JP2009038738A - Density/lightness tone correction method, and saturation tone correction method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To solve a problem that color measurement/light measurement and the like required for determination is difficult although tone and color reproducibility are required to be switched according to a recording medium used (regular paper, gloss paper for photographs, OHP, thermal transfer paper, CDR, DVD, and the like) in a color printer and the like, and the tone and color reproducibility are required to be determined when a new recording medium is used. <P>SOLUTION: Tone/color reproduction with higher accuracy is realized by using color measurement/light measurement values for several to several tens of patches printed on a recording medium which is difficult to perform color measurement/light measurement, color measurement values of patches printed on another recording medium which is easy to measure a color by the same signal and the same recording member to predict color measuring values in points except the patches. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a color reproduction method in a recording technology apparatus.

  In recent years, with the spread of personal computers (hereinafter referred to as PCs) and digital cameras (hereinafter referred to as DSCs), there is an increasing demand for high-quality printing at home. In order to meet such demand, the image quality of printers has increased more rapidly than before.

In order to meet a variety of needs, the types of recording media printed by printers are not limited to paper, but also support non-paper media such as OHP, thermal transfer paper, and CD and DVD.
Japanese Patent Laid-Open No. 2001-186368 JP 2001-119594

  However, if the density / brightness characteristics and saturation characteristics of the recording medium are different, the amount of recording material applied / discharged to achieve good color reproduction differs. It was necessary to prescribe.

  Further, as described in Patent Document 1, in order to express the entire color reproduction space, image data (patch) as shown in FIG. 1 is output, and the color reproduction space (gamut) of each output device is based on the output result. ) Must be expressed by a three-dimensional grid in the RGB space that is associated with coordinate points in the uniform color space, but a sufficient size cannot be obtained depending on the non-paper medium, and the coordinates of the grid points However, it is extremely difficult to obtain colorimetric values of 9 ^ 3 = 729 points represented by combinations of signal values for each color = "0", "16", "32", ..., "224" and "255" Become. As a result, it has been difficult to obtain colorimetric / photometric values with sufficient accuracy.

  In order to avoid such complications, for example, a method as described in Patent Document 2 has been proposed.

  However, the conventional method achieves color reproduction by reducing the number of colorimetry / photometry for recording media having relatively similar characteristics. However, the signal value of the recording medium with known recording characteristics is used as the colorimetric characteristic ratio. Accordingly, if the absorption characteristics of the recording material in the recording medium are significantly different just by adjusting the signal amount, it is difficult to realize it due to physical restrictions to prevent image quality deterioration due to excessive implantation of the recording material.

  Furthermore, even if 729 colorimetric values are obtained with a recording medium having a sufficient size as shown in FIG. 1, if the obtained value is significantly different from the colorimetric / photometric characteristics on the recording medium to be obtained, There is a problem that the gradation of brightness, density, and saturation of the obtained image is impaired.

  In order to solve the above-described problem, the configuration according to claim 1 is a density / lightness gradation correction method on a plurality of types of recording media on which the recording apparatus can record an image, and a sufficient number Record several to several tens of patches on the first recording medium where it is difficult to record patches, and use the same recording material under the same signal value as the first process of colorimetry / photometry. The second process of recording patches and measuring colors / photometrically on a second recording medium with different characteristics and standardizing the colorimetric values obtained in the second process in the range of 0-1 A third process for obtaining a curve fitted with elementary functions such as a power function including an exponential function of the order, and a fourth process for converting the characteristic curve obtained in the third process into an inverse function for the expected density / lightness characteristic. And the product of the inverse function curve and the signal value curve obtained in the fourth process (if the obtained characteristics are discrete values, Product) to obtain a density / lightness correction curve.

  As described above, according to the present invention, color reproduction on a recording medium in which colorimetry / photometry is difficult can be improved.

  Preferred embodiments of the present invention will be described below with reference to the accompanying drawings.

  FIG. 2 is a diagram schematically showing a configuration of an image processing system according to a preferred embodiment of the present invention. This image processing system includes a computer 100 such as a PC, a monitor 5, and a printer 106 such as an ink jet printer.

  The computer 100 includes application software 101 such as document processing, spreadsheet, and internet browser, an OS (Operating System) 102, and various drawing command groups (image drawing commands) related to print commands issued by the application software 101 to the OS 102. Printer driver 103 that generates print data by processing text drawing commands and graphics drawing commands), monitor driver 104 that processes various drawing commands issued by application software 101 and displays an image on monitor 106, etc. Software is incorporated.

