JP2004159307A - Color conversion table generating method, print controller, color conversion table generator, and color conversion table generating program - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve the problem in a color conversion table forming method that the resolution of a specific gradation is improved to result in a relative reduction of the resolutions of other gradations. <P>SOLUTION: For forming a color conversion table specifying the corresponding relation of ink value data for specifying ink quantities of colors used for a printer to color component values of colors used for other image apparatus, a correction for improving the interpolation accuracy is previously executed so that colors in low lightness regions become more than those in high lightness regions to compensate the resolution relatively lowering in the low lightness regions due to a correction for improving the resolution at the interpolation accuracy, then the correction for improving the resolution is executed to generate ink value data, and the colors of patches printed according to the ink value data are measured to generate a color conversion table. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a color conversion table creation method, a printing control device, a color conversion table creation device, and a color conversion table creation program.

  2. Description of the Related Art Image devices such as displays and printers generally use color image data in which the color of each pixel is expressed in gradations with a specific color component. For example, an RGB color space using three colors of R (red), G (green), and B (blue) or a CMY color space using colors of C (cyan), M (magenta), and Y (yellow) systems (Lc: light cyan, lm: light magenta, DY: dark yellow, K: black, etc.) are defined as image data by defining colors in various color spaces. Since these colors are device-dependent colors that are unique to the image device, a color conversion table (LUT) that defines the correspondence between colors in each device in order to enable the same image to be output in the same color between various image devices. ) Is used.

Due to the difference in color properties between the display using the RGB color space and the printer using the CMY system color space, such as the difference between the additive color mixture method and the subtractive color mixture method or the color gamut of each other. However, it is not always easy to perform natural color conversion with high accuracy. Accordingly, the resolution of a specific gradation is substantially improved by a configuration in which the gradation value is temporarily increased when performing color conversion, and the excess is eliminated when performing halftone processing (for example, see Patent Document 1). )).
Japanese Patent Application No. 2000-307859

Assuming that the storage capacity used for expressing the gradation of each color is fixed, when the resolution of a specific gradation is improved by performing a premium as in the above-described conventional example, the resolution of other gradations is relatively high. Sometimes decreased.
The present invention has been made in view of the above problems, and has a color conversion table creation method, a print control device, a color conversion table creation device, and a color conversion table creation that can define a correspondence relationship for performing color conversion with high accuracy in the entire brightness range. To provide the program.

  In order to achieve the above object, in the present invention, when generating a color conversion table, a correction for improving interpolation accuracy is performed on a reference value before correction extracted from a predetermined gradation value before correction, and a correction for improving interpolation accuracy is performed. The patch data of the color specified by generating the subsequent magnitude of the gradation value and the magnitude of the ink amount substantially linearly is generated. Further, the tone value in the patch data is defined as an integer value existing in a predetermined value range, and the tone value corresponding to the high brightness range is set to be lower than the tone value corresponding to the lower brightness range. The ink value is defined so that the change of the ink recording rate corresponding to the unit change of the tonal value becomes small, and this is defined as the ink value data. Then, halftone image data is generated while interpreting the ink amount corresponding to the reference value of the patch data according to the definition of the gradation value, and the color conversion table is generated.

  In other words, in the ink value data, the gradation value corresponding to the high lightness region and the gradation value corresponding to the low lightness region are defined so that the amount of change in the ink recording rate, which means a unit change in the gradation value, is different. By correcting, the change in the ink recording rate can be expressed more finely in the high brightness region. However, in this case, a comparison is made with a case where the unit change of the gradation value and the change amount of the ink recording rate are made substantially the same at the gradation value (for example, 8 bits, 256 gradations) represented by the limited capacity. Thus, the resolution in the low brightness range is lower than the resolution in the high brightness range. Therefore, in the present invention, correction for improving the interpolation accuracy is performed in advance, and the patch data corresponding to the color in the low lightness region is increased to be the color to be measured.

  At this time, a pre-correction reference value smaller than the total number of gradations of the pre-correction gradation value is extracted from a predetermined pre-correction gradation value for each ink color and combined. As a result, a plurality of data sets composed of a combination of a plurality of pre-correction reference values are obtained, and corrections for improving the interpolation accuracy are made. In the correction for improving the interpolation accuracy, the reference value before correction is increased. Therefore, assuming that the magnitude of the value and the amount of ink correspond substantially linearly in the reference value before correction, the color indicated by each of the data sets and the color indicated by the corrected data for improving the interpolation accuracy are shown. The latter has more colors in the low brightness range.

  By printing the patch data using more colors in the low brightness range than in the high brightness range, the interpolation accuracy when creating the color conversion table is improved in the low brightness range. That is, interpolation is used to associate the ink value data with the color component values of the respective colors used in the other image devices according to the colorimetric results. However, if there are many colors in the low brightness range, the low High-precision interpolation calculations can be performed on colors in the brightness range. In addition, since the interpolation process is also performed when color conversion is performed using this color conversion table, color conversion can be performed with high accuracy by specifying colors with high accuracy in the low brightness region.

  As the pre-correction gradation value, for example, a gradation value of each color component in a color space including three color components can be adopted. When the intensity of each color component is linearly represented by a gradation value, in order to adopt a color that is substantially evenly distributed over the entire color space, even if the gradation values obtained by dividing the gradation value range substantially equally are combined. However, when the ink value data is defined as described above, the resolution in the low lightness region relatively decreases.

  Therefore, if the correction for improving the resolution is performed on the gradation values obtained by substantially dividing the gradation value range, the representative colors are so distributed that the colors in the low brightness range are more densely distributed than in the high brightness range as compared with before the correction. Can be extracted. Of course, if the gradation value range is not a combination of substantially equally divided gradation values as described above, for example, even if data obtained by performing separation described below is corrected for interpolation accuracy improvement, By increasing the number of colors in the low lightness region, it is possible to improve the interpolation accuracy in the low lightness region.

  In any case, the correction for improving the interpolation accuracy and the definition of the ink value data make it possible to express both finer changes in the ink recording rate in the high-brightness region and prevent the resolution from decreasing in the low-brightness region. . As a result, it is possible to define the correspondence for performing color conversion with high accuracy in the entire brightness range in the color conversion table. In addition, since the ink value data is defined as an integer value existing within a predetermined value range, the total number of gradations is limited. For example, if the gradation value is expressed by an 8-bit capacity, the total number of gradations is 256 for each color, and the gradation can be defined by an integer value of 0 to 255 or the like.

Also, patch data is created for the number of patches to be measured, but it is virtually impossible to extract reference values corresponding to all the gradation numbers of the ink value data to obtain the patch data. That is, assuming 256 gradations, which is the most common gradation number at present, 256 ^x (x is the number of ink colors) in order to extract reference values corresponding to all gradation numbers and make them into patch data. Since patch data is required, it is practically impossible to measure the color of this vast number of patches. Therefore, for each ink, a smaller number of reference values than the total number of gradations are extracted, and for example, 1000 pieces of patch data are created.

  The gradation value of the ink value data is associated with the ink amount. That is, the ink amount is determined by the gradation value. Therefore, when the gradation values are combined for each ink color, the amount of ink in each ink color is specified, and the color when printing is performed using this amount of ink is specified. Further, in the ink value data, the change in the ink recording rate corresponding to the unit change of the gradation value becomes smaller as the gradation value corresponds to the higher brightness region than the gradation value corresponding to the lower brightness region. It is sufficient if it can be defined.

  As a specific example, when the unit change of the gradation value is the minimum change amount of the integer value “1”, when the gradation value changes by “1” in the high lightness region, the ink recording rate changes by m% or less. This can be realized by defining the ink recording rate to change more than m% when the gradation value changes by “1” in the low brightness range. With the above definition, even if the gradation value is defined as an integer value existing in a predetermined value range, the ink recording rate can be defined with high accuracy in a high brightness range, and based on this definition, By creating the color conversion table, color conversion can be performed with high accuracy even with high brightness.

  That is, when defining a gradation value by a computer, its value range is limited, and the value is generally defined by an integer. In this definition, if the change of the ink recording rate corresponding to the unit change of the gradation value is defined to be substantially constant in the entire lightness region as in the related art, a high brightness is caused due to the characteristic of the lightness change with respect to the change of the ink recording rate. Accuracy is relatively lower in the degree range than in the low brightness range. However, by defining the gradation values as in the present invention, it is possible to define the ink recording rate with high brightness and high accuracy without increasing the range of the gradation values.

  For example, with a storage capacity of 8 bits, 256 gradations can be expressed. However, if 0 to 100% of the ink recording rate is equally allocated to each of the 0 to 256 gradations, the resolution is uniform, but n% A value larger than 256 * n / 100 is made to correspond to the ink recording rate (n is an example of a high brightness area). For example, a case where 40 (instead of 13 (≒ 256 * 5/100)) for 5% ink recording rate and 61 (instead of 26 (≒ 256 * 10/100)) for 10% ink recording rate are used. Then, the ink recording rate of 5 to 10% can be expressed by 21 gradations instead of 13 gradations. When 256 gradations are expressed by 8 bits, each gradation value is an integer value, and the decimal part is rounded down or rounded off.

