Backward-Compatible HDR to SDR Encoding via Luma-Dependent Chroma Correction
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Solution Overview
Problem
Current methods for encoding HDR pictures to SDR often result in color shifts and quality degradation, particularly in preserving color appearance values like hue and saturation during the dynamic range reduction process.
Innovation Solution
A method and device that map HDR pictures to SDR, determine color correcting parameters to preserve specific color appearance values, and encode the color-corrected SDR pictures, using techniques such as inverse EOTF, color space conversion, and adaptive transfer functions to maintain perceptual quality and backward compatibility.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If HDR picture is mapped to SDR picture to achieve backward compatibility, then compatibility with SDR devices is improved, but color appearance values (hue and saturation) are distorted
Solution Approach 1:
The patent applies color correction in the luma domain before encoding, using pre-computed correction factors stored in lookup tables. This preliminary correction of chroma components based on luma values prevents color distortion from propagating through the encoding process, thereby preserving color appearance while maintaining SDR compatibility
Solution Approach 2:
The patent modifies chroma parameters (Cb and Cr components) based on luma parameters using correction factors. By changing the chroma parameters as a function of luma values through the formula: corrected_chroma = original_chroma + correction_factor[luma_index] * (original_luma - reference_luma), the patent compensates for color shifts introduced by HDR-to-SDR mapping
2Manufacturing precision
If color correction is applied to preserve color appearance, then color accuracy is improved, but encoding complexity increases
Solution Approach 1:
The correction factors are pre-computed and stored in lookup tables during encoding setup. This preliminary preparation avoids complex real-time calculations during the actual encoding process, reducing computational complexity while maintaining color accuracy
Solution Approach 2:
The patent uses simple parameter adjustments rather than complex color transformation matrices. By modifying only the chroma parameters based on luma values with a straightforward correction formula, the patent achieves color accuracy with minimal additional computational overhead compared to full color space transformations
3Adaptability or versatility
If dynamic range is reduced from HDR to SDR, then compatibility is improved, but perceptual quality deteriorates
Solution Approach 1:
The patent applies different correction factors to different luma ranges using lookup tables indexed by luma values. This local correction approach tailors the color correction to specific brightness regions, preserving perceptual quality in different parts of the image while adapting to SDR display characteristics
Solution Approach 2:
The patent introduces an intermediate corrected SDR picture as a mediator between the original HDR picture and the final encoded SDR stream. This intermediate representation preserves color appearance through luma-domain correction before encoding, ensuring perceptual quality is maintained in the compatibility layer
Data Source
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AI summary
A method for coding an HDR picture is disclosed that comprises: - mapping the HDR picture to obtain a SDR picture; - determining color correcting parameters; - color correcting said SDR picture responsive to said color correcting parameters; and - encoding the color corrected SDR picture in a stream. In the method, determining color correcting parameters comprises: - determining an intermediate SDR picture so that at least one color appearance value is preserved between said HDR picture and said intermediate SDR picture; - determining the color correcting parameters from said intermediate SDR picture and from said SDR picture.