JP2006031376A - Image processing apparatus and color conversion method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide color conversion that makes the best use of a color reproduction range of a printer.
A printing condition is set (S101), and input data and a profile for a printing apparatus are used to convert RGB input data into CMYK data in a second color space composed of four basic colors for printing ( In step S103, a 4D LUT corresponding to the printing condition is selected (S104), and the CMYK data is converted into data in a color space including colors of six colors (S105). High-efficiency compression is applied to the lookup table, and the number of profiles is further reduced to reduce the total amount of data.
[Selection] Figure 3

Description

  The present invention relates to an image processing apparatus and a color conversion method, and, for example, to color conversion when image data is output to a color printer or the like.

  There are inkjet printers that achieve high image quality by using two types of light and dark inks such as cyan and magenta. Among them, six color inks (C, LC, M, LM, Y, K) are used, in which cyan is divided into dark cyan (C) and light cyan (LC), and magenta is divided into dark magenta (M) and light magenta (LM). Inkjet printers are typical.

  When printing RGB data of color representation that is used in many operating systems (OS) as standard with such an inkjet printer, there is a method of developing RGB data into multicolor data such as six colors using a three-dimensional lookup table. Yes (see, for example, Patent Document 1). While there are systems based on RGB input as described above, data based on output from a printing press is often input in CMYK.

  For example, PDL data submitted in a page description language (PDL) typified by Adobe's PostScript (R) is often standardized CMYK colors that are assumed to be output by a specified printer. It is configured as spatial PDL data. Such PDL data in the CMYK color space is color-converted into data in the CMYK color space of the printer when the PDL data is converted into raster data by a raster image processor (RIP) when printing with a general printer.

  This color conversion processing is often performed by a combination of an input profile describing an input color space and an output profile describing an output destination color space. That is, color conversion is performed on input data in the CMYK color space by combining a CMYK input profile describing characteristics of the CMYK color space and an output profile describing characteristics of the CMYK color space of the output destination printer. Also, if the input data includes RGB color space data, color conversion is performed by combining the RGB input profile that describes the characteristics of the RGB color space and the output profile that describes the CMYK color space characteristics of the output printer. Do.

  For the input profile, an ICC profile, which is a standardized general format, or a CSA (Color Space Array) defined by PostScript (R) is often used. Also, an ICC profile, which is a standardized general-purpose format, and a CRD (Color Rendering Dictionary) defined by PostScript (R) are often used for the output profile.

  Through the color conversion process as described above, the input data is rasterized into data in the CMYK color space of the output printer, and then printed by performing a halftone process.

  Here, when outputting the color-converted data to a printer that uses dark and light ink, for example, C and M are subjected to light and dark separation of the rasterized data, and C, LC, M, LM, Y, K Expands to six ink colors. The density separation is realized by two one-dimensional lookup tables (1D LUT) for dark ink and light ink. That is, in the case of cyan, the rasterized data C value is input, the value output from the 1D LUT for dark ink is set to the value of dark cyan C, and the value output from the 1D LUT for light ink is set to light cyan Perform light / dark decomposition to LC value.

  In many printers, the total amount of ink and toner that can be input per unit area of recording paper is limited. For example, the total ink placement amount of an ink jet printer is often limited to a maximum of 150 to 250%. Further, the amount of ink applied varies depending on the type of recording paper and the printing speed.

  The color reproduction range will be wider if more ink is applied, but if the ink is applied beyond the maximum ink injection amount, the image will bleed, the image will be lost due to ink dripping, and the print head will be rubbed. Problems such as “damage” occur. Accordingly, it is necessary to appropriately control the printing amount so as to be equal to or less than the prescribed maximum ink driving amount.