  If this concept is applied when creating a color conversion table, it is possible to perform color conversion with very high accuracy in a high lightness region where the change rate of lightness with respect to a change in ink amount is large. According to the above definition, when the gradation value is associated with the ink recording rate, it is possible to specify the color to be printed by defining the gradation value for each ink color. On the other hand, by combining the color component values of the respective colors (for example, the color component values of the RGB colors, the color component values of the CMYK colors, etc.) used in the other image device, the colors in the other image device are specified. Therefore, by associating the combination of the gradation value with the combination of the color component value, it becomes possible to create table data and a profile for color conversion of both.

  In the halftone process, the presence or absence of a dot to be recorded for each pixel in the dot matrix is determined according to the ink recording rate defined by the gradation value. Therefore, by defining the gradation value as described above and performing printing in accordance with the output value of the halftone processing module, it becomes possible to execute printing while controlling the ink recording rate at high brightness. Further, by performing color conversion on the color component values of the other image devices with reference to the color conversion table created according to the above definition, it is possible to perform high-precision color conversion with high accuracy. The ink recording rate corresponds to the area of the dots recorded per unit area or the dot count. 0% indicates that no dots are recorded per unit area, and the maximum number of dots is recorded per unit area. Is 100%.

  Note that the change in brightness with respect to a unit change in the ink recording rate at a high brightness is larger than the change in brightness with respect to a unit change in the ink recording rate at a low brightness. For this reason, in a general ink, the lightness hardly changes even if the ink recording rate is changed in a predetermined lightness region including the lowest lightness that can be expressed by each ink color. Therefore, it is also possible to define the ink value data excluding a predetermined lightness region including the lowest lightness that can be expressed by such an ink color.

  That is, the above-described ink value data is defined by allocating all gradation values to a part of the value range of the ink recording rate. As a result, it is possible to exclude the ink recording rate at which a substantial change in lightness cannot be expressed on a print medium, and to assign a gradation value only to an ink amount whose lightness can substantially change. Therefore, it is possible to more effectively express the gradation of the ink amount with a limited capacity, and to express a more subtle gradation change.

  Here, the value range of the ink recording rate is defined as the minimum ink recording rate when no ink is recorded on the print medium, and as the maximum ink recording rate when ink is maximally recorded on the print medium. For the sake of simplicity, a common value range for all ink colors may be adopted as a part of the value range of the ink recording rate, but differs for each ink in view of the different rate of change in brightness for each ink. The value range may be adopted as a part of the value range of the ink recording rate.

  Further, in the present invention, a change in the ink recording rate corresponding to a unit change of the gradation value is smaller for a gradation value corresponding to a high brightness region than for a gradation value corresponding to a lower brightness region. It suffices if it is possible to compensate for the relatively low resolution of the low brightness by defining the above. For this reason, in the first tone value data for specifying the color of the colorimetric target while making the ink amount and the magnitude of the tone value substantially linearly correspond to each other, a large number of colors in the low lightness region are provided. It may be included. That is, according to the above-described correction for improving the resolution, the resolution of the high brightness region is improved, but the possibility of dividing the low brightness region is reduced, so that the resolution that relatively decreases in the low brightness region is compensated with interpolation accuracy. First, the first gradation value data is extracted in advance so that the colors in the low lightness region are greater than the colors in the high lightness region.

  More specifically, in a gradation value in which the ink amount and the gradation value are approximately linearly associated with each other, the larger the gradation value is, the larger the ink amount is, and the color becomes lower in brightness. Is equivalent to increasing the low lightness color. Therefore, by preliminarily increasing the number of low lightness colors, the color specified by the first gradation value data is measured, and when a color conversion table is created, interpolation is performed with high accuracy in the low lightness region. Can be implemented.

  Assuming that the first tone value data in this state is corrected at a larger increase rate than the tone value corresponding to the lower brightness range as the tone value corresponding to the higher brightness range becomes the ink value data, Ink value data is defined such that the change in the ink recording rate corresponding to the unit change of the gradation value becomes smaller as the gradation value corresponding to the high lightness region becomes smaller than the gradation value corresponding to the lower lightness region. can do. Therefore, the resolution in the high brightness range can be improved.

  When printing a colorimetric object based on this ink value data, halftone processing may be performed while reflecting a deviation corresponding to a decimal number or less when inversely correcting the correction for improving the resolution. As a result, the data after the inverse correction is the same as a state in which 0 to 100% of the ink recording rate is evenly allocated to each of the gradations 0 to 256. Can be determined. However, in this case, since the halftone processing is performed by reflecting a deviation corresponding to a decimal number or less when the inverse correction is performed, a state of high brightness and high resolution is maintained.

  Of course, it is not possible to consider infinitely the deviation equivalent to the decimal fraction when the inverse correction is performed, but if the value of the predetermined number of decimals or less is considered according to the capability at the time of the halftone processing, the color can be accurately calculated. It can be specified. In addition, here, it is sufficient that a deviation corresponding to a decimal number or less when inversely corrected is taken into consideration. In addition to the configuration for actually performing inverse correction, the ink recording rate corresponding to each of the above-described corrected ink value data is further increased. The value specified by the high bit number may be stored in advance. Also in the above configuration, color conversion can be performed with high accuracy in the entire brightness range. Note that although the correction for improving the resolution is performed on the ink value data, the inverse correction is performed by the halftone process, so that the color indicated by the ink value data and the color indicated by the first gradation value data are the same.

  In the present invention, a color separation process may be performed when determining the first gradation value data. That is, a printing apparatus is often configured to be able to execute printing using inks of more than three colors of CMY, for example, inks of six colors or seven colors, and the colorimetric target in a six-dimensional or seven-dimensional space is often used. Is difficult to identify. This is because, when six or seven colors of ink are used, inks that are used alternatively, such as cyan and light cyan, are included, and even if different combinations of six or seven colors are used, the colors are almost the same in many cases.

  Further, it is difficult to create a predetermined conversion formula such as a matrix for uniquely converting a coordinate value in a six-dimensional space into a coordinate value in a three-dimensional space. It is easy to create a conversion formula that uniquely converts to a coordinate value in a six-dimensional space. Therefore, it is very easy to specify a color to be measured by three colors of CMY, and to perform color separation in which a combination of the three colors is converted into a combination of six or seven colors by a specific conversion formula. is there.

  Therefore, in a configuration in which a color to be measured is first specified by CMY values, separation is performed, and CMY values are converted, the CMY values before separation or the values after separation are corrected to improve resolution and low. The colors in the low lightness region are made more than the colors in the high lightness region so as to compensate for the relatively lower resolution in the lightness region with interpolation accuracy. That is, by performing color separation, the ink value of the patch to be printed can be easily specified, and the present invention can be easily applied. In addition, since the original color at the time of performing the color separation process is represented by a combination of each of the CMY colors, such a configuration is preferable because any color can be represented. However, it is needless to say that each color ink used in the printing apparatus is used. A color system other than the CMY space (for example, an RGB color system) is adopted as long as the configuration converts coordinates in a predetermined color space composed of color components smaller than the number of ink colors. Then, the first gradation value data may be created.

  At the time of color measurement, it suffices to be able to acquire the color value of the print result, and a Lab color space (L, a, b is usually marked with *, but is omitted in this specification for simplicity. The same applies hereinafter. ), Etc., as long as colorimetric data indicating coordinates in a device-independent color space can be obtained. When creating a color conversion table, use the colorimetric data indicating the coordinates in the device-independent color space and the coordinates in the device-independent color space of the color indicated by the color data used by another image device. do it.

  That is, if the coordinates in the device-independent color space are known for a plurality of colors, it is possible to calculate the ink value data of an arbitrary color and the color component value of each color used in another image device by interpolation or the like. Therefore, the color conversion table can be defined by calculating the correspondence between the two colors for an arbitrary color. Since it is necessary to obtain the coordinates in the device-independent color space for the color component values of each color used in other image devices, the sRGB standard that can calculate the coordinates in the device-independent color space by a predetermined formula Is preferable. Of course, the color displayed on another image device may be measured.

