US RE41527 E1 Abstract A method for transforming a first image defined by a first multi-dimensional color space (RGB) into second image defined by a second multi-dimensional color space (CMYK). The method computes the transformation using information derived from a previous transformation of said second image into said first image. The method then minimizes the error produced while transforming the second image back into the first image. As such, an image editing system can display on a video monitor an image that is defined in one multi-dimensional color space (RGB), print using a printer that prints images using a second multi-dimensional color space (CMYK) and edit an image using any multi-dimensional color space (either RGB or CMYK) that facilitates rapid and accurate image editing.
Claims(36) 1. For use in a method for editing color data wherein original CMYK data is converted to RGB color data and the RGB color data is edited to provide modified RGB color data, a method for converting modified RGB color data to modified CMYK color data, the method comprising:
providing original CMYK data; and utilizing at least a portion of the original CMYK color data for converting modified RGB color data to modified CMYK color data; wherein said utilizing comprises constructing an RGB-to-CMYK conversion table that contains a plurality of RGB table entries, and said constructing an RGB-to-CMYK conversion table includes,
transforming at least a portion of the original CMYK data to RGB data;
locating, for at least one of the plurality of RGB table entries, candidate CMYK data, from the original CMYK data, which transforms into RGB data in proximity to the RGB table entries; and
associating with at least one of the plurality of RGB table entries corresponding CMYK color data, based upon the candidate CMYK data.
2. The method of
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4. The method of
5. A method for converting RGB color data to converted CMYK color data, the RGB color data being derived from original CMYK color data, the method comprising:
providing original CMYK color data; and utilizing at least a portion of the original CMYK color data for converting RGB color data to converted CMYK color data; wherein said utilizing comprises constructing an RGB-to-CMYK conversion table that contains a plurality of RGB table entries, and said constructing an RGB-to-CMYK conversion table includes,
transforming at least a portion of the original CMYK data to RGB data;
locating, for at least one of the plurality of RGB table entries, candidate CMYK data, from the original CMYK data, which transforms into RGB data in proximity to the RGB table entries; and
associating with at least one of the plurality of RGB table entries corresponding CMYK color data, based upon the candidate CMYK data. 6. The method of
RGB table entries, corresponding CMYK color data, according to a default RGB-to-CMYK transformation. 7. The method of
8. The method of
9. For use in editing color data wherein CMYK data is converted to RGB color data and the RGB color data is edited to provide modified RGB color data, a system for converting modified RGB color data to modified CMYK color data, the system comprising:
a source of original CMYK data; and a color converter, utilizing at least a portion of the original CMYK data for converting modified RGB color data to modified CMYK color data; wherein said color converter includes a table constructor, constructing an RGB-to-CMYK conversion table, that contains a plurality of RGB table entries, and the table constructor includes, a color transformer, transforming at least a portion of the original CMYK data to RGB data; a data finder, locating, for at least one of the plurality of RGB table entries, candidate CMYK data, from the original CMYK data, which transforms under said color transformer to RGB data in proximity to the RGB table entries; and a data processor, associating with at least one of the plurality of RGB table entries corresponding CMYK color data, based upon the candidate CMYK data. 10. The system of
11. The system of
12. The system of
data smoother, smoothing at least a portion of the corresponding CMYK data associated with at least one of the plurality of RGB table entries, by means of a smoothing filter. 13. A system for converting RGB color data to converted CMYK color data, the RGB color data being derived from original CMYK color data, the system comprising:
a source of original CMYK color data; and a color converter, utilizing at least a portion of the original CMYK color data for converting RGB color data to converted CMYK color data; wherein said color converter comprises a table that contains a constructor, constructing an RGB-to-CMYK conversion table a plurality of RGB table entries, and said table constructor includes, a color transformer, transforming at least a portion of the original CMYK data to RGB data; a data finder, locating, for at least one of the plurality of RGB table entries, candidate CMYK data, from the original CMYK data, which transforms under said color transformer to RGB data in proximity to the RGB table entries; and a data processor, associating with at least one of the plurality of RGB table entries corresponding CMYK color data, based upon the candidate CMYK data. 14. The system of
15. The system of
16. The system of
17. A method for converting second multi-dimensional color space data to converted first multi-dimensional color space data, the second multi-dimensional color space data being derived from original first multi-dimensional color space data, comprising:
utilizing at least a portion of the original first multi-dimensional color space data for converting second multi-dimensional color space data to converted first multi-dimensional color space data, wherein said utilizing comprises constructing a second multi-dimensional color space data-to-first multi-dimensional color space data conversion table that contains a plurality of second multi-dimensional color space data table entries, and said constructing the second multi-dimensional color space data-to-first multi-dimensional color space data conversion table includes: transforming at least a portion of the original first multi-dimensional color space data to second multi-dimensional color space data; locating, for at least one of the plurality of second multi-dimensional color space data table entries, candidate first multi-dimensional color space data, from the original first multi-dimensional color space data, which transforms into second multi-dimensional color space data in proximity to the second multi-dimensional color space data table entries; and associating with at least one of the plurality of second multi-dimensional color space data table entries corresponding first multi-dimensional color space data, based upon the candidate first multi-dimensional color space data. 18. The method of