  When the input CMYK or RGB PDL data is rasterized into the CMYK color space and then separated into a larger number of ink colors, such as six colors, using multiple 1D LUTs, the total ink is obtained by color separation using the 1D LUT. Assuming that the hit amount increases, the LUT to be converted into the CMYK color space is created in advance so that the ink hit amount is smaller than the maximum ink hit amount. Therefore, in a color gamut where dark and light ink is not used much, there is a problem that ink can be applied only up to an impact amount lower than the maximum ink impact amount, and the color reproduction range of the printer cannot be utilized to the maximum.

  Another problem is the amount of data occupied by the profile.

  Inkjet printers and the like support various types of recording paper and print quality, and the maximum ink ejection amount and color reproduction range differ depending on the combination of recording paper and print quality. Therefore, CMYK output profiles are required for combinations of types of recording paper supported by the printer and corresponding print quality. In addition, when a plurality of halftone processing methods are implemented, the CMYK output profile increases by combining the halftone processing methods.

  As described above, general-purpose data formats such as CRD and ICC profiles are often used for the CMYK output profile, and such data is uncompressed. When the data of one grid point is a general CMYK ICC profile of 2 bytes per color, the file size is about 800 kB, and it is necessary to store this data as it is in a storage medium. This 800kB breakdown consists of a CMYK / Lab conversion table type (4 input / 3 output 17 grid points) of about 500kB (= 2 × 17 × 17 × 17 × 17 × 3), Lab / CMYK conversion table type ( The size of 3 input / 4 output 33 grid points) is about 290kB (= 2x33x33x33x4), and other headers etc. are about 40kB.

Furthermore, for example, three types of print quality can be set for each of ten types of recording paper, and the profile data amount of software RIP that supports a printer having two types of halftone processing methods is as follows.
800kB / × 10 × 3 × 2 = about 47MB

In addition, some RIP software supports multiple types of printers. For example, when supporting five models, the amount of data necessary to record all profiles is as follows.
47 x 5 = about 235MB

Japanese Patent Laid-Open No. 2003-110864

  The present invention solves the above-described problems individually or collectively, and aims at color conversion that makes the most of the color gamut of a printer.

  The present invention has the following configuration as one means for achieving the above object.

  The present invention sets printing conditions, uses input data and a profile for a printing apparatus, and converts the input data represented by the first color space into a second color space consisting of four basic colors for printing. Using the color conversion profile corresponding to the printing conditions, the data of the second color space is converted into data of a third color space composed of colors of four or more colors used by the printing apparatus. It is characterized by converting to data.

  According to the present invention, color conversion that makes the most of the color gamut of the printer can be achieved.

  Hereinafter, image processing according to an embodiment of the present invention will be described in detail with reference to the drawings.

  FIG. 1 is a block diagram illustrating a configuration example of a color conversion apparatus according to an embodiment. In the following, an example in which the color conversion device is configured by general computer equipment will be described.

  The computer apparatus 100 is an apparatus such as a personal computer. The CPU 101 uses the RAM 102 as a work memory, executes a program including color conversion processing, which will be described later, stored in the ROM 103 and the hard disk drive (HDD) 107, and configures other configurations via the system bus 105. Control.

  The CPU 101 displays a processing status and result, a graphic user interface, and the like on a monitor 109 such as a CRT or LCD via a monitor interface (I / F) 104. The CPU 101 transfers data to be executed to a wired or wireless network 114 via a storage medium such as a CD-ROM mounted on the HDD 107 or the storage medium drive 108, or a network interface card (NIC) 113. Acquired from the server device above, and writes the processing results to a storage medium such as the HDD 107 or CD-R installed in the storage media drive 108, sent to the server device on the network 114, or the serial bus The data is output to the printer 110 connected to the serial bus 115 such as USB (Universal Serial Bus) or IEEE1394 via the I / F 106. The printer 110 may be connected to the network 114. In this case, the print data is transmitted to the network 114 via the NIC 113.

  When executing processing, the CPU 110 displays on the monitor 109 a processing condition setting screen and a user interface including a processing progress display screen and a processing result display screen. In accordance with the user interface displayed on the monitor 109, the user operates the keyboard 111 and the mouse 112 to specify input data and output data (or data input destination and output destination), set processing conditions, and process Enter start instructions.

