LMCS LUT Extension and Clipping for Video Reshaping
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Solution Overview
Problem
Existing video compression systems face inefficiencies due to deviations between forward and inverse mapping functions, particularly when these functions are uniformly sampled, leading to reduced reshaping performance and potential skewing of results.
Innovation Solution
The method involves determining a non-uniformly sampled inverse mapping function from a uniformly sampled forward mapping function by swapping coordinates of pivot points, interpolating between swapped pivot points, and aligning pivot points on a uniformly sampled grid to refine the inverse mapping function, while also replacing pivot points and clipping values to maintain accuracy and reduce deviation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a uniformly sampled inverse mapping function is used for simplicity, then implementation complexity is reduced, but mapping accuracy and compression performance deteriorate due to deviations from the forward mapping function
Solution Approach 1:
The patent changes the sampling parameter of the inverse mapping function from uniform to non-uniform, specifically positioning pivot points at swapped coordinates of the forward mapping function's pivot points. This parameter change allows the inverse mapping to better match the forward mapping characteristics, improving mapping accuracy while maintaining implementation feasibility through systematic pivot point selection and linear interpolation between points.
2Measurement precision
If pivot points are swapped to create non-uniformly sampled inverse mapping function, then mapping accuracy is improved, but computational complexity increases due to irregular sampling intervals
Solution Approach 1:
The patent segments the mapping function into multiple linear segments defined by pivot points. By dividing the continuous mapping function into discrete segments connected at pivot points, the complex non-uniform mapping can be implemented through simple linear interpolation within each segment, reducing computational complexity while maintaining accuracy.
Solution Approach 2:
The patent performs preliminary action by pre-defining the pivot points of the inverse mapping function based on the forward mapping function's pivot points. This preliminary setup of key control points allows the inverse mapping to be constructed systematically before actual video encoding/decoding operations, reducing runtime computational complexity.
3Device complexity
If uniform sampling is applied to both forward and inverse mapping functions, then implementation is simplified, but reshaping performance deteriorates due to inherent deviations between the two functions
Solution Approach 1:
The patent introduces asymmetry in the sampling approach: the forward mapping function uses uniform sampling while the inverse mapping function uses non-uniform sampling with pivot points swapped from the forward function. This asymmetric design allows each mapping function to be optimized for its specific role, improving overall reshaping performance while maintaining implementation feasibility.
Data Source
AI summary
Methods and apparatus provide mapping functions for video encoding or decoding by replacing a pivot point of a uniformly sampled inverse mapping function by another pivot point not located on an original non-uniformly sampled inverse mapping function. In one embodiment a last pivot point of the inverse mapping function is extended based on a previous slope whose curve is plotted between two previous pivot points or based on the preservation of the slope which is dual to the existing slope of the equivalent section in the forward mapping function. Syntax is provided that assists the mapping and inverse mapping and clipping.