Method and apparatus for color difference transform
View Patent ↗Efficient image compression for video data characterized by a non-neutral dominant white point is achieved by transforming the input video signal into a de-correlated video signal based on a color difference encoding transform, wherein the color difference encoding transform is adapted based on the dominant white point using an algorithm. The adapting algorithm is designed for optimizing low-entropy output when the white point is other than a neutral or equal-energy value.
1. A method for encoding a video signal, the method comprising:
receiving an input video signal by a processor, wherein one or more frames of the input video signal are characterized by a dominant white point;
transforming the input video signal into a de-correlated video signal based on a Y′D z D x color difference encoding transform for transforming XYZ tri-stimulus values of a device-invariant color space wherein ‘Y’ designates a luminance value and ‘X’ and Z′ designate chromaticity values into Y′D z D x color difference values, using the processor; and
adapting the color difference encoding transform based on the dominant white point, wherein the adapting is directed to enabling a more efficiently compressible output by adapting calculation of D z D x in the color difference transform.
2. The method of claim 1 , wherein the adapting the color difference encoding transform is further based on a pair of coefficients multiplied by respective ones of the D z D x values.
3. The method of claim 2 , further comprising determining the pair of coefficients using respective ratios of a normalized encoding function operating on image chromaticity inputs.
4. The method of claim 2 , further comprising determining the pair of coefficients based simultaneously on the dominant white point and at least one of average or median picture levels.
5. The method of claim 1 , wherein the adapting the color difference encoding transform causes the compressed video signal to be more highly compressed than without the adapting.
6. The method of claim 1 , further comprising determining the dominant white point based on the one or more frames of the input video signal, using the processor.
7. The method of claim 6 , wherein the processor changes values of the dominant white point between sets of the one or more frames, based on the determining.
8. The method of claim 6 , wherein determining the dominant white point comprises obtaining a value of the dominant white point from metadata associated with the one or more frames.
9. The method of claim 6 , wherein determining the dominant white point comprises analyzing at least one image in the one or more frames, using the processor.
10. The method of claim 1 , wherein the input signal is based on an International Commission of Illumination (CIE) XYZ color space encoded using a non-linear encoding function.
11. The method of claim 10 , wherein adapting the color different encoding transform is further based on the non-linear encoding function.
12. An apparatus comprising a processor coupled to a memory, the memory holding instructions for:
receiving an input video signal, wherein one or more frames of the input video signal are characterized by a dominant white point;
transforming the input video signal into a de-correlated video signal based on a Y′D z D x color difference encoding transform for transforming XYZ tri-stimulus values of a device-invariant color space wherein ‘Y’ designates a luminance value and ‘X’ and Z′ designate chromaticity values into Y′D z D x color difference values; and
adapting the color difference encoding transform based on the dominant white point, wherein the adapting is directed to enabling a more efficiently compressible output by adapting calculation of D z D x in the color difference transform.
13. A method for decoding a video signal, the method comprising:
receiving an encoded input video signal by a processor, wherein one or more frames of the input video signal are characterized by a dominant white point;
transforming the input video signal into a correlated video signal based on a Y′D z D x color difference decoding transform for transforming XYZ tri-stimulus values of a device-invariant color space wherein ‘Y’ designates a luminance value and ‘X’ and Z′ designate chromaticity values into Y′D z D x color difference values, using the processor; and
adapting the color difference decoding transform based on the dominant white point, wherein the adapting is directed to enabling a more efficiently compressible output by adapting calculation of D z D x in the color difference transform.
14. The method of claim 13 , wherein the adapting the color difference decoding transform is further based on a pair of coefficients multiplied by respective ones of the D z D x values.
15. The method of claim 14 , further comprising determining the pair of coefficients using respective ratios of a normalized encoding function operating on image chromaticity inputs.
16. The method of claim 14 , further comprising determining the pair of coefficients based simultaneously on the dominant white point and at least one of average or median picture levels.
17. The method of claim 14 , further comprising determining the dominant white point based on the one or more frames of the encoded input video signal, using the processor.
18. The method of claim 17 , wherein the processor changes values of the dominant white point between sets of the one or more frames, based on the determining.
19. The method of claim 17 , wherein determining the dominant white point comprises obtaining a value of the dominant white point from metadata associated with the one or more frames.
20. The method of claim 17 , wherein determining the dominant white point comprises analyzing at least one image in the one or more frames, using the processor.
21. The method of claim 13 , wherein the input signal is based on an International Commission of Illumination (CIE) XYZ color space encoded using a nonlinear encoding function.
22. The method of claim 13 , wherein adapting the color different encoding transform is further based on the non-linear encoding function.
23. An apparatus comprising a processor coupled to a memory, the memory holding instructions for:
receiving an encoded input video signal, wherein one or more frames of the input video signal are characterized by a dominant white point;
transforming the input video signal into a correlated video signal based on a Y′D z D x color difference decoding transform for transforming XYZ tri-stimulus values of a device-invariant color space wherein ‘Y’ designates a luminance value and ‘X’ and Z′ designate chromaticity values into Y′D z D x color difference values; and
adapting the color difference decoding transform based on the dominant white point, wherein the adapting is directed to enabling a more efficiently compressible output by adapting calculation of D z D x in the color difference transform.