid points adjacent above the horizontal axis The lightness after the color conversion processing is shown as the value on the vertical axis.

  In addition, the graph (light line) indicated by K_Off is obtained by performing color conversion processing using the lookup table LUT (AB1), and the graph (thick line) indicated before K_On improvement is the lookup table LUT (AB2). When only the used color conversion processing is performed, the graph (thin line) shown after the K_On improvement is the chromatic color after the processing described based on FIG. 4, that is, the color conversion using the lookup table LUT (AB2). The actual processing results are shown for each of the above changes.

  As is apparent from the graph in the figure, in the case of K_Off, the look-up table LUT (AB1) designed so as to produce an image with excellent color reproducibility and appearance at the time of output is used by the designer. The brightness increases from the lattice point on the K axis to the adjacent lattice point at a substantially uniform rate, indicating a good result. On the other hand, in the case before K_On improvement of only color conversion using the lookup table LUT (AB2), it can be seen that the brightness is far from it and the continuity of the brightness is disturbed. However, in the case after the improvement of K_On, as described above, since the same brightness processing as in the case of K_Off is performed, the continuity of the brightness is ensured and a good result is obtained.

  Furthermore, in this method, when changing the value of the chromatic color, the relative relationship between the colors is not changed, so the hue (balance) of the chromatic color is not changed from that before the change. Therefore, in the area near the K axis where colors other than black are conspicuous, the balance of chromatic colors that continuously change from the grid point on the K axis to the adjacent grid point on the K axis can be maintained. Is also good.

  In the above-described embodiment, the color conversion processing near the K axis is executed each time. However, the lattice point interval near the K axis in the lookup table LUT (AB2) is made smaller than other regions, You may make it match | combine the value of the result of the conversion process containing the change of the chromatic color mentioned above to the lattice point.

  In the above-described embodiment, the four look-up tables are designed in advance. However, the look-up tables LUT (B) and LUT (AB1) are used in a case where the density characteristics of the printer 2 are taken into account. ), The LUT (AB2) may be dynamically generated by the controller 21 at a predetermined timing and held in the RAM 26 or the like.

  As described above, the color conversion process of the printer 2 according to the present embodiment provides the advantage that only achromatic colors can be converted into only achromatic colors, and the lightness discontinuity around the K axis that appears at that time. It can be solved and a good output result can be obtained. Further, as described above, the continuity of the hue is also ensured.

  Further, as described above, the processing time can be shortened by providing a lookup table reflecting the result of such processing in the vicinity of the K axis in advance.

  In the above embodiment, the printer 2 performs the color conversion process. However, the host computer 1 may be provided with a similar color conversion device so that the color conversion is performed on the host computer 1 side. Is possible.

  The protection scope of the present invention is not limited to the above-described embodiment, but covers the invention described in the claims and equivalents thereof.

  1 Host computer, 2 Printer, 11 Printer driver, 21 Controller, 22 Printing mechanism, 23 I / F, 24 CPU, 25 ROM, 26 RAM, 27 Engine I / F

Claims (4)

A color conversion device that converts color data expressed in chromatic and achromatic colors,
In the first conversion information for converting input input color data to output color data, and the first conversion information, when the input color data indicates an achromatic color and a chromatic color, The output color data after the conversion has a second conversion information configured to indicate an achromatic color and a color that does not have a chromatic color,
When the conversion process is performed using the second conversion information, the conversion using the second conversion information is performed on the neighboring colors having the achromatic color and not having the chromatic color. After obtaining the output color data, the color indicated by the output color data after changing the value of each color of the chromatic color in the output color data without changing the relative ratio of the value of each color The color conversion device performs the conversion process by changing the brightness so that the brightness is the same as the brightness of the color indicated by the output color data when the conversion is performed using the first conversion information. . In claim 1,
Each of the first conversion information and the second conversion information is information that associates each color in the color space of the output color data with each color in the color space of the input color data, and associates the colors with the second conversion information. Each color is the same as each color associated in the first conversion information except for a color associated with the achromatic color and not having a chromatic color. A color conversion method in a color conversion device that performs conversion processing of color data expressed in chromatic and achromatic colors,
In the first conversion information for converting input input color data to output color data, and the first conversion information, when the input color data indicates an achromatic color and a chromatic color, Second conversion information is provided so that the output color data after the conversion has an achromatic color and a color that does not have a chromatic color;
When the conversion process is performed using the second conversion information, the conversion using the second conversion information is performed on the neighboring colors having the achromatic color and not having the chromatic color. After obtaining the output color data, the color indicated by the output color data after changing the value of each color of the chromatic color in the output color data without changing the relative ratio of the value of each color The color conversion method is characterized in that the conversion processing is performed by changing the lightness of the color to be the same as the lightness of the color indicated by the output color data when the conversion is performed using the first conversion information. . A color conversion program for causing a control device to perform conversion processing of color data expressed in chromatic and achromatic colors,
In the first conversion information for converting input input color data to output color data, and the first conversion information, when the input color data indicates an achromatic color and a chromatic color, Second conversion information is provided so that the output color data after the conversion has an achromatic color and a color that does not have a chromatic color;
The control means is
When the conversion process is performed using the second conversion information, the conversion using the second conversion information is performed on the neighboring colors having the achromatic color and not having the chromatic color. After obtaining the output color data, the color indicated by the output color data after changing the value of each color of the chromatic color in the output color data without changing the relative ratio of the value of each color Is changed so that the brightness of the color indicated by the output color data when the conversion is performed using the first conversion information, and the conversion process is performed.
A color conversion program characterized in that the conversion process is executed as described above.