Coding of HDR video signals in the ICtCp color format
Methods to improve the quality of coding high-dynamic range (HDR) signals in the ICtCp color space are presented. Techniques are described to a) generate optimum chroma Offset and scaling parameters, b) compute chroma mode decisions by optimizing mode selection distortion metrics based on chroma saturation and hue angle values, and c) preserving iso-luminance by maintaining in-gamut values during chroma down-sampling and chroma up-sampling operations.
1. A method to maintain constant luminance or iso-luminance during chroma processing of a video sequence, the method comprising:
receiving input pixel values in a first color format;
converting the input pixel values from the first color format to first pixel values in a second color format, wherein in the second color format a pixel value comprises a luma value, a first chroma value, and a second chroma value;
determining whether first chroma values or second chroma values of the first pixel values in the second color format are within a specified color gamut, and for one or more chroma pixel values of the first pixel values in the second color format that are not within the specified color gamut:
generating adjusted chroma pixel values within the specified color gamut in the second color format, wherein generating the adjusted chroma pixel values comprises:
for a chroma pixel value outside of the specified color gamut:
computing a saturation value and a hue angle value based on the first chroma value and the second chroma value of the chroma pixel;
generating a scaled saturation value based on the saturation value, wherein the scaled saturation value is within the specified color gamut; and
adjusting the first chroma value and the second chroma value based on the scaled saturation value and the hue angle value to generate an adjusted first chroma value and an adjusted second chroma value, wherein generating the adjusted first and second chroma pixel values (Ct 1 , Cp 1 ) comprises computing
Ct 1 =sat 1 *cos(θ), and
Cp 1 =sat 1 *sin(θ),
where θ denotes the hue angle value for the chroma pixel and sats denotes the scaled saturation value of the chroma pixel; and
generating output pixel values in the second color format comprising the luma value of the first pixel values and the adjusted chroma pixel values.
2. The method of claim 1 , wherein generating the scaled saturation value (sat 1 ) comprises computing
sat 1 =sat 0 *α,
where sat 0 denotes the saturation value, and α is a scaling factor.
3. The method of claim 1 , further comprising performing chroma mode decisions based on the adjusted chroma pixel values.
4. An apparatus comprising a processor and configured to perform the method recited in claim 1 .
5. A non-transitory computer-readable storage medium having stored thereon computer-executable instructions for executing with a processor a method in accordance with claim 1 .