JP2006173823A - Image processing apparatus and method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for improving color reproduction accuracy of important colors such as gray and skin colors by controlling a grid in the case of generating a LUT used for color conversion. <P>SOLUTION: The image processing method comprises: an image input step of receiving image data for denoting an input image; a grid data generating step of generating uneven interval grid data; a colorimetry step of generating the LUT in response to the uneven interval grid data on the basis of a result of the colorimetry processing applied to an outputted patch image; and a color space conversion step of carrying out color space conversion by using the generated LUT. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a technique for creating a lookup table when performing color reproduction of an output image using a lookup table.

In an image output device such as a display or a printer, when performing image output, a color system unique to the normal device is used. For example, an RGB color system is used for CRT displays and liquid crystal displays, and a CMY color system is used for inkjet printers and laser printers. In the image output apparatus, in order to express the same color between different devices, for example, a CRT display and an ink jet printer, conversion from the RGB coordinate system of the display to the CMY coordinate system of the printer is necessary. However, in general, the RGB → CMY conversion has strong non-linearity and is difficult to express with a unique mathematical expression or the like. Therefore, in order to perform the RGB → CMY conversion with high accuracy, a technique using a lookup table (hereinafter referred to as LUT) is widely used. The LUT defines the correspondence of the color system between different devices. Usually, the correspondence of N 3 points (N is the number of grids of each component of the LUT) as shown in FIG. When the points between the grids are input, the output values are obtained by interpolation from the surrounding points. However, in many cases, in order to provide general versatility, instead of directly obtaining the correspondence relationship of RGB → CMY directly, it passes through a color system such as CIELab, which does not depend on the device, and RGB → Lab, Lab → In general, conversion is performed like CMY. In the conversion using the LUT, when a color that is not on the grid of the LUT is input, interpolation is performed using linear interpolation such as tetrahedral interpolation to obtain an output value. When the nonlinearity is strong, in order to perform conversion with sufficient accuracy, the number of grids has to be increased, and there is a problem that enormous cost is required.
JP-A-9-46542

  However, in the color conversion method using the LUT, as shown in FIG. 11, grid points are arranged in a cubic lattice pattern in the RGB color space or the CMY color space, and the grid points in the LUT are converted into CIELab uniform colors. When viewed in space, as shown in FIG. 12, the grid is clearly divided into a dense part and a sparse part in the space.

  As the adjacent grids are denser, the interpolation accuracy is clearly higher. Therefore, it is preferable to make the grid intervals denser for important colors such as gray, skin color, and sky color. When the data is created, there is a problem that the grid interval in the CIELab space is not the preferred interval.

  The present invention has been made in view of the problems as described above. When creating an LUT for performing color conversion, a grid is created in consideration of the distribution in the CIELab space. It is an object of the present invention to provide a method for improving the color reproduction accuracy of important colors.

  In order to achieve the above object, in the color image processing apparatus, when digital image data is output, the present invention provides an image input means for inputting an image, a grid data creation means for creating non-uniformly spaced grid data, Color measurement means for measuring the output patch image and creating an LUT, color conversion means for converting color data from the color space of the image data to the color space of the output space, and image output means for outputting the image It is characterized by comprising.

  According to the present invention, grid data of the LUT can be used efficiently, and high-precision color reproduction can be performed.

(First embodiment)
Hereinafter, an embodiment according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a block diagram showing a configuration of an image processing apparatus according to an embodiment of the present invention. 1 is an image processing apparatus according to an embodiment of the present invention. An image data storage unit 2 stores image information to be output, such as a hard disk or a CD-ROM. 3 is an image input unit for reading image data from the image data storage unit 2 into the image processing apparatus 1. Reference numeral 4 denotes a histogram creation unit that creates a histogram of an image read by the image input unit 3. A grid data creation unit 5 creates LUT grid data based on the histogram created by the histogram creation unit 4. 6 is an LUT storage unit for storing the created LUT. 7 is for converting RGB data expressed in the color space unique to the input image into data in the Lab color space, which is a device-independent color space, using the LUT stored in the LUT storage unit 6. RGB → Lab converter. Reference numeral 8 denotes Lab → for converting the Lab data converted by the RGB → Lab conversion unit 7 into data in the CMY color space which is a color space unique to the printer using the LUT stored in the LUT storage unit 6. CMY conversion unit. Reference numeral 9 denotes an image output unit for outputting CMY data converted by the Lab → CMY conversion unit 8. Reference numeral 10 denotes an image output apparatus that actually performs printing based on the CMY data output from the image output unit 9. 11 is a colorimetric device for measuring the color of the patch data output from the image output device 10, such as a spectral reflectance measuring machine. A colorimetric value input unit 12 reads the colorimetric values measured by the colorimetric device 11 into the image processing apparatus 1.