  As described above, the first gradation is set so that the colors in the low-brightness region are larger than the colors in the high-brightness region in advance so that the resolution that is relatively reduced in the low-brightness region by the correction for improving the resolution is compensated with the interpolation accuracy. Various corrections can be adopted to extract the value data or to perform the above-described correction for improving the resolution. As an example of the configuration, γ correction can be used. That is, the γ correction is a correction that inputs a numerical value in a predetermined value range and outputs a result obtained by converting the input value by a predetermined function, and uses a function that provides a γ curve. According to this γ curve, only by adjusting the value of γ, a smaller input value can be easily corrected and output at a larger increase rate than a larger input value, and the correction degree can be easily adjusted. It can be conveniently adjusted. When actually performing the γ correction, an input value may be substituted for a function that gives a γ curve, or data obtained by tabulating the results of the γ correction may be referred to. In the present invention, the correction for obtaining the first gradation value data is referred to as (γ) correction for improving the interpolation accuracy. However, even if the correction is for improving the resolution, the interpolation accuracy is improved.

  Of course, in the configuration for performing the correction such as the γ correction, it is also possible to adopt a configuration in which the tone value is assigned only to the ink amount whose brightness can change substantially as described above. That is, when the gradation value indicating the lowest lightness in the first gradation value data corresponds to the maximum ink recording rate printable on a print medium, the predetermined gradation including the gradation value indicating the lowest lightness When the value range is excluded, a part near low brightness is excluded from the gradation value range.

  When the correction is performed so that the remaining tone value range matches the entire tone value range of the ink value data, the remaining tone value range, that is, the entire tone value of the ink value data is changed to a substantially changing brightness. Can be assigned. Therefore, it is possible to more effectively express the gradation of the ink amount with a limited capacity, and to express a more subtle gradation change.

  As described above, the present invention generates ink value data indicating the amount of ink when printing a patch to be subjected to color measurement, performs printing by interpreting the amount of ink indicated by the ink value data in halftone processing, The color conversion table is created by measuring the colors of the obtained patches. The color conversion table is also referred to to perform color conversion and function as a print control device for printing. That is, by referring to this color conversion table, printing can be executed while color conversion is performed with high accuracy for high-brightness and low-brightness colors, and tone jump in the entire brightness range can be prevented.

  Furthermore, it can be said that the above-described apparatus for creating a color conversion table also utilizes the technical idea of the present invention, and a color conversion table creating apparatus according to claim 3 can be configured as in claim 8. Of course, it is also possible to adopt a configuration corresponding to claims 4 to 7. In addition, such a color conversion table creating apparatus may be implemented independently, or may be implemented together with another apparatus or method while being incorporated in a certain device. The present invention is not limited to this, and includes various aspects, and can be appropriately changed, such as software or hardware.

  If the software for implementing the method and apparatus for creating a color conversion table is realized as an embodiment of the idea of the present invention, such software naturally functions and is used as an invention. Therefore, the present invention can also be realized as a color conversion table creation program according to the ninth aspect. Of course, it is also possible to adopt a configuration corresponding to claims 4 to 7. The recording medium may be a magnetic recording medium, a magneto-optical recording medium, or any recording medium to be developed in the future. Further, the duplication stages of the primary duplicated product, the secondary duplicated product, and the like are completely indispensable.

  In addition, the present invention is not limited to the case where the present invention is used even when the supply is performed using a communication line. Further, even when a part is implemented by software and a part is implemented by hardware, the concept of the present invention is not completely different, and a part is stored on a recording medium and appropriately It may be in a form that can be read. Also, not all functions are necessarily realized by the program itself, but may be realized by an external program or the like. This is because, even in such a case, any function can be realized by a computer.

Here, embodiments of the present invention will be described in the following order.
(1) Outline of color conversion table creation:
(2) Apparatus and processing for creating a color conversion table:
(3) Printing using the LUT created according to the present invention:
(4) Other embodiments:

(1) Outline of color conversion table creation:
FIG. 1 is an explanatory diagram schematically illustrating the steps of a color conversion table creation method according to the present invention. Since this step requires a lot of arithmetic processing, it is preferable to use a computer. In addition, since printing is actually performed, it is preferable to perform printing using a printer that performs printing using the created color conversion table. Halftone processing (HT) described below is also used as halftone processing (HT). It needs to be the same algorithm as the tone processing.

The color conversion table in the present embodiment, 17 for ^three reference points is a table which defines the correspondence relationship between the RGB data and the CMYKlclm data, any color by performing an interpolation process with reference to these reference points Can be associated with RGB data and CMYKlclm data. In the present embodiment, the RGB data is data conforming to the sRGB standard used in a display for a computer, and is represented by 256 gradations of each color (data in which the gradation of each color is specified by an integer value of 0 to 255). The colors are expressed by combinations of the RGB colors. The CMYKlclm data is data for specifying the amount of ejected ink in the printer according to the present embodiment, and is 256 gradations for each color (data in which gradations of each color are specified by integer values from 0 to 255). The colors are expressed by combinations.

  In order to perform printing by a printer, it is necessary to associate the above RGB data and CMYKlclm data in the color conversion table. However, since the CMYKlclm data is a device-dependent color of the printer, it is generally necessary to create a color conversion table. Measure the actual print result on the printer. Then, a color conversion table is created by associating the RGB data with the CMYKlclm data in a device-independent color space.

In the present embodiment, a Lab color space (usually, this space is described as L ^* a ^* b ^* , but is abbreviated to * for simplicity in this specification; the same applies hereinafter) as the device-independent color space. In the process of creating the color conversion table, first, coordinate values in the Lab color space are specified for each of the RGB data and the CMYKlclm data. The RGB data complies with the sRGB standard as described above, and the sRGB data can be converted into coordinate values in the Lab color space by a predetermined conversion formula. In FIG. 1, the coordinates after the conversion are represented as L ₀ a ₀ b ₀ , and at this stage, a plurality of RGB data are converted into coordinate values in the Lab color space.

In the color conversion table, the reference points are preferably distributed over substantially the entire color gamut of the display and the printer so that color conversion can be performed for any color represented by RGB data and CMYKlclm data. . However, since the color gamut of the display and the printer are generally different, color gamut mapping for converting the color on the display into a color that can be expressed by the printer is performed. Also, when outputting images, it is often better to convert to colors that are closer to human memory colors than to output actual colors, such as skin color or blue sky, as they are. For, the actual color is converted to the memory color. In FIG. 1, the coordinates in the Lab color space obtained by converting the coordinates L ₀ a ₀ b ₀ in this manner are represented as L ₁ a ₁ b _1, and the plurality of RGB data is represented by the L ₁ a ₁ b _1. It is associated with a ₁ b ₁ .

  On the other hand, CMYKlclm data is ink value data for specifying an ink amount and is a device-dependent color. Therefore, the color values in the Lab color space are obtained by measuring the colors of the actually printed patches with a colorimeter. However, the CMYKlclm data is data expressing an arbitrary color by appropriately combining the respective ink amounts of the six colors of ink, and a very similar color can be expressed by a large number of combinations.

Although colorimetry 10 ³ patches in this embodiment, since similar colors be different coordinates in the ink amount space is very much present, the ink amount of the combination to be any rule without colorimetry target Is determined, it is distributed over substantially the entire color gamut of the printer, and it is difficult to select a preferable combination of six colors when performing color conversion for an arbitrary color. Therefore, in general, a color separation process for converting the virtual CMY values into six color ink amounts is performed, and the present embodiment also uses the concept of the color separation process.

In the color separation process, a three-dimensional space (virtual CMY space) having each color as an orthogonal axis while expressing each of the three colors of CMY in 256 gradations is considered, and coordinates indicating colors to be measured in this virtual CMY space. The value is determined, and the coordinate value is converted into a six-dimensional ink value by a predetermined conversion formula. That is, the coordinate values in a three-dimensional space so easy to create a conversion formula for converting the coordinate values in the six-dimensional space, first identify 10 ³ the object of color measurement in the three-dimensional virtual CMY space The CMYKlclm data is determined by performing three-dimensional to six-dimensional conversion using this conversion formula.

  In this separation processing, conversion is performed so that the ink recording rate can be specified from the CMYKlclm data after the separation processing. In the simplest case, the ink recording rate is converted so that the ink recording rate can be specified from each ink value by equally assigning 0 to 100% of the ink recording rate to each of the gradations 0 to 255. Separation can be performed by a conversion formula that takes into account various restrictions such as a restriction on a recording amount and a restriction on the use of black ink. In any case, the ink recording rate is specified from the CMYKlclm data after the separation processing.

When specifying the CMYKlclm data as described above, will be 10 ^three sets of coordinate values to be object of color measurement in the ink amount space of each color 256 gradations can be obtained, printing a patch of color indicated by the coordinate values. In an ink-jet printer, the number of gradations of 2 to 4 for each dot, that is, 2 gradations in a state where ink droplets are recorded and in a state where recording is not performed, and 4th floor in a state where ink droplets are not recorded and a state where large, medium and small dots are recorded Since tone expression is performed by tone or the like, halftone processing is performed on each of the 256 color ink amounts to convert the data into data representing the tone of each dot in the printer. Is performed when 10 ³ patches printed on the basis of this data is obtained, it is possible to specify the coordinate values of the Lab color space for ¹⁰³ patches by colorimetry by these colorimeter. In FIG. 1, this coordinate value is shown as L ₂ a ₂ b ₂ .

