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US 6,268,939 B1Granted
Method and apparatus for correcting luminance and chrominance data in digital color images
AssigneeXerox Corporation
InventorsR. Victor Klassen, Thyagarajan Balasubramanian
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Abstract
The present invention is directed to a method and apparatus for improving the efficiency of color correcting subsampled luminance and chrominance based data. More specifically, the present invention is directed to a method of applying a full conversion between color spaces for one pixel in a selected pixel block. Chrominance values are assigned to the remaining pixels based upon their luminance values relative to the luminance value of the converted pixel. The invention may be used to convert between various types of color spaces, and may be used in the processing of both device dependent and device independent data.
Claims41
A method of correcting data in an input device dependent digital color image generated at an input device, preparatory to reproduction at a plural channel output device, comprising the steps of:
- a) providing input device dependent color image signals in an input device dependent color space said device dependent color image signals having been sub-sampled;
- b) providing information about the color response characteristic of the input device and the output device;
- c) using said information, mapping said input device dependent color image signals to output device dependent color image signals at said plurality of channels in an output device dependent color space, said output device dependent color space being an RGB space, with a complex mapping process which includes color correction, wherein said output device dependent color image signals correspond to said input device dependent color image signals said processing comprising:
- i) deriving a set of input device dependent master signals from said input device dependent color image signals by designating a signal set at a given location within each block wherein said input device dependent color imaging signals are received at three channels, wherein a first channel L receives luminance signals at a luminance channel resolution, a second channel Cr receives red-green chrominance signals at a red-green chrominance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance resolution;
- ii) mapping said input device dependent master signal set to an output device dependent master signal set in said output device dependent color space, said mapping comprising:
- dividing the digital color image into blocks, each block including a plurality of device dependent signal sets, wherein each signal set includes a signal from each of said channels, and wherein each block includes one master signal set;
- and applying a look up table conversion to transform said master signal set from said device dependent color space to said output device dependent color space;
- and
- iii) defining output device dependent color image signals corresponding to a subset of said input device dependent color signals, said defining step further comprises creating output device dependent signals for each signal set in said block according to a luminance signal for each signal set relative to a luminance of said master signal set;
- and wherein said creating step comprising:
- setting output device dependent signal Rij=(Rm/Lm)Lij;
- setting output device dependent signal Gij=(Gm/Lm)Lij;
- and setting output device dependent signal Bij=(Bm/Lm)Lij for each signal set Pij in said block, wherein ii denotes a location of each set within said block, Lij is said luminance signal of signal set Pij, Rm, Gm, and Bm are master signal set output device dependent signals and Lm is said master signal set luminance signal;
- and
- d) converting said output device dependent color image signals with a simple color conversion method to output device dependent coordinate signals which correspond to output device colorants.
12 more elements
An apparatus for correcting data in a device dependent digital color image generated at an input device, preparatory to reproduction at a plural channel output device, comprising:
- a) means for providing input device dependent color image signals in an input device dependent color space said device dependent color image signals having been sub-sampled;
- b) means for mapping said input device dependent color image signals to output device dependent color image signals at a plurality of channels in an output device dependent color space, said output device dependent color space being an RGB space, with a complex color mapping process which includes color correction said processing means further comprises:
- i) means for deriving a set of input device dependent master signals from said input device dependent color image signals by designating a signal set at a given location within each block wherein said input device dependent color imaging signals are received at three channels, wherein a first channel L receives luminance signals at a luminance channel resolution, a second channel Cr receives red-green chrominance signals at a red-green chrominance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance resolution;
- ii) means for mapping said input device dependent master signal set to an output device dependent master signal set in said output device dependent color space, said means for mapping comprising:
- means dividing the digital color image into blocks, each block including a plurality of device dependent signal sets, wherein each signal set includes a signal from each of said channels, and wherein each block includes one master signal set;
- and
- means for applying a look up table conversion to transform said master signal set from said device dependent color space to said output device dependent color space;
- and
- iii) means for defining output device dependent color image signals corresponding to a subset of said input device dependent color signals, said means for defining comprising means for creating output device dependent signals for each signal set in said block according to a luminance signal for each signal set relative to a luminance of said master signal set;
- and wherein said means for creating comprises:
- means for setting output device dependent signal Rij=(Rm/Lm)Lij;
- means for setting output device dependent signal Gij=(Gm/Lm)Lij;
- and means for setting output device dependent signal Bij=(Bm/Lm)Lij for each signal set Pij in said block, wherein ij denotes a location of each set within said block, Lij is said luminance signal of signal set Pij, Rm, Gm, and Bm are master signal set output device dependent signals and Lm is said master signal set luminance signal;
- and
- c) means for providing information about the color response characteristic of the input device and the output device, and
- d) means, using said information provided, for converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants with a simple color space conversion method.
13 more elements
Dependencies are inferred from the claim text.