<Overall processing>
Here, FIG. 2 is a flowchart of image processing performed in the image processing apparatus 1. Hereinafter, an embodiment according to the present invention will be described in detail with reference to the drawings. First, in S201, an LUT in which RGB grids are arranged at equal intervals is created and stored in the LUT storage unit 6 by a conventional method. In step S202, the image input unit 3 reads image data from the image data storage unit 2. In S203, using the image information read in S202, a non-equally spaced grid LUT is created (details will be described later) and stored in the LUT storage unit 6. In S204, using the LUT information stored in the LUT storage unit 6, the RGB data of the image read in S202 is converted into Lab data in the RGB → Lab conversion unit 7, and then the Lab → CMY conversion unit. At 8 the data is converted to CMY data. In S205, the image data converted into CMY data in S204 is output from the image output unit 9 and printed by the image output device 10.

<Method for creating non-uniformly spaced grid LUT>
Next, details of the non-uniformly spaced grid LUT creation processing in step S203 will be described with reference to FIG.

  In S301, the histogram creation unit 4 creates a histogram for each RGB channel of the image data read by the image input unit. In S302, non-uniformly spaced grid data is created based on the histogram calculated in S301 (details will be described later). In S303, the non-uniformly spaced RGB grid data created in S302 is converted into Lab data by the RGB → Lab converter 7 and further converted into CMY data by the Lab → CMY converter 8 to obtain an image output unit. In 9, printing is performed by the image output device 10. In step S304, the color measurement device 11 performs color measurement on the patch output in step S303, and the color measurement value input unit 12 reads the patch into the image processing device 1. In S305, a non-uniform spacing grid LUT is created using the non-uniform spacing grid data created in S302 and the colorimetric values measured in S304.

<How to create non-uniformly spaced grid data>
Next, details of the non-uniformly spaced grid LUT creation processing in step S302 will be described.

  First, a cumulative histogram (FIG. 5) is created for image data with respect to the obtained histogram (FIG. 4). Further, an inverse transformation (FIG. 6) of the cumulative histogram as shown in FIG. 6 is created, and the approximate inverse transformation is used as a new grid interval. By performing the above processing for each of the RGB channels, a non-uniform grid for each of the RGB channels is generated. After that, patches for all combinations of RGB values of the non-uniformly spaced grid in each channel are created, and output and colorimetric are performed. As shown in FIG. 7, a grid near a color that appears frequently in the output image. It is possible to create a non-uniformly spaced grid LUT that is dense.

  As described above, according to the first embodiment, in the Lab space, an LUT that creates a dense grid near colors that frequently appear in the output image is created and used for image output. It is possible to improve the average value of the color reproduction accuracy in the entire image without changing the size of the LUT.

(Second embodiment)
Hereinafter, a second embodiment according to the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 8 is a block diagram showing a configuration of an image processing apparatus according to the second embodiment of the present invention. An image processing apparatus 801 is an embodiment of the present invention. 802 is an image database for storing various images such as a hard disk and a CD-ROM as a database. An image data storage unit 803 stores image information to be output, such as a hard disk or a CD-ROM. An image input unit 804 reads image data from the image database 802 and the image data storage unit 803 into the image processing apparatus 801. A histogram creation unit 805 creates a histogram of the image read by the image input unit 804. A grid data creation unit 806 creates LUT grid data based on the histogram created by the histogram creation unit 805. An LUT storage unit 807 stores the created LUT. An 808 uses the LUT stored in the LUT storage unit 807 to convert RGB data expressed in the color space unique to the input image into data in the Lab color space, which is a device-independent color space. RGB → Lab converter. Reference numeral 809 denotes Lab for converting the Lab data converted by the RGB → Lab conversion unit 808 into data in the CMY color space, which is a printer-specific color space, using the LUT stored in the LUT storage unit 807. CMY conversion unit. Reference numeral 810 denotes an image output unit for outputting CMY data converted by the Lab → CMY conversion unit 809. 811 is an image output apparatus that actually performs printing based on CMY data output from the image output unit 810. Reference numeral 812 denotes a color measuring device for measuring the color of patch data output from the image output device 811 such as a spectral reflectance measuring machine. A colorimetric value input unit 813 reads the colorimetric values measured by the colorimetric device 812 into the image processing device 801.

<Overall processing>
Here, FIG. 9 is a flowchart of image processing performed by the image processing apparatus 801. Hereinafter, a second embodiment according to the present invention will be described in detail with reference to the drawings. First, in S901, an LUT in which RGB grids are arranged at equal intervals is created and stored in the LUT storage unit 807 by a conventional method. In step S902, the image input unit 804 reads image data from the image database 802. In S903, using the image database information read in S902, a non-uniformly spaced grid LUT is created and stored in the LUT storage unit 807 as in the first embodiment. In S904, the image input unit 804 reads output image data from the image data storage unit 803. In S905, using the LUT information stored in the LUT storage unit 807, the RGB data of the image read in S904 is converted into Lab data by the RGB → Lab conversion unit 808, and then the Lab → CMY conversion unit. In 809, it is converted into CMY data. In S906, the image data converted into CMY data in S904 is output from the image output unit 810 and printed by the image output device 811.