It is possible to identify the coordinate value L ₁ a ₁ b ₁ corresponding to the RGB data of the coordinate values L ₂ a ₂ b ₂ with 256 gradations corresponding to CMYKlclm data of the 256 gradations by the above steps, these From the RGB data and the CMYKlclm data. Although the coordinate values L ₂ a ₂ b ₂ and the coordinate values L ₁ a ₁ b ₁ is not necessarily show the same color, because the 10 ^three coordinate values are present in the color space, the coordinate values L ₂ a ₂ b ₂ from a can be calculated any CMYKlclm data by interpolation, it is possible to calculate any RGB data by interpolation from the coordinate values L ₁ a ₁ b _1. Therefore, the correspondence between the RGB data and the CMYKlclm data can be defined by the interpolation calculation, and as a result, the above-described color conversion table can be determined.

  Although the color conversion table can be determined by the above steps, the color conversion table cannot perform color conversion for a specific color without causing a tone jump. That is, in general, when the ink amount increases at a constant rate, the brightness does not change at a constant rate. In the above separation, it is possible to specify a color conversion table accurately corresponding to the brightness change due to the change in the ink amount. could not.

  FIG. 2 is a diagram showing the relationship between the recording rate (%) of ink droplets recorded per unit area for each color ink and its lightness L, and shows a specific example for each of the KClc inks. As shown in the figure, the change in lightness with respect to the change in the ink recording rate is not constant, and the curve is convex downward for all colors. That is, in a high brightness region where the number of ink droplets is small, the brightness changes greatly with an increase in the number of ink droplets, but in a low brightness region, the change in brightness becomes slower with an increase in the number of ink droplets.

  Further, as the color of the ink itself becomes darker, the tendency of the lightness change rate accompanying the ink recording rate in the low lightness area increases. Further, in the color conversion table, the correspondence between RGB data and CMYKlclm data is not defined for all gradation values, but the correspondence between them is defined for a plurality of reference points as described above. Is different for each value of CMYKlclm data.

  That is, the interpolation accuracy of the CMYKlclm data is higher as the color when printing is performed using the CMYKlclm data obtained by interpolation and the color corresponding to the RGB data of the conversion source are higher. Tone jumps can occur. For example, if the reference points are provided substantially equally in the value range of each color ink amount, the interpolation accuracy in the high brightness region where each color ink amount is small is lower than the interpolation accuracy in the low brightness region where each color ink amount is large. In this case, the brightness change with respect to the recording rate change between the reference points corresponding to the high brightness is not linear, and the absolute value of the brightness also fluctuates greatly, so that a slight difference in the amount of ink appears as the actual brightness change. is there.

  When the virtual CMY values are converted into ink values by a uniform conversion formula as in the above separation, it is difficult to cope with such a characteristic of each ink amount value. After or during the separation processing, the CMYKlclm data is γ-corrected to improve the resolution, so that colors are accurately described at reference points corresponding to the CMYKlclm data indicating high brightness. That is, as shown by the dashed line in FIG. 2, an upwardly convex input / output characteristic curve which is inverse to the characteristic of the ink (when converting the input value of the ink amount gradation value into a predetermined output value, the ink recording rate The γ-correction is performed using a curve (a curve shown by a broken line in FIG. 2 when the brightness range and the output range are matched). When halftone processing is performed on the CMYKlclm data after the resolution enhancement γ correction, γ interpretation (inverse γ correction), which will be described later, is performed, and the value corresponding to the decimal point in the numerical system before the resolution enhancement γ correction is also obtained. This allows for printing at high resolution.

On the other hand, as described above, it is the color of the high brightness region that enables printing at high resolution by the γ correction for improving the resolution, and the resolution of the color of the low brightness region relatively decreases, In order to compensate for this decrease, the CMYKlclm data giving low brightness has more reference points than the CMYKlclm data giving high brightness. That is, at a reference point corresponding to CMYKlclm data that gives low brightness, an interpolation operation can be performed using a reference point having a close value, and the interpolation accuracy is improved. FIG. 3 is an explanatory diagram for explaining how CMYKlclm data giving low brightness has more reference points than CMYKlclm data giving high brightness. FIG. ³ is a schematic diagram showing 103 colorimetric objects in the virtual CMY space. The cube shown in the figure shows a virtual sky CMY space with each color of CMY as an axis, and the existence of reference points is indicated by grid points in the cube.

  When the gradation values of each color of CMY are divided into nine equal parts and ten gradation values of each color are considered, and the gradation values of each color are arbitrarily combined, the cubic lattice point shown in FIG. In the present invention, the interpolation value improving γ correction is performed on the gradation value of each color, so that the higher the gradation value (corresponding to the lower brightness), the larger the change in the value before and after the correction. As a result, the higher the gradation value, the larger the value. For example, a reference point where the combination of each of the CMY colors is (255, 255, 198) before performing the interpolation accuracy improving γ correction is (255, 255, 218) by the interpolation accuracy improving γ correction.

  In the present embodiment, as shown in the lower right of the graph in which the input value is the horizontal axis and the output value is the vertical axis, the γ correction for improving the interpolation accuracy is performed by an upwardly convex curve. Since the gradation value 198 is converted into the gradation value 218, the combination of each of the CMY colors is converted as described above. By performing the interpolation accuracy improving γ correction for each color in this manner, it is possible to increase the number of low brightness colors. In the present invention, since a colorimetric patch is printed for the color specified by the CMYKlclm data obtained by performing the γ correction for improving the interpolation accuracy, low brightness is used when creating a color conversion table using this colorimetric result. The colors in the gamut can be accurately defined.

  Therefore, when color conversion is performed using the resulting color conversion table, it is possible to accurately convert colors in the low brightness range, and it is possible to improve interpolation accuracy at low brightness. The gamma correction for improving the interpolation accuracy may be performed before the separation processing or during the separation processing. As described above, in the present embodiment, the resolution is improved in the high brightness region and the interpolation accuracy is improved in the low brightness region by using the γ correction for improving the resolution and the γ correction for improving the interpolation accuracy. Of course, it is possible to improve the interpolation accuracy by using only the interpolation value improving γ correction and using the ink value data with a large number of low brightness colors.

(2) Apparatus and processing for creating a color conversion table:
In the present invention, the occurrence of tone jump is reduced by the γ-correction for improving the interpolation accuracy and the γ-correction for improving the resolution. Hereinafter, an apparatus and process for that purpose will be described more specifically. FIG. 4 is a flowchart showing a process for creating a color conversion table (LUT) according to the present invention, and FIG. 5 is a block diagram showing a configuration of a computer for executing the process. FIG. 6 is a diagram showing an example of a value change when the virtual CMY value is subjected to interpolation accuracy improving γ correction, divided into CMYKlclm data, and further subjected to resolution improving γ correction.

  The computer 10 includes an arithmetic processing unit 11 that executes arithmetic processing and an HDD 12 that stores data. The computer 10 is connected to the printer 20 via an interface (not shown), and can output print data from the computer 10 and execute printing. Further, the computer 10 can take in colorimetric data obtained by performing colorimetry with the colorimeter 30. Various modes can be adopted, such as inputting this colorimetric data with a predetermined input device, inputting via a recording medium, or inputting by connecting and transferring data via a predetermined interface. It is.

In the arithmetic processing unit 11, the arithmetic processing by executing a predetermined program for creating a color conversion table can be run, separation processing section 11b virtual CMY for 10 ^three colorimetric step S100 Get the value. In the present embodiment, the virtual CMY values for colorimetry correspond to values of grid points evenly arranged in the virtual CMY space. That is, as shown at the left end of FIG. 6, the virtual CMY values for color measurement are values obtained by arbitrarily combining the tone values obtained by dividing each color tone value range into approximately nine equal parts. The gradation value range of this virtual CMY is 0 to 255, and the gradation value is defined by an integer. Further, when three CMY inks are assumed, it is considered that the ink amount and the gradation value correspond linearly.

  The colorimetric virtual CMY values are input to the interpolation accuracy improving γ correction unit 11a, and in step S105, γ correction for interpolation accuracy improvement is performed. Since the interpolation accuracy improving γ correction is performed for each color using the upwardly convex input / output specific curve as described above, when the CMY values are small as shown in the table in the second column from the left in FIG. The rate of increase before and after correction is larger than when each value is large. (For example, the value 0 to 25 of C becomes 0 to 60, but the value 170 to 193 of Y becomes 198 to 215). In addition, it is considered that both of the corrected gradation values and the ink amount linearly correspond to each other. Therefore, compared with the combination of the gradation values obtained by equally dividing the gradation value range, the number of low-brightness colors is increased after the γ correction for improving the interpolation accuracy.