  FIG. 2 is a block diagram illustrating the configuration of color conversion processing and the processing data. A block surrounded by a broken line indicates data, and a block surrounded by a solid line indicates a functional block (software module or hardware module).

  The color management module (CMM) 3 is an arithmetic module that handles an ICC profile and performs color conversion. The profile set 4 is a set of a plurality of ICC profiles describing characteristics of input data, and is stored in a memory such as the HDD 107. The input profile 5 represents an input profile used by the CMM 3 from the profile set 5.

  The profile set 6 is a set of ICC profiles that define the characteristics of the output CMYK color space, which is the conversion result of the CMM 3, and is stored in a memory such as the HDD 107. The output profile 7 represents an output profile used by the CMM 3 from the profile set 6.

  With such a configuration, the CMM 3 inputs CMYK data 1 or RGB data 2, and based on the characteristics described in the input profile 5, the input data is a device-independent color space (for example, L * a * b * color space) ), And the device-independent color space data based on the characteristics described in the output profile 7 is output as CMYK data 8 in the device-dependent output CMYK color space. That is, the CMYK data 8 is image data as a result of color conversion by the CMM 3.

  The four-dimensional lookup table (4D LUT) calculation unit 9 is a calculation module that inputs CMYK data and outputs six-color (C, LC, M, LM, Y, K) data including dark and light inks. The 4D LUT set 11 is a set of 4D LUTs used by the 4D LUT calculation unit 9, and is stored in a memory such as a hard disk of a computer device. The 4D LUT 12 represents a 4D LUT used by the 4D LUT calculation unit 9.

  With such a configuration, the 4D LUT calculation unit 9 inputs CMYK data 8 and outputs data 10 for ink of six colors (C, LC, M, LM, Y, K).

  The printing condition setting unit 13 is a printing condition setting module that generates a graphic user interface and sets printing conditions based on the user's operation on the graphic user interface.

  FIG. 3 is a flowchart for explaining the color conversion process.

  First, printing conditions are set by the printing condition setting unit 13 (S101). The printing conditions include the type of recording paper used for printing, such as plain paper, coated paper, and glossy paper. In addition, the print quality, for example, “High speed”, “Standard”, “Priority on image quality”, etc. Contains items. Depending on the setting contents, the operating conditions of the printer 110 change and the printing characteristics also change.

  Next, an output profile 7 used by the CMM 3 for color conversion is selected from the profile set 6 (S102). This selection may be performed based on, for example, preset model information of the printer 110, or may be performed based on information obtained by communication between the CPU 101 and the printer 110.

  Here, when the change in the printing characteristics of the printer 110 depending on the setting contents of the printing conditions is absorbed by the color conversion of the CMM 3, profiles corresponding to all the printing conditions are prepared in the profile set 7, and It is necessary to select an output profile corresponding to the set printing conditions. In this embodiment, since the 4D LUT calculation unit 9 shares part of the color conversion, the output profile does not necessarily need to be linked to the setting of the printing condition. In other words, as long as the printer models are the same, one output profile 7 may be fixed regardless of the print condition setting. Alternatively, for example, the size of the color gamut depending on the printer model may be classified into, for example, three levels of large, medium, and small, and limited to three types of output profiles corresponding to each. In this way, the number of profiles to be held in the profile set 6 can be greatly reduced.

  Next, the CMM 3 performs first color conversion using the input profile 5 and the output profile 7 (S103). The CPU 101 can switch the input profile 4 used by the CMM 3 in units of objects included in the input data. If the input data is CMYK data 1, the CPU 101 sets a CMYK ICC profile that matches the color space of the data, and if the input data is RGB data 2, the RGB ICC profile that matches the color space of the data Are sequentially selected and supplied to the CMM 3 as the input profile 5. If the input profile is embedded in the input data, the CPU 101 instructs the CMM 3 to use the embedded input profile. Therefore, in such a case, the profile set 4 is not always necessary.

