Coding of HDR Video Signals in the ICtCp Color Format
AssigneeDolby Laboratories Licensing Corporation
InventorsTaoran LU, Fangjun PU, Peng YIN, Tao CHEN
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Abstract
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.
Claims19
In a video encoder, a method to generate chroma offset parameters for chroma quantization, the method comprising:
- determining target chroma bit rates for a first chroma component and a second chroma component;
- for a chroma quantization model, given a set of model parameters comprising a luminance base quantization parameter (baseQP), a first subset of chroma offset parameters, and a second subset of chroma offset parameters:
- setting the second subset of chroma offset parameters to zero, and
- for one or more video sequences, computing actual chroma bit rates for one or more values of the baseQP parameter and one or more values of the first subset of chroma offset parameters;
- for the one or more video sequences, computing based on the target chroma bit rates an ideal mapping between the one or more values of the baseQP parameter and values of the first subset of chroma offset values;
- for values of the set of model parameters and based on the chroma quantization model, computing a target mapping between the one or more values of the baseQP parameter and values of the first subset of chroma offset values;
- and
- generating an optimum set of model parameters by minimizing an error between the ideal mapping and the target mapping.
In a video encoder, a method to improve chroma mode decision, the method comprising:
- receiving luminance and chroma pixel values for one or more pictures in a video sequence, wherein a chroma pixel value comprises a first chroma pixel value and a second chroma pixel value;
- computing saturation and hue angle values for the chroma pixel values based on the chroma pixel values;
- and determining an output chroma mode based on a cost function comprising a cost parameter based on the saturation and hue angle values for the chroma pixel values.
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;
- 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.
Dependencies are inferred from the claim text.