  The virtual CMY values subjected to the interpolation accuracy improvement γ correction in step S105 are input to the color separation processing unit 11b, and the color separation processing is performed. The color separation processing is performed by substituting the virtual CMY values after the above-described interpolation accuracy improving γ correction into a predetermined color separation function. This separation function is defined by the separation function data 12a stored in the HDD 12. Of course, the color separation processing may be performed by interpolation processing using an LUT instead of substitution into a function.

  In the separation, it is assumed that the colors before and after the separation match, although not strictly, and the value of the CMYKlclm data after the separation is an integer value of 0 to 255. It is also assumed that the magnitude of the CMYKlclm data after the separation and the ink amount linearly correspond to each other. As a result, the CMYKlclm data after the separation also shows the same color as that after the above-described interpolation accuracy improving γ correction, and is compared with the virtual CMY value (reference value before correction) shown at the left end of FIG. And low-brightness colors are increasing. Therefore, when the result of printing with the CMYKlclm data is subjected to colorimetry and an interpolation operation is performed, it is possible to perform an interpolation process using a reference point having a value close to a low lightness color as compared with a high lightness color, A color conversion table capable of low-brightness and high-precision color conversion can be created. If γ correction for improving the interpolation accuracy is performed after the separation processing, each gradation value of the CMYKlclm data after the separation processing corresponds to the pre-correction gradation value in the above claim, and the gradation value is extracted. The combination of the reference values corresponds to the pre-correction reference value.

  When the separation processing unit 11b generates CMYKlclm data by separation, the resolution improving γ correction unit 11c uses the characteristic curve such as a broken line shown in FIG. 2 for the CMYKlclm data for each color in step S120 for the CMYKlclm data. Perform gamma correction. That is, the γ correction is performed by the input / output specifying curve that is convex upward. Accordingly, as shown in the right end of FIG. 6, the correction is made at a larger increase rate as the value is smaller than other gradation values.

  In the gradation value after the resolution improvement γ correction, the magnitude of the gradation value and the ink amount do not linearly correspond to each other, and the gradation value corresponding to the high lightness region corresponds to the lower lightness region. It is defined such that the change in the ink recording rate corresponding to the unit change of the gradation value is smaller than the corresponding gradation value. That is, in the present embodiment, the CMYKlclm data after the separation corresponds linearly to the ink recording rate, but the meaning of the value differs in the data after the γ correction for improving the resolution, and the data corresponds nonlinearly to the ink recording rate. I have. As a result, as the gradation value becomes smaller, it becomes possible to execute the halftone processing by reflecting the value to the decimal point, as will be described later, and the accuracy of color conversion is improved. The gamma-corrected ink value data 12b obtained by performing the gamma correction by the resolution improving gamma correction unit 11c is stored in the HDD 12.

  Note that the minimum value and the maximum value of the input value are not changed before and after the interpolation accuracy improving γ correction and the resolution improving γ correction, and the output value is an integer. It is an integer, and the value range is 0 to 255. Here, since the value after the correction is an integer, as a result of the conversion into integers, as the value of the CMYKlclm data after the separation for all the values is smaller, the change in the value before and after the γ correction is not necessarily larger, At least in the stage of the γ correction, the correction is performed such that the smaller the value, the larger the change in the value before and after the γ correction. In addition, since the data shown in the second column and the right end from the right in FIG. 6 are also data for each ink color of CMYKlclm, in this specification, the former is CMYKlclm data after separation, and the latter is ink value data. To be distinguished.

  Further, in the present embodiment, the CMYKlclm data after the separation corresponds to the first gradation value data in the claims, since the magnitude of the gradation value and the ink amount correspond substantially linearly. In this case, the color space formed by the virtual CMY corresponds to a predetermined color space composed of color components smaller than the number of ink colors. The ink value data is a value input to the halftone processing unit 11c, and specifies the patch to be subjected to the color measurement, and thus is the patch data in the above claim. Since it is not essential to perform the separation, only the data after the separation does not correspond to the first gradation value data. If the gradation value and the ink amount correspond substantially linearly. It can be the first gradation value data. For example, when the color separation process is not performed, the virtual CMY after the above-described interpolation accuracy improving γ correction corresponds to the first gradation value data.

  In step S130, the halftone processing unit 11d performs a halftone process of specifying the ink droplets to be ejected by the printer 20 based on the CMYKlclm color ink amounts after the resolution improvement γ correction. The halftone processing unit 11d includes a γ interpretation unit 11d1 and a gradation number reduction unit 11d2, and the γ interpretation unit 11d1 calculates a value before the γ correction for improving the resolution is performed on the ink value data 12b. Can be. The gradation number reduction unit 11d2 performs halftone processing based on the value before the resolution improvement γ correction, and generates intermediate data specifying the ejection ink droplet for each pixel in the printer 20. However, in this halftone process, the value after the above-mentioned γ correction for resolution improvement is grasped to a value corresponding to a decimal number before the γ correction, and the halftone process is performed while reflecting a numerical value difference smaller than the decimal number.

  In other words, in the halftone processing, the ejection ink of each pixel in the printer 20 is specified in order to express the gradation by adjusting the dot count recorded per unit area, but the CMYKlclm is expressed in 256 gradations. In this case, the change of the dot count corresponding to the one gradation change is not 1, but a large number of dot changes. Therefore, in the halftone processing, a subtle change originally corresponding to the decimal number or less can be expressed, and if data having a number corresponding to the decimal number or less is included, a color can be output with higher accuracy. .

  FIG. 7 is an explanatory diagram for explaining the halftone process. The left end of the figure shows the ink value data of the C ink, and the left center column of the figure shows the data after interpretation by the gamma interpretation unit 11d1. In the present embodiment, numerical values including decimals and smaller numbers shown in the same column can be calculated by performing inverse γ correction having input / output characteristics opposite to the above resolution improving γ correction on the ink value data of the C ink. it can. In the inverse γ correction, the minimum value and the maximum value of the input value are not changed when the input value is converted into a predetermined output value.

  Therefore, the value range of the value after the inverse γ correction is also 0 to 255. In the present embodiment, the magnitude of the dot count recorded per unit area is represented by the magnitude of the value after the inverse γ correction, the value 0 represents the ink recording rate 0%, the value 255 represents the ink recording rate 100%, and the value 1 Indicates the ink recording rate of 100 × 1/255%. As shown in the left center column of FIG. 7, there are seven values of 0.036 to 0.0810 below the value of 1.002. Therefore, halftone processing can be performed while reflecting a difference in ink recording rate of 100 × 1/255% or less.

  Further, as shown in FIG. 7, the smaller the value of the ink value data, the smaller the change in the ink recording rate with respect to the unit change of the ink value data (that is, the change of “1” in the ink value data). For example, when the ink value data changes from "1" to "2", the corresponding ink recording rate changes from "0.036" to "0.109", and the change is "0.073". On the other hand, when the ink value data changes from "34" to "35", the corresponding ink recording rate changes from "10.126" to "0.480", and the change is "0.480". In the former, the change in the ink recording rate is smaller.

  Therefore, in the ink value data according to the present embodiment, it can be said that a smaller change in the ink recording rate can be expressed in the high brightness area where the ink recording rate is small, and the resolution of the ink value data is high in the high brightness area. It can also be said. In the case where the CMYKlclm data after separation is subjected to resolution improvement γ correction and the γ interpretation is not performed at the time of halftone processing as in the present invention, numerical values after the decimal point as shown in the left center column of FIG. Are all rounded by rounding. For this reason, as shown in the right end of FIG. 7, an ink recording rate of 100 × 1/255% or less can be expressed only as 0, and a minute change in the ink recording rate cannot be expressed in a high brightness region. Therefore, it is not possible to define a correspondence for performing color conversion with high accuracy in a high brightness region.

  In addition, the data corresponding to the decimal places after the inverse γ correction may be handled as a decimal point as shown in the left center column of FIG. 7, or may be converted to an integer value and used as a matter of course. For example, as shown in the center right column of FIG. 7 as 16 bits, the data may be converted into a gradation value described with a capacity of 16 bits. That is, the ink value data may be converted into 16-bit data at the time of interpreting the γ, and the ink recording rate may be expressed by a value of 65535 gradations. In this case, the inverse γ correction is performed in the same manner as described above. There is no difference in performing the halftone processing while reflecting a deviation corresponding to a decimal number or less.

  When the ink value data is converted into 16-bit data, it can be realized by, for example, creating a table that associates the ink value data with the converted 16-bit data, and referencing the table. In any case, the tone number reduction unit 11d2 of the halftone processing unit 11d adopts a value including a numerical value smaller than the decimal or a value defined by 16 bits as a threshold value for determining whether or not to eject a dot. Accordingly, halftone processing can be performed while reflecting a deviation corresponding to a decimal number or less.