19. The method of
20. The method of
21. The method of
22. The method of
23. For use in editing color data wherein first multi-dimensional color space data is converted to second multi-dimensional color space data and the second multi-dimensional color space data is edited to provide modified second multi-dimensional color space data, a system for converting modified second multi-dimensional color space data to modified first multi-dimensional color space data, the system comprising:
a color converter capable of utilizing at least a portion of original first multi-dimensional color space data and converting modified second multi-dimensional color space data to modified first multi-dimensional color space data, wherein said color converter includes a table constructor capable of constructing an second multi-dimensional color space data-to-first multi-dimensional color space data conversion table that contains a plurality of second multi-dimensional color space data table entries, and the table constructor includes: a color transformer capable of transforming at least a portion of the original first multi-dimensional color space data to second multi-dimensional color space data; a data finder capable of locating, for at least one of the plurality of second multi-dimensional color space data table entries, candidate first multi-dimensional color space data, from the original first multi-dimensional color space data, which transforms under said color transformer to second multi-dimensional color space data in proximity to the second multi-dimensional color space data table entries; and a data processor capable of associating with at least one of the plurality of second multi-dimensional color space data table entries corresponding first multi-dimensional color space data, based upon the candidate first multi-dimensional color space data. 24. The system of
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29. The system of
30. A system for converting second multi-dimensional color space data to converted first multi-dimensional color space data, the second multi-dimensional color space data being derived from original first multi-dimensional color space data, comprising:
a color converter capable of utilizing at least a portion of the original first multi-dimensional color space data and converting second multi-dimensional color space data to converted first multi-dimensional color space data; wherein said color converter comprises a table that contains a constructor capable of constructing a second multi-dimensional color space data-to-first multi-dimensional color space data conversion table that contains a plurality of second multi-dimensional color space data table entries, and said table constructor includes: 31. The system of
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36. The system of
Description ThisMore than one reissue application has been filed for the reissue of U.S. Pat. No. 1. Field of the Invention The invention relates to image processing systems. More particularly, the invention relates to a method for accurately transforming color information between two color spaces having differing dimensions, e.g., between a red-green-blue (RGB) color space and a cyan-magenta-yellow-black (CMYK) color space and vice versa. 2. Description of the Background Art In printing, image retouching and image processing, it is often necessary to convert colors from one representation (color space) into another. Many computer video monitors and scanners, for example, use red-green-blue (RGB) representations for colors, while printers typically represent colors in terms of the amounts of a variety of differently colored inks (for example, cyan-magenta-yellow-black (CMYK)). As such, in a typical computer system, the RGB color space used to produce an image upon a computer screen must be converted into a CMYK color space to facilitate printing of the image depicted on the screen. However, for any particular two color spaces, it is in many instances much easier to convert in one direction than the other, e.g., convert from CMYK to RGB. For example, converting from CMYK to RGB is relatively easy because the CMYK space has more dimensions than the RGB space. Specifically, an important task in photocompositing is to take a set of images in CMYK format, modify them, and output the result in CMYK. Many of the intermediate operations (image modifications) are more easily or effectively accomplished in RGB space, so it is often necessary to convert from CMYK to RGB and then back to CMYK. One problem with such a transformation is that a CMYK color space is a four-dimensional space and an RGB color space is a three-dimensional space, so the transformation from CMYK to RGB, though relatively simple, inherently loses color information. In particular, the color information produced by “black generation” during creation of the CMYK image is lost. Black generation describes an amount of black ink substituted for equal parts of Cyan, Magenta and Yellow for printing purposes. Consequently, it is very important that an image processing system be able to convert from CMYK to RGB and back to CMYK and produce a black component of the CMYK image that closely approximates the black component in the original CMYK image. Thus, a difficult and widely needed color transformation is the transformation from an RGB color space to a CMYK color space that retains, as closely as possible, the black generation of the original CMYK image. U.S. Pat. No. 4,500,919 discloses a particular method for converting from RGB to CMYK which is called the Ink Correction Model (ICM). The patent mentions that the ICM “. . . could be implemented in one huge lookup table, but this would be uneconomic and would also give no clue as to how to find the data to be stored in the LUT short of an impossibly large printing test.” [11:21] Since the time of filing of '919 patent, the cost of memory has been reduced sufficiently