US 6,239,886 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.

Claims45

Dependent claims

A method of correcting device independent luminance and chrominance data in a digital color image, comprising the steps of:

a) providing device independent color image signals to a plurality of channels in a device independent color space, said device independent color image signals being sub-sampled and corresponding to color signals;
b) deriving a set of device independent master signals from said device independent color image signals, said device independent master signal set including a signal from each of said channels and wherein said device independent color imaging signals have been sub-sampled and are received at three channels, 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 said red-green chrominance channel resolution being less than said luminance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance channel resolution, said blue-yellow chrominance channel resolution being less than said red-green chrominance channel resolution, and wherein said master signal deriving step further comprises:
i.) dividing the digital color image into a plurality of blocks, and further dividing said blocks into a plurality of sub-blocks, wherein each block includes a plurality of signal sets that have a substantially constant blue-yellow resolution, and each sub-block includes a plurality of signal sets that have substantially constant red-green chrominance and blue-yellow resolutions:
ii.) selecting a master sub-block;
and
iii.) choosing a signal set within said master sub-block as said master signal set;
c) mapping said device independent master signal set to an output device dependent master signal set in an output device dependent color space;
d) defining output device dependent color image signals corresponding to a subset of said device independent color signals wherein said output device dependent signals are created 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
e) converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants.

7 more elements

An apparatus for correcting device independent luminance and chrominance data in a digital color image, comprising:

a) means for providing device independent color image signals to a plurality of channels in a device independent color space, said device independent color image signals corresponding to color signals;
b) means for deriving a set of device independent master signals from said device independent color image signals, said device independent master signal set including a signal from each of said channels and wherein said device independent color image signals have been sub-sampled and are received at three channels, 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 channel resolution wherein said red-green channel resolution is less than said luminance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow channel resolution said blue-yellow channel resolution is less than said red-green channel resolution and wherein said master signal deriving means further comprises;
i.) means for dividing the digital color image into a plurality of blocks, and further dividing said blocks into a plurality of sub-blocks, wherein each block includes a plurality of signal sets that have a substantially constant blue-yellow resolution, and each sub-block includes a plurality of signal sets that have substantially constant red-green chrominance and blue-yellow resolutions;
ii.) means for selecting a master sub-block;
and
iii.) means for choosing a signal set within said master sub-block as said master signal set;
c) means for mapping said device independent master signal set to an output device dependent master signal set in an output device dependent color space wherein said mapping means further comprises:
i.) means for dividing the digital color image into blocks, each block including a plurality of device independent signal sets, wherein each signal set includes a signal from each of said channels, and wherein each block includes one master signal set:
and
ii.) means for applying a look up table conversion to transform said master signal set from said device independent color space to said output device dependent color space;
d) means for defining output device dependent color image signals corresponding to a subset of said device independent color signals wherein said defining means further comprises 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
e) means for converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants.

10 more elements

Dependencies are inferred from the claim text.

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Abstract

A method of correcting device independent luminance and chrominance data in a digital color image, comprising the steps of:

a) providing device independent color image signals to a plurality of channels in a device independent color space, said device independent color image signals being sub-sampled and corresponding to color signals;
b) deriving a set of device independent master signals from said device independent color image signals, said device independent master signal set including a signal from each of said channels and wherein said device independent color imaging signals have been sub-sampled and are received at three channels, 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 said red-green chrominance channel resolution being less than said luminance channel resolution, and a third channel Cb receives blue-yellow chrominance signals at a blue-yellow chrominance channel resolution, said blue-yellow chrominance channel resolution being less than said red-green chrominance channel resolution, and wherein said master signal deriving step further comprises:
i.) dividing the digital color image into a plurality of blocks, and further dividing said blocks into a plurality of sub-blocks, wherein each block includes a plurality of signal sets that have a substantially constant blue-yellow resolution, and each sub-block includes a plurality of signal sets that have substantially constant red-green chrominance and blue-yellow resolutions:
ii.) selecting a master sub-block;
and
iii.) choosing a signal set within said master sub-block as said master signal set;
c) mapping said device independent master signal set to an output device dependent master signal set in an output device dependent color space;
d) defining output device dependent color image signals corresponding to a subset of said device independent color signals wherein said output device dependent signals are created 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
e) converting said output device dependent color image signals to output device dependent coordinate signals which correspond to output device colorants.

7 more elements