  By performing the color conversion with reference to the color conversion table created by the present invention having the above-described mechanism, it is possible to prevent the occurrence of a tone jump in the entire brightness range. That is, by performing γ correction for interpolation accuracy improvement on the virtual CMY value, CMYKlclm data corresponding to low brightness can be calculated by interpolation from ink value data close to that value, thereby improving the interpolation accuracy. Here, it suffices if the interpolation accuracy can be improved so as to compensate for the low-brightness resolution that is relatively reduced by the resolution improvement γ correction. Further, by performing gamma correction for resolution improvement on the CMYKlclm data and performing color conversion using the value after the gamma correction for resolution improvement, halftone processing can be performed in consideration of values corresponding to decimal places. This improves the resolution when expressing high-brightness colors. The above both improve the accuracy of color conversion.

The print data generation / output unit 11e performs processing such as arranging the data after the halftone processing in the order of ink droplet ejection from each nozzle of the printer 20, and generates print data for printing a patch corresponding to CMYKlclm data. Then, the data is output to the printer 20. As a result, the printer 20 prints a 10 ^three color measurement patches (step S140). After printing the colorimetric patches, the colorimetric patches are measured by the colorimeter 30 (step S150). The colorimeter 30 is a device that acquires Lab coordinate values of a colorimetric object as colorimetric data, and the acquired colorimetric data is taken into the LUT creation unit 11g of the computer 10.

With the above processing, coordinate values (corresponding to L ₂ a ₂ b ₂ in FIG. 1 above) in the CMYKlclm data corresponding to the colorimetric virtual CMY values in the Lab color space, which is a device-independent color space, are obtained. Become. On the other hand, after step S160, a process for acquiring coordinate values in the Lab color space corresponding to the RGB data is performed. Steps S160 and S170 may be executed before step S100.

In step S160, (corresponding to L ₀ a ₀ b ₀ of FIG. 1) of sRGB data converting unit 11f acquires a prepared sRGB values are converted into coordinate values in the Lab color space by a predetermined conversion formula. The number of conversion targets by the sRGB data conversion unit 11f is about 10 ³ , and the conversion target may be specified in advance by arbitrarily combining coordinates obtained by dividing the value range of each color of RGB into nine equal parts. In step S170, the sRGB data conversion unit 11f performs correction in consideration of the color gamut mapping, the storage color, and the like. Consequently, the coordinate values corresponding to L ₁ a ₁ b ₁ of FIG. 1 can be obtained.

These coordinate values are taken into the LUT creation unit 11g. LUT generator 11g at step S170 and the step S150 is acquired and L ₁ a ₁ b ₁ and L ₂ a ₂ b ₂ shown in FIG. 1, a plurality of reference points by interpolation in step S180 Defines the correspondence between RGB data and CMYKlclm data. Here, the correspondence between the RGB data and the CMYKlclm data may be defined. Since RGB data and CMYKlclm data corresponding to arbitrary coordinates in the Lab color space can be calculated by interpolation, it is possible to define the correspondence between the two data for an arbitrary color via the Lab coordinate values. In step S190, the association and the RGB data and the CMYKlclm data for 17 ^three reference points, and generates table data indicating the correspondence relationship is stored in the HDD12 (LUT12c).

(3) Printing using the LUT created according to the present invention:
The LUT 12c is referred to for performing a color conversion process when printing is performed by the printer 20, and a configuration for performing the printing will be described below. FIG. 8 is a block diagram illustrating a configuration example of a computer that uses the LUT 12c at the time of printing. The computer 110 is a general-purpose personal computer, and a printer driver (PRTDRV) 210, an input device driver (DRV) 220, and a display driver (DRV) 230 are incorporated in the OS 200. The display DRV 230 is a driver for controlling the display of image data and the like on the display 180. The input device DRV 220 receives a code signal from the keyboard 310 and the mouse 320 input through the serial communication I / O 190a and receives a predetermined signal. A driver that accepts input operations.

  The APL 250 is an application program that can execute retouching of a color image and the like, and a user can operate the operation input device to print the color image on the printer 20 under the execution of the APL 250. When printing such a color image, the LUT 12c created by the present invention is referred to. The color image data 120a of the color image created by the APL 250 is dot matrix data in which each color component of RGB is expressed in gradation, is data conforming to the sRGB standard, and is stored in the HDD 120.

  The PRTDRV 210 includes an image data acquisition module 210a, a color conversion module 210b, a halftone processing module 210c, and a print data generation module 210d to execute printing. Further, the LUT 12c created by the present invention is stored in the HDD 120. When the user issues a print execution instruction when executing the APL 250, the image data 120a to be printed is acquired by the image data acquisition module 210a, and the image data acquisition module 210a activates the color conversion module 210b. The color conversion module 210b is a module that converts RGB data into CMYKlclm data, and converts arbitrary RGB data into CMYKlclm data using a reference point of the LUT 12c.

  Since the CMYKlclm data is obtained by subjecting the virtual CMY values to γ correction and color separation, the LUT 12c is created in a state where colors in the low brightness range can be color-converted with very high accuracy. Therefore, the correspondence between the RGB data defined by the LUT 12c and the CMYKlclm data is also very accurate, and color conversion can be performed with high accuracy. FIG. 9 is an explanatory diagram illustrating a mechanism that enables high-accuracy color conversion in a low brightness range. In the figure, the manner in which CMYKlclm data corresponding to an arbitrary Lab value is calculated by interpolation is schematically shown. The interpolation processing in the conventional color conversion table creation process is on the left, and the interpolation processing in the present invention is on the right. Is shown in Position A indicates an interpolation target position in the Lab color space.

There are various methods for the interpolation processing. In the figure, four reference points close to position A are extracted, and the coordinate components at each reference point are weighted according to the distance from each reference point to position A. A method is shown in which the coordinates of the position A are added to be added. In the present invention, as described above, since the colors in the low-brightness region are dense, four reference points closer to the position A as compared with the conventional situation in which the γ correction for improving the interpolation accuracy is not performed. Can be extracted to perform interpolation processing. That is, the distance l ₁ <the distance l ₀ .

The coordinate value of the position A obtained by the interpolation processing usually includes an error. When the error in the conventional method is α and the error in the present invention is β, as described above, the distance l ₁ <l _{Since 0} , the error β <α. Therefore, according to the present invention, it is possible to accurately associate the coordinates in the Lab color space with the CMYKlclm data in the low lightness region. As a result, when creating the LUT 12c, the RGB data and the CMYKlclm data can be accurately associated. Conversely, the fact that the two are accurately associated means that, when color conversion is performed using the LUT 12c, arbitrary RGB data can be accurately associated with CMYKlclm data, and color conversion is performed with high precision. It becomes possible.

  When the color conversion module 210b performs color conversion to generate CMYKlclm data, the CMYKlclm gradation data is transferred to the halftone processing module 210c. The halftone processing module 210c is a module that performs the same processing as the halftone processing unit 11d, and includes a γ interpretation unit 210c1 and a gradation number reduction unit 210c2. Therefore, when the CMYKlclm data is transferred to the halftone processing module 210, the γ interpreting unit 210c1 calculates a value before the γ correction for improving the resolution is performed on the ink value data, and the tone number reducing unit 210c2 The halftone process is performed while also taking into account the value corresponding to the decimal part.

  As a result, ink ejection / non-ejection at each nozzle can be determined while reflecting the brightness change with respect to the ink recording rate change for each color of CMYKlclm. The print data generation module 210d receives the data after the halftone processing, and performs rasterization for rearranging the data in the order used by the printer 20. After this rasterization, print data is generated by adding predetermined information such as the resolution of the image, and output to the printer 20 via the parallel communication I / O 190b. The printer 20 prints the image displayed on the display 180 based on the print data.

In this printing process, since the color conversion is performed with reference to the LUT created by the present invention, it is possible to perform the color conversion with high accuracy over the entire color gamut of the display 180 and the printer 20, and it is possible to perform the tone jump. It is possible to perform high-quality printing without any problem. Hereinafter, a mechanism for reducing the tone jump will be described with reference to FIG. The left side of FIG. 10 shows an example of conventional halftone processing. In this example, ejection / non-ejection of ink droplets of each pixel is determined so that the gradation value C ₀ of the ink value data becomes a dot count h per unit area by halftone processing (HT), and the gradation value ( The ejection / non-ejection of ink droplets of each pixel is determined so that C ₀ +1) becomes a dot count h + 100 per unit area by halftone processing.

  In the conventional processing in which the γ correction for improving the resolution is not performed, even if there is a difference of “100” in the dot count after the halftone processing, the gradation value of the ink value data only has a difference of “1”. The ink value data corresponding to an arbitrary color can be obtained by interpolation calculation with reference to the LUT. However, the number of gradations of each color handled by the computer is 256, and if the data capacity is not changed, the gradation value of the ink value data is "1". The difference within "cannot be expressed. Therefore, in the conventional halftone processing, the minimum pitch of the dot count becomes “100”.