  The computer 100 includes a central processing unit (CPU) 108, a hard disk driver (HD) 107, a random access memory (RAM) 109, a read-only memory (ROM) 110, and the like as various hardware capable of operating these software.

  As a specific configuration example of the image processing system shown in FIG. 1, an example in which an OS is installed on a generally popular PC, an application having a printing function is installed, and a monitor and a printer are connected to the PC. Is mentioned.

  In the computer 100, based on the display image displayed on the monitor, the application 101 classifies the document data into text such as characters, graphics data classified into graphics such as graphics, and natural images. Output image data is created using image data or the like. When printing an image based on the output image data, the application 101 issues a print output request to the OS 102, the graphics data portion is a graphics drawing command, and the image data portion is a drawing composed of an image drawing command. An instruction group is issued to OS102.

  The OS 102 receives an application output request and issues a drawing command group to the printer driver 103 corresponding to the output printer. The printer driver 103 processes the print request and drawing command group input from the OS 102, creates print data that can be printed by the printer 105, and transfers the print data to the printer 105. When the printer 105 is a raster printer, the printer driver 103 sequentially performs image correction processing on rendering commands from the OS 102, and then rasterizes them sequentially into RGB 24-bit page memory (RAM), and rasterizes all rendering commands. Later, the contents of the RGB 24-bit page memory are converted into a data format printable by the printer 105, for example, CMYK data, and transferred to the printer.

  FIG. 3 is a diagram showing the configuration of the printer driver 103. As shown in FIG. The image correction processing unit 120 performs image correction processing on the color information included in the drawing command group supplied from the OS 102. In this image correction process, RGB color information is converted into a luminance / color difference signal, exposure correction processing is performed on the luminance signal, and the corrected luminance / color difference signal is inversely converted into RGB color information.

  The printer correction processing unit 121 rasterizes the rendering command based on the RGB color information subjected to the image correction processing, generates a raster image on the RGB 24-bit page memory, performs color reproduction space mapping processing, color separation processing to CMYK, gradation Correction processing is performed to generate CMYK data depending on the color reproducibility of the printer for each pixel and transfer it to the printer 105.

  FIG. 4 is a diagram illustrating a configuration of the printer correction processing unit 121. As illustrated in FIG. Hereinafter, processing in the printer correction processing unit 121 will be described with reference to FIG.

  The input signal appropriately adjusted in the image correction processing unit 120 (FIG. 3) is input to the pre-stage color signal conversion unit B2 via the image input unit B1. In the preceding stage color signal converter B2, RGB signals are dependent on the device (in this case, the printer) through an operation called gamut compression and gamut mapping to adjust the difference between the color gamuts of the monitor and the printer. Convert to G'B 'signal. Next, the post-stage color signal converter B3 receives the converted R′G′B ′ and converts it into a recording material unique to the printer, for example, 8-bit CMYK. That is, the maximum gamut, density / brightness, and chroma gradation of the printer are determined depending on the signal in the subsequent color signal processing unit B3 that directly defines the amount of recording material.

By the way, the maximum printer gamut required for the conversion processing in the preceding stage color signal processing unit B2 is to measure and measure a patch as shown in FIG. 1 in an L ^* a ^* b ^* uniform color space and a space interchangeable with it. Can express. Here, for example, the color corresponding to each color signal intensity is measured using a color measuring device such as SPM100-II manufactured by GRETAG, and for example, an L ^* a ^* b ^* value can be obtained.

  The following is the greatest feature of the present invention, but depending on a certain non-paper recording medium, a combination of “0”, “16”, “32”,..., “224” and “255” as shown in FIG. It is extremely difficult to obtain a color gamut consisting of 9 ^ 3 = 729 colorimetric values expressed by. In the following description, a CDR is used as a specific example.

  For example, take an example in which a signal sequence from white (R′G′B ′ = 255, 255, 255) to red (R′G′B ′ = 255, 0, 0) is woven. (R'G'B ') = (255, 255, 255), (255, 240, 240), (255, 224, 224), ..., (255, 16, 16), (255, 17 patches consisting of 0,0) are printed on a recording medium that is large enough to print / colorimetry FIG. At this time, desired colorimetric / photometric characteristics are satisfied on the recording medium. Here, as an example, it is assumed that the lightness is desired to be a linear shape. Such a recording medium is referred to as a provisional recording medium in this case.