  CMM 3 uses input profile 5 and output profile 7 to color-convert input data. Normally, however, data written in PDL is also rasterized at the same time as color conversion, so CMYK data 8 is rasterized data. Is generated as

  Next, the 4D LUT used by the 4D LUT computing unit 9 is selected from the 4D LUT set 11 (S104). Here, the optimum 4D LUT is selected according to the contents set by the printing condition setting unit 13 in order to absorb the change in the printing characteristics of the printer 110 depending on the setting contents of the printing conditions.

  After generating the rasterized CMYK data 8, if C, LC, M, LM, Y, and K are generated by multiple 1D LUTs to separate the dark and light inks, as described above, the total ink is generated by color separation using the 1D LUT. Assuming that the hit amount increases, the LUT to be converted into the CMYK color space is created in advance so that the ink hit amount is smaller than the maximum ink hit amount. Therefore, in a color gamut where dark and light ink is not used much, there is a problem that ink can be applied only up to an impact amount lower than the maximum ink impact amount, and the color reproduction range of the printer cannot be utilized to the maximum.

  In the present embodiment, after the rasterized CMYK data 8 is generated, color conversion to multiple colors using a 4D LUT is further performed. Accordingly, it is possible to relatively freely adjust the combination of the ink ejection amounts of the respective inks in a state where the specified maximum ink ejection amount is utilized to the maximum. In other words, color conversion can be performed without sacrificing the color gamut of the printer 110.

  In many cases, the CMM 3 is based on a general-purpose profile such as an ICC profile, and is configured integrally with a PDL interpreter and a rasterizer. Therefore, the degree of freedom in designing CMM 3 is relatively low. On the other hand, the 4D LUT calculation unit 9 can be configured as a simple module for converting raster data into raster data, and thus has a high degree of freedom in design. Therefore, it is possible to perform high-efficiency data encoding on the 4D LUT set 11 and to incorporate the decoder into the 4D LUT calculation unit 9. As a result, the data amount of the 4D LUT set 11 can be greatly reduced. It becomes possible.

  Next, the 4D LUT calculation unit 9 performs color conversion using the 4D LUT 12 on the CMYK data 8 to generate data 10 developed into six colors of C, LC, M, LM, Y, and K (S105). ).

  In this way, after performing color conversion to CMYK four colors, it is configured to convert to multiple colors (for example, six colors) by 4D LUT, thereby maximizing the maximum ink ejection amount specified for printer 110 This makes it possible to make full use of the color conversion that makes the best use of the color gamut of the printer 110. Furthermore, by reducing the total number of profiles included in the profile set 6 of the uncompressed format used for color conversion to CMYK four colors, a 4D LUT is prepared for the 4D LUT set 11 subjected to high-efficiency data compression. Therefore, it is possible to reduce the total amount of data (including profile set 4, profile set 6, and 4D LUT set 11).

  In the above description, the case of a six-color configuration in which dark and light inks are applied to cyan and magenta, such as C, LC, M, LM, Y, and K, has been described. Color conversion is possible.

  Focusing on reducing the total data amount, even if the conversion result from the 4D LUT is CMYK four-color data, the total number of profiles included in the uncompressed profile set 7 is reduced, and high-efficiency data compression is achieved. By configuring a possible 4D LUT set 11, the total amount of data can be reduced.

  Thus, after converting the data of the first color space to CMYK data of the second color space (first conversion), the 4D LUT converts the CMYK data to data of any number of colors (the second color space). Conversion), it is possible to freely combine the ink ejection amounts up to the maximum ink ejection amount of the printer in the entire color space, and realize color conversion that makes maximum use of the printer's color gamut. be able to.

  Further, in order to obtain data of an arbitrary number of colors only by the first conversion, it is necessary to switch the output profile of CMYK used in the first conversion by the number of printing conditions. However, in this embodiment, the output profile of CMYK used in the first conversion is fixed or limited to a limited number of output profiles, and the absorption of printing characteristics that differ depending on the printing conditions is the second conversion. It can be carried out.