On the other hand, the right side of FIG. 10 shows an example of the halftone process of the present invention. In this example, data obtained by performing γ correction for resolution improvement on the gradation value C ₀ is indicated as γC _0, and data obtained by performing γ correction on the gradation value (C ₀ +1) is represented by γ (C ₀ +1). The γ correction is a method of correcting an input value and an output value with a γ curve. In a graph in which the input value is a horizontal axis and the output axis is a vertical axis, the correction is not performed on a straight line that passes through the origin and has a slope of ２. Is shown. In step S120, correction is performed using a γ curve that is upwardly convex in this graph and the input and output values do not change between the origin and the maximum gradation value. Therefore, the correction is performed at a larger increase rate as the gradation value is smaller.

In the example shown in FIG. 10, γ (C ₀ +1) is equal to γC ₀ +10. That is, nine gradation levels (γC ₀ +1 to γC ₀ +9) exist between the gradation value γC ₀ and the gradation value γ (C ₀ +1). Therefore, when the interpolation process is performed with reference to the LUT 12c in which the reference point is defined by the data subjected to the resolution improvement γ correction, not only the gradation values γC ₀ and γC ₀ +10 but also the values between them can be expressed. it can.

The halftone processing module 210c performs the halftone processing while grasping the value before the γ correction in step S120 by the γ interpretation, so that γC ₀ becomes the dot count h and γ (C ₀ +1) becomes the dot count h + 100. while treatment can be processed so that the dot count h + 10~h + 90 also between the tone values γC ₀ + 1~γC ₀ +9. That is, the minimum pitch of the dot count becomes extremely small. Further, in the present invention, the correction is performed using the convex γ curve as described above so that the smaller the gradation value, the larger the original value. Therefore, strictly speaking, as shown in FIG. 10, the change pitch of the dot count after the halftone is not constant, but the halftone is expressed while expressing a subtle color change with respect to a color having a large variation in brightness with respect to the ink recording rate. There is no difference that the processing can be performed. Therefore, tone jump can be reduced.

(4) Other embodiments:
The embodiment described above is an example, and the resolution of the high brightness area is relatively higher than the resolution of the low brightness area, and as a result, the resolution of the low brightness area is compensated for by the interpolation accuracy so as to compensate for the relatively reduced low brightness resolution. It is sufficient that the number of colors can be increased, and various configurations can be adopted. According to the applicant's experiments, the case where the above-described virtual CMY space is combined with gradation values evenly distributed in the virtual CMY space (separation is performed if necessary) to perform the γ correction for improving the resolution, It has been found that the color conversion can be performed with higher accuracy in the entire brightness range when the gamma correction for improving the interpolation accuracy is performed on evenly distributed gradation values.

  In addition, if attention is focused only on improving the interpolation accuracy in the low lightness region, the color in the low lightness region can be changed as necessary without combining with the configuration in which the value corresponding to the decimal point of the gradation value is considered as described above. It is also possible to adopt only the configuration for increasing the number. Of course, the lightness range in which colors are increased is not limited to the low lightness range. Further, since it is sufficient that the color of the patch to be measured is unevenly distributed in a specific lightness region, various methods can be adopted as a method for giving a bias, not limited to γ correction.

  Furthermore, an algorithm that performs correction with the same γ curve for all colors of the virtual CMY values can perform γ correction with very simple calculation. Of course, a different γ curve may be used for each color. Further, the γ curve may be changed depending on the print medium or print mode. In these cases, a different color conversion table is created for each γ curve.

  Further, in the above-described embodiment, the γ correction for improving the interpolation accuracy is performed on the data obtained by performing the separation processing. However, it is not essential to use the data after the separation processing. That is, in the present invention, it is sufficient that the color conversion accuracy can be improved by increasing the number of colors in a specific brightness range. Therefore, the first gradation value data may be obtained by performing gamma correction on the color measurement object obtained by a method other than the color separation processing.

  The number of gradation levels obtained after the resolution improvement γ correction is not limited to 9 gradation levels as shown in FIG. That is, when the accuracy is improved in the high brightness range, the number of gray levels that can be expressed in the low brightness range is reduced. Therefore, the γ correction for improving the resolution may be performed so as to be in the most convenient state in a balance between the two. . Actually, even in a configuration in which the resolution in the high-brightness region is at most about three times the conventional resolution by the above-described resolution improving γ correction, that is, the tone value “1” of the CMYKlclm data is corrected to the tone value “3” Color conversion can be performed with very high accuracy.

  Further, in the above-described embodiment, the γ correction for improving the resolution is performed so that the ink value data is associated with all the value ranges of the ink recording rate. However, as shown in FIG. In, the lightness does not change significantly with changes in the ink recording rate. That is, in a predetermined recording rate value range including the highest ink recording rate, it is not possible to express a brightness gradation with a substantial change. Therefore, in order to use a part of the value range of the ink recording rate, a part of the high ink recording rate corresponding to low brightness may be excluded from the separated CMYKlclm data to be subjected to the resolution improvement γ correction. .

  FIG. 11 is an explanatory diagram illustrating an example in which a part of the high ink recording rate corresponding to low brightness is excluded from the CMYKlclm data after the separation. The solid line curve shown in FIG. 6 shows the relationship between the ink recording rate and the lightness of the K ink, and in the K ink, the lightness hardly changes in a value range of 70% to 100% of the ink recording rate. Therefore, the CMYKlclm data after separation corresponding to the ink recording rate of 0% to 100% is not converted into ink value data by the γ correction for improving the resolution as in the above embodiment shown by the broken line in FIG. The CMYKlclm data (gradation values 0 to 179) corresponding to the ink recording rate of 0% to 70% is converted into 256-gradation ink value data by gamma correction for resolution improvement as indicated by the dashed line.

  FIG. 12 is an explanatory diagram illustrating the former and the latter embodiments. The upper part of the figure shows an example in which the resolution of the gradation values "0" to "1" is tripled by the γ correction for improving the resolution, and the lower part of the figure shows the value range of the ink recording rate of 0% to 70%. Is shown. In the example shown at the top of the figure, the C data after the separation is an integer value of 0 to 255. On the other hand, as described above, if the correction value is corrected such that the smaller the value is, the higher the rate of increase after correction is, and the obtained result is converted into an integer, the ink value data shown in the upper part of FIG.

  That is, “0” and “1” of the C data after the separation are converted into “0” and “3” of the ink value data, respectively. When printing the above-mentioned patch or color image based on the ink value data, the halftone processing module interprets γ. This gamma interpretation is a correction corresponding to the inverse characteristic of the above-described gamma correction for resolution improvement, and "0", "1", "2", and "3" of the ink value data are "0" and "0.254", respectively. "," 0.603 "and" 1.000 ". Therefore, it is possible to express the gradation at three gradations with respect to the ink recording rate corresponding to greater than "0" and equal to or less than "1" of the C data after the separation.

  On the other hand, the C data after separation defined by integer values of 0 to 255 as shown in the lower part of FIG. 12 is compared with the resolution improving γ correction shown in the upper part of FIG. Implement γ correction for resolution improvement with a changed value range. That is, in a situation where the values 0 to 255 of the separated C data linearly correspond to the ink recording rates of 0% to 100%, the values 0 to 179 of the separated C data are input and the floor of the ink value data is set. The adjustment value is set to be 0 to 255. Further, the γ value (corresponding to the index at the time of the γ correction for resolution improvement) is the same value as the γ correction for resolution improvement shown in the upper part of FIG.

  As a result of the resolution improvement γ correction, “0” and “1” of the C data after the separation are converted into ink value data “0” and “4”, respectively. Further, when interpreting γ by the halftone processing module in the example shown in the lower part of FIG. 12, correction corresponding to the inverse characteristic of γ correction for improving resolution is also performed. As a result, “0”, “1”, “2”, “3”, and “4” of the ink value data are “0”, “0.178”, “0.422”, “0.699”, It is converted to "1.000". Therefore, it is possible to express gradations in four gradations with respect to the ink recording rate corresponding to greater than "0" and less than or equal to "1" of the C data after separation.

  Note that the maximum value 255 of the ink value data is interpreted as “179” by gamma interpretation. However, as described above, even if an ink recording rate greater than 70% corresponding to the value “179” is used, the printed matter of the printed matter is not used. Since the brightness does not substantially change, even if the halftone image data is generated with the maximum value “179”, it is possible to perform printing with sufficiently rich gradation. Further, in FIG. 12, 70% to 100% is excluded because the ink recording rate of 0% to 70% substantially changes the brightness with the K ink. %, But can be changed according to the characteristics of the ink and the gradation required on the high brightness side and the low brightness side.