that it is no longer uneconomic to use “one huge lookup table”. Furthermore, the '919 patent states that, in using a table based transformation, a large printing test must be conducted to facilitate color space transformation calibration. However, such printing tests are time consuming and complicate the transformation process. Therefore, a need exists in the art for a method that rapidly and accurately transforms images from a first multi-dimensional color space, e.g., RGB, into a second multi-dimensional color space, e.g., CKYK, without using a printing test and which preserves as closely as possible the black generation of an original CMYK image. The present invention overcomes the disadvantages heretofore associated with the prior art. Specifically, the present invention converts pixel values from one color space to another, e.g., RGB to CMYK, using a table of interpolated values. The values in the table are filled using data derived from sample images which have been previously converted in the other direction, e.g., CMYK to RGB. The invention infers from those sample images enough about the forward transformation to build an inverse transformation in the table. In order to convert from RGB to CMYK while retaining as closely as possible the black generation of the original CMYK files, the present invention examines the CMYK files and implicitly infers a black generation model. It does this by creating a table in RGB space of the CMYK values found in the files. At the beginning, each sample of the RGB table is initialized with a value determined from a default transformation of RGB into CMYK using any default black generation strategy. The choice of this transformation is not very important because it is highly modified in the following steps performed by the invention. Next, each pixel of each CMYK image used for creation of the table is converted into RGB, and then the appropriate entries in the RGB-space table are modified so that the interpolation of the table entries at the RGB values yields a value as close as possible to the CMYK pixel color. Once the table has been constructed, it may be low-pass filtered (smoothed), so that the values are highly continuous and no visible artifacts can be identified in the conversion. If the CMYK values of the input images are converted to RGB and the resulting RGB values are converted back to CMYK using the table described above, the original CMYK values with their original black generation are reconstructed with high accuracy as long as all the input images used the same black-generation strategy. If several input images are used that were created with different black-generation strategies (different UCR, GCR, and the like), the table is constructed using an average of the different strategies. A key advantage of the current invention is that the user need not know anything about the black-generation strategy used in the CMYK file. It is inferred automatically by the inventive method. In situations where people are collaborating over long distances and it is impractical to do a large series of printing tests to facilitate optimization of the color space transformation process, the invention has great advantages over the prior art. The teachings of the present invention can be readily understood by considering the following detailed description in conjunction with the accompanying drawings, in which: FIG. 1 depicts a computer system capable of executing an image processing program as well as a color space transformation program in accordance this the present invention; FIG. 2 depicts a flow chart of a color space transformation table generation routine as executed upon the computer system shown in FIG. 1; and FIG. 3 depicts a flow chart of a color space transformation routine that uses the table generated using the routine depicted in FIG. 2. To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. FIG. 1 is a block diagram of a conventional computer system Specifically, the computer system In operation, a user typically manipulates the user command input device Broadly speaking, the transformation program Inverting T is problematic. Since T is many-to-one, there is no full inverse T The present invention makes use of the observation that even though R The invention, which is embodied in a software routine The routine begins by creating an m-dimensional grid which samples R At step In order to establish the values H, the routine looks for colors in R Specifically, the source for the colors C is a source image containing a collection of pixels x in R If, at step It is possible that the table resulting from the second phase, i.e., an updated table G containing modified H values, can be insufficiently smooth if the colors in the source image do not cover the entire color space. Consequently, there will be a transition region between the portion of G(P) which still contains the original default mapping and the portion that contains the updated mapping based on the source image. To ensure that there are no objectionable artifacts produced by this boundary, the routine, at step Using the grid G and the interpolation function S, any input pixel value from an m-dimensional color space is accurately transformed into a pixel value in an n-dimensional color space. FIG. 3 depicts a flow chart of a transformation routine The routine is entered at step In operation, the two routines (FIGS. 2 and 3) have obtained quite satisfactory results transforming colors from RGB to CMYK using the conventional tri-linear interpolant. The resulting transformation has the property for RGB to CMYK transformation that it reproduces very accurately the black generation in the original CMYK file. In experiments, the observed errors typically on the order of one or two percent of the original CMYK values. Using the present invention within an image editing system, an operator can convert a series of images from CMYK to RGB, re-touch or edit the images in RGB, and then transform the result back into CM, knowing that the black-generation of the resulting CMYK image will very closely match the original black-generation. Although one embodiment incorporating the teachings of the present invention has been shown and described in detail herein, those skilled in the art can readily devise many other varied embodiments that still incorporate these teachings. Patent Citations
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