  And since the 4D LUT of the second conversion can be made into a profile with high-efficiency data compression, the profile (4D LUT) is compared to the case where many uncompressed profiles (including 1D LUT) are prepared. The total amount of data is reduced.

For example, the data amount of 6D output and 4D LUT profile in which data for one grid point is 2 bytes per color and each axis corresponding to four colors is 17 grid points is as follows.
2 × 17 × 17 × 17 × 17 × 6 = about 980kB

When highly efficient data compression is applied to this data, it can be reduced to about 60% of the original data size, so the actual data amount is as follows.
980kB × 0.6 = about 590kB

The number of ICC profiles of approximately 800kB to be applied to the output profile 7 is three types (common to all models). Furthermore, three types of print quality can be set for each of the ten types of recording paper for the 4D LUT. The profile data amount of software RIP that supports five printers with tone processing methods is as follows.
800kB × 3 + 590kB × 10 × 3 × 2 × 5 = about 180MB

  As described above, the conventional method requires a data amount of about 235 MB. However, according to the present embodiment, the data amount of about 25% can be reduced to a data amount of about 180 MB.

[Other embodiments]
Note that the present invention can be applied to a system including a plurality of devices (for example, a host computer, an interface device, a reader, and a printer), and a device (for example, a copying machine and a facsimile device) including a single device. You may apply to.

  Also, an object of the present invention is to supply a storage medium (or recording medium) on which a program code of software that realizes the functions of the above-described embodiments is recorded to a system or apparatus, and the computer (or CPU or Needless to say, this can also be achieved by the MPU) reading and executing the program code stored in the storage medium. In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention. Further, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an operating system (OS) running on the computer based on the instruction of the program code. It goes without saying that a case where the function of the above-described embodiment is realized by performing part or all of the actual processing and the processing is included.

  Furthermore, after the program code read from the storage medium is written into a memory provided in a function expansion card inserted into the computer or a function expansion unit connected to the computer, the function is determined based on the instruction of the program code. Needless to say, the CPU of the expansion card or the function expansion unit performs part or all of the actual processing and the functions of the above-described embodiments are realized by the processing.

  When the present invention is applied to the storage medium, the storage medium stores program codes corresponding to the flowcharts described above.

A block diagram showing a configuration example of a color conversion device of an embodiment, Block diagram for explaining the configuration and processing data of color conversion processing, It is a flowchart explaining a color conversion process.

Claims (9)

Set the printing conditions,
Using the input data and the profile for the printing device, the input data represented in the first color space is converted into data in the second color space consisting of the basic four colors for printing,
Using the color conversion profile corresponding to the printing conditions, converting the data of the second color space into data of a third color space composed of colors of four or more colors used by the printing apparatus. A color conversion method characterized by the above.   2. The color conversion method according to claim 1, wherein the color conversion profile is selected based on the printing conditions from a plurality of color conversion profiles prepared in advance.   3. The color conversion method according to claim 2, wherein the plurality of color conversion profiles are a collection of four-dimensional lookup tables.   4. The color conversion method according to claim 3, wherein the aggregate is data-compressed.   5. The color conversion method according to claim 1, wherein the printing condition includes at least one of a recording paper type and a printing quality.   6. The color conversion method according to claim 1, wherein the input data and the profile for the printing apparatus are color conversion profiles in a general format.   7. A program for controlling the information processing apparatus to realize the color conversion described in any one of claims 1 to 6.   8. A recording medium on which the program according to claim 7 is recorded. Setting means for setting printing conditions;
First conversion means for converting the input data represented in the first color space into data in a second color space consisting of four basic colors for printing using the input data and the profile for the printing apparatus; ,
The second color space data is converted into data of a third color space composed of colors of four or more colors used by the printing apparatus using a color conversion profile corresponding to the printing condition. An image processing apparatus comprising: a second conversion unit.

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