FIG. 9 is an explanatory diagram schematically illustrating steps of a color conversion table creation method. FIG. 5 is a diagram illustrating a relationship between a recording rate (%) of an ink droplet and its lightness L. FIG. 9 is an explanatory diagram for explaining how to reduce the change pitch of a reference point. 9 is a flowchart illustrating a color conversion table creation process. It is a block diagram of a computer. It is a figure showing an example of a change of a value when performing γ correction. FIG. 9 is an explanatory diagram of a halftone process. FIG. 14 is a block diagram of a computer that uses an LUT during printing. FIG. 4 is an explanatory diagram illustrating a mechanism for performing color conversion. FIG. 4 is an explanatory diagram illustrating a mechanism for reducing tone jump. FIG. 9 is an explanatory diagram illustrating an example in which a part of a high ink recording rate is excluded. FIG. 9 is an explanatory diagram of correction excluding a part of a high ink recording rate.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 10 ... Computer, 11 ... Operation processing part, 11a ... Gamma correction part for improving interpolation precision, 11b ... Separation processing part, 11c ... Gamma correction part for improving resolution, 11d ... Halftone processing part, 11d1 ... Gamma interpretation part, 11d2 ... Gradation number reduction section, 11e print data generation / output section, 11f RGB data conversion section, 11g LUT creation section, 12 HDD, 12a separation function data, 12b ink value data, 12c LUT, 20: Printer, 30: Colorimeter

Claims (9)

A plurality of patches output by the printing apparatus are measured, and the colorimetric values of each color used by other image devices and the ink value data corresponding to the ink amount of each ink color used by the printing apparatus are measured based on the colorimetric results. In a color conversion table creating method for creating a color conversion table defining a correspondence,
Extracting and combining reference values smaller than the total number of gradations of the ink value data for each ink color, generating patch data specifying the plurality of patches,
The same patch data is input, the halftone process is performed to convert the patch data into halftone image data indicating the presence or absence of ink dots, and the plurality of patches are printed,
In generating the color conversion table based on colorimetric data obtained by measuring the colors of a plurality of printed patches,
The colors of the patch data are extracted and combined with a pre-correction reference value that is smaller than the total number of gradations of the pre-correction gradation value from a predetermined pre-correction gradation value for each ink color. A color specified by performing a correction for increasing the interpolation accuracy to be increased, and making the magnitude of the corrected value and the ink amount correspond substantially linearly,
The above-mentioned ink value data corresponds to a unit change of the gradation value as compared with a gradation value corresponding to an integer value and a high lightness region which is present in a predetermined value region and a lower lightness region. Defined so that the change in ink recording rate is small,
In the halftone processing, a halftone image data is generated by interpreting an ink amount corresponding to a reference value of the patch data according to the definition of the gradation value. 2. The method according to claim 1, wherein the ink value data is defined by assigning all gradations to a part of a value range of an ink recording rate. In a color conversion table creating method for creating a color conversion table that defines a correspondence between ink value data specifying an ink amount of each color used in a printing apparatus and color component values of each color used in another image device,
A first gradation value data is generated by extracting and combining a gradation value in which an ink amount and a gradation value are approximately linearly associated with each ink color, and generating the first gradation value data. The higher the tone value corresponding to the high lightness region, the higher the gradation value corresponding to the lower lightness region. The halftone process is performed while reflecting a deviation corresponding to a decimal number or less when the inverse correction of the correction for the image is performed, printing is performed, and the ink value is determined based on the colorimetric data obtained by measuring the print result. In generating a color conversion table in which data and color component values of respective colors used in the other image devices are associated with each other, the first gradation value data is relatively corrected in a low lightness region by the correction for improving the resolution. Interpolation resolution The color conversion table generation method characterized in that the color of the previously low brightness region to compensation are extracted to be larger than the color of the high brightness region in. The gradation value of the first gradation value data is obtained by converting coordinates in a predetermined color space composed of color components smaller than the number of ink colors into a gradation value indicating the amount of each color ink by a predetermined conversion formula. 4. The method according to claim 3, wherein the color conversion table is created based on the following. The tone value of the first tone value data is compared with a tone value that is smaller than a tone value in which the ink amount and the tone value are approximately linearly associated with each other. The method according to claim 3 or 4, wherein the color conversion table is obtained by performing γ correction for correcting and outputting at a large increase rate. The tone value indicating the lowest brightness in the first tone value data corresponds to the maximum ink recording rate printable on a print medium, and the correction for improving the resolution includes the tone value indicating the lowest brightness. 6. The method according to claim 3, wherein a predetermined gradation value range is excluded, and correction is performed such that the remaining gradation value ranges match all the gradation value ranges of the ink value data. How to create a color conversion table. Referring to a color conversion table that defines the correspondence between the ink value data specifying the ink amount of each color used in the printing apparatus and the color component value of each color used in another image device, the display image on the image device is displayed. A print control device that generates print data indicating an output image in the printing device from the image data to be displayed and executes printing.
Image data acquisition means for acquiring image data in which the color of each pixel in a dot matrix form is expressed in gradation for an image in the other image device,
A first gradation value data is generated by extracting and combining a gradation value in which an ink amount and a gradation value are approximately linearly associated with each ink color, and generating the first gradation value data. The higher the tone value corresponding to the high lightness region, the higher the gradation value corresponding to the lower lightness region. The halftone process is performed while reflecting a deviation corresponding to a decimal number or less when the inverse correction of the correction for the image is performed, printing is performed, and the ink value is determined based on the colorimetric data obtained by measuring the print result. In generating a color conversion table in which data and color component values of respective colors used in the other image devices are associated with each other, the first gradation value data is relatively corrected in a low lightness region by the correction for improving the resolution. Interpolation resolution In order to compensate for the above, the colors in the low lightness region are extracted in advance so as to be greater than the colors in the high lightness region, and the image data is referred to the corresponding ink value data with reference to the color conversion table generated in this manner. Color conversion means for converting the color into
Halftone processing means for interpreting the ink amount indicated by the ink value data from the color-converted ink value data and converting it into pseudo halftone data expressing gradation with the recording density of ink droplets to be recorded on a print medium,
Print data generating means for generating print data for driving the printing apparatus to eject ink droplets according to the recording density specified by the pseudo halftone data for each pixel,
A print data output unit that outputs the print data to a printing apparatus. In a color conversion table creation device that creates a color conversion table that defines the correspondence between ink value data specifying the amount of ink of each color used in the printing device and color component values of each color used in other image devices,
First tone value data acquiring means for acquiring first tone value data obtained by extracting and combining, for each ink color, a tone value in which the ink amount and the magnitude of the tone value are substantially linearly associated;
The first tone value data is subjected to a correction for improving the resolution at a larger increase rate than a tone value corresponding to a lower brightness range as the tone value corresponding to the higher brightness range, and the ink value data is corrected. Ink value data acquisition means for acquiring as
A print execution unit that performs halftone processing and performs printing while reflecting a deviation corresponding to a decimal number or less when the inverse correction of the resolution improvement correction is performed on the ink value data,
A print result color measurement unit for measuring the color of the printed result;
A color conversion table that generates a color conversion table that associates the ink value data with the color component values of each color used in the other image device based on colorimetric data obtained by measuring the color of the printed result. Generating means,
The first gradation value data is set so that the colors in the low lightness region are preliminarily greater than the colors in the high lightness region so that the resolution relatively reduced in the low lightness region by the correction for improving the resolution is compensated with interpolation accuracy. An apparatus for creating a color conversion table, wherein the apparatus has been extracted. In a color conversion table creation program that creates a color conversion table that defines the correspondence between ink value data that specifies the amount of ink of each color used in the printing apparatus and color component values of each color that is used in another image device,
A first gradation value data acquisition function for acquiring first gradation value data obtained by extracting and combining, for each ink color, a gradation value in which the amount of ink and the magnitude of the gradation value correspond substantially linearly;
The first tone value data is subjected to a correction for improving the resolution at a larger increase rate than a tone value corresponding to a lower brightness range as the tone value corresponding to the higher brightness range, and the ink value data is corrected. An ink value data acquisition function to acquire as
A print execution function of performing a halftone process and performing printing while reflecting a deviation corresponding to a decimal number or less when the inverse correction of the resolution improvement correction is performed on the ink value data,
A print result color measurement function that measures the color of the printed result,
A color conversion table that generates a color conversion table that associates the ink value data with the color component values of each color used in the other image device based on colorimetric data obtained by measuring the color of the printed result. In making a computer realize the generation function,
The first gradation value data is set so that the colors in the low lightness region are preliminarily greater than the colors in the high lightness region so that the resolution relatively reduced in the low lightness region by the correction for improving the resolution is compensated with interpolation accuracy. A color conversion table creation program characterized by being extracted.

Images