  As described above, according to the second embodiment, in the Lab space, the color distribution of the image database possessed by the user is analyzed, and an LUT is created so that the grids near the colors with frequent appearances are dense. The average value of the color reproduction accuracy of the entire image can be improved without changing the size of the LUT.

(Other embodiments)
<Color space>
In the first and second embodiments, the color space of the input image is the RGB space, the intermediate color space that does not depend on the device is the Lab space, and the color space of the output device is the CMY space. For example, a YCC color space may be used as the input color space, an XYZ color space may be used as the intermediate color space, and an RGB color space may be used as the output color space.

<How to create non-uniformly spaced grid data>
In the first and second embodiments, the method of using the histogram of the image has been described as the method for creating the non-uniform grid data. However, the present invention is not limited to this. For example, discrimination of human eyes A MacAdam ellipse representing a threshold may be used, and in a region where the ellipse is large, the grid interval is sparse, and conversely, in a region where the ellipse is small, the grid interval is dense.

  That is, the specific content of the output image is not limited as long as it is a method that can make the grid interval closer to the color that is more important in the color space.

<Output device>
In the first embodiment and the second embodiment, the image output device is a printer. However, for example, it may be a CRT display or a liquid crystal projector.

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

  Another object of the present invention is to supply a storage medium storing software program codes for realizing the functions of the above-described embodiments to a system or apparatus, and the computer (or CPU or MPU) of the system or apparatus stores the storage medium. Needless to say, this can also be achieved by reading and executing the program code stored in.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention.

  As a storage medium for supplying the program code, for example, a floppy (registered trademark) disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CD-R, magnetic tape, nonvolatile memory card, ROM or the like is used. I can do it.

  In addition, by executing the program code read by the computer, not only the functions of the above-described embodiments are realized, but also an OS (operating system) running on the computer based on the instruction of the program code. It goes without saying that a part of the actual processing is performed and the functions of the above-described embodiments are realized by the processing.

  Further, after the program code read from the storage medium is written to a memory provided in a function expansion board inserted into the computer or a function expansion unit connected to the computer, the function expansion is performed based on the instruction of the program code. It goes without saying that the CPU or the like provided in the board or the function expansion unit performs part or all of the actual processing, and the functions of the above-described embodiments are realized by the processing.

1 is a block diagram illustrating an image processing apparatus according to a first embodiment. 6 is a flowchart for explaining a processing flow in the image processing apparatus according to the first embodiment. 6 is a flowchart for explaining a method of creating a non-uniform grid LUT. An example of a created histogram. An example of the created cumulative histogram. An example of a created non-uniformly spaced grid LUT. An example of distribution in the Lab space of the grid of a non-equal interval LUT. The block diagram which shows the image processing apparatus of 2nd embodiment. 9 is a flowchart for explaining a processing flow in the image processing apparatus according to the second embodiment. The conceptual diagram of RGB-> Lab conversion and Lab-> CMY conversion. An example of distribution in Lab space of the grid of equally spaced LUTs.

Claims (8)

  When digital image data is output in a color image processing device, an image input means for inputting an image, a grid data creation means for creating non-uniformly spaced grid data, and a color measurement of the output patch image to create an LUT An image processing apparatus comprising: a colorimetric unit that performs color conversion; a color conversion unit that converts color data from a color space of image data to a color space of an output space; and an image output unit that outputs an image.   The grid data creation means is characterized in that, based on a histogram of an input image, a region having a high appearance frequency in the color space has a dense grid interval, and conversely a region having a low appearance frequency has a narrow grid interval. The image processing apparatus according to 1.   2. The grid data creation unit according to claim 1, wherein, based on the MacAdam ellipse, the grid interval is sparse in a region where the ellipse is large, and conversely, the grid interval is dense in a region where the ellipse is small. Image processing device.   2. The image processing apparatus according to claim 1, wherein the image input means inputs RGB image data, and the image output means outputs image data corresponding to the type of ink or toner color of the printer. An image input process for inputting image data indicating an input image;
Grid data creation process for creating non-uniformly spaced grid data;
A colorimetric process for creating an LUT corresponding to the non-uniformly spaced grid data from the colorimetric result of the output patch image;
An image processing method comprising: a color space conversion step of performing color space conversion on the image data using the created LUT. The grid data creation step includes
6. The image processing according to claim 5, wherein, based on a histogram of the input image, a region having a high appearance frequency in the color space has a dense grid interval, and conversely a region having a low appearance frequency has a sparse grid interval. Method. The grid data creation step is based on a MacAdam ellipse, and in a region where the ellipse is large, the grid interval is sparse, and conversely, in a region where the ellipse is small, the grid interval is dense. Image processing method. A program for realizing the image processing method according to claim 6 or 7 on a computer.

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