  Next, with the signal values designed to satisfy the above, (R′G′B ′) = (255, 255, 255), (255, 224, 224), ( Nine patches consisting of 255, 192, 192), ..., (255, 32, 32), (255, 0, 0) are printed and measured.

  Next, consider a case where the obtained colorimetric / photometric values on the CDR are normalized by 0 to 1 as shown in FIG. Here, the normalized colorimetric lightness on the vertical axis is assumed to be f (s). At this time, s is a number from 0 to 8 corresponding to the measured patch. This discrete value is approximated by a function that is considered preferable. Here, for the sake of explanation, it is assumed that it is approximated by the simplest exponential function. The resulting curve can be expressed as f (x) = x ^ γ (0 ≦ x ≦ 1) as shown in FIG. Further, the curve (b) in FIG. 7 is illustrated as a primary linear type based on the above assumption.

  In order to obtain the desired colorimetric characteristics on the CDR, the normalized characteristic curve g (x) for the colorimetric values on the temporary recording medium is an inverse function with respect to the curve (b) in FIG. It is preferable. Therefore, an inverse function g (x) of f (x) shown in FIG. 7 (a) with respect to FIG. 7 (b) is obtained. In this case, g (x) = x ^ (1 / γ) (FIG. 7 (c)). Substituting s ′ which is a discretized state of x into g (x), a coefficient between 0 and 1 is obtained. Note that s ′ can be discretized into a desired number based on a desired signal interval. Here, it is assumed that the number is discretized into 17 as printed / colorimetrically recorded on the temporary recording medium.

  The signal intensity is corrected by taking the inner product of the coefficient obtained in this way and each of the subsequent signal values C, M, Y, and K that realize the primary linear characteristics on the provisional recording medium. The lightness characteristic at the point leads to a desired characteristic. This operation applies not only to white → red in gamut but also to red → black, white → green ... and red green blue cyan magenta yellow and the axis connecting white and black at the shortest (usually called the gray axis). Then, if a patch as shown in FIG. 1 is printed on the provisional recording medium from the obtained signal value and colorimetry / photometry is performed, a subsequent signal value that maintains linearity on the CDR can be realized.

  In the above example, the brightness is taken as an example, but it is also possible to carry out the saturation.

  In the above example, the CDR is taken as an example, but it goes without saying that other recording media including a DVD are also possible.

  Furthermore, in the above example, an exponential function was taken as an example, but the same argument can be applied to an elementary function for which an inverse function can be obtained.

  In the above example, the desired characteristic is obtained from the primary linearity, but the same technique can be developed even for other characteristics.

  In the above example, the number of colorimetric / photometric patches is limited. However, the number is not limited to the number in the example.

A chart used to determine the gamut of printers, monitors, etc. (By outputting this and performing colorimetry / photometry, it is possible to associate RGB color signal intensity with L ^* a ^* b ^* values) is there. 1 is a diagram illustrating a schematic configuration of an image processing system according to a preferred embodiment of the present invention. FIG. 6 is a diagram illustrating processing performed by a printer driver. FIG. 6 is a diagram illustrating color processing performed by a printer driver. FIG. 5 is a schematic diagram when color measurement / photometry is performed on a limited patch on a recording medium in which it is difficult to perform color measurement / photometry directly on a large area. It is a specific example of a standardized colorimetric value. The approximate curve and inverse function of the standardized colorimetric value.

Claims (5)

  A density / lightness gradation correction method on a plurality of types of recording media on which a recording apparatus can record an image, wherein density / lightness gradation characteristics obtained from a first recording medium are recorded on a second recording medium. A density / lightness gradation correction method comprising: correcting and reflecting the correction result on a first recording medium.   A saturation gradation correction method on a plurality of types of recording media on which a recording apparatus can record an image, wherein the saturation gradation characteristics obtained from the first recording medium are corrected on the second recording medium. A saturation gradation correction method, wherein the correction result is reflected on the first recording medium.   The gradation correction method, wherein the first recording medium does not have a sufficient print area for colorimetry / photometry of the number of patches necessary for gradation correction, or The gradation correction method according to claim 2.   3. The gradation correction method according to claim 1, wherein the second recording medium can receive a recording material equivalent to or higher than the first recording medium. Gradation correction method.   4. The gradation correction method according to claim 3, wherein the required number of patches is 729.

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