Chroma Adjustment in Video Coding Color Spaces
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Solution Overview
Problem
In video coding, severe artifacts in saturated colors occur due to nonlinear transfer functions and 4:2:0 subsampling, making it challenging to maintain accurate chroma values, especially when converting between different color representations like Y′Cb′Cr′ and RGB, which affects compression efficiency and visual quality.
Innovation Solution
A method is introduced to process pixels by setting upper and lower limits for color components in one color space to obtain a filtered value that is within a specific range in another color space, ensuring the color remains perceptually equivalent, thus reducing artifacts and improving encoding efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If nonlinear transfer function and 4:2:0 subsampling are used in video coding, then compression efficiency is improved, but severe artifacts appear in saturated colors and chroma values become inaccurate
Solution Approach 1:
The patent applies preliminary action by processing chroma values before the nonlinear transfer function and subsampling operations. Specifically, it adjusts chroma values in advance to compensate for the artifacts that will be introduced during encoding, ensuring that the final reconstructed colors remain accurate despite the use of nonlinear transfer functions and subsampling.
Solution Approach 2:
The patent changes the parameter representation by working in a different color space (Yu'v' instead of Y'CbCr) where the chroma components are transformed to be less sensitive to luminance variations. This parameter transformation allows the chroma values to be encoded with fewer bits while maintaining color accuracy, resolving the contradiction between compression efficiency and chroma value accuracy.
2Measurement precision
If luma adjustment is applied to correct luminance artifacts, then luminance accuracy is improved, but the complexity of the encoding process increases
Solution Approach 1:
The patent merges the luma adjustment operation with the chroma processing by performing both adjustments in a unified manner within the same color space transformation framework. Instead of separately adjusting luma and chroma components, it processes them together through the Yu'v' color space conversion, reducing the overall complexity while maintaining luminance accuracy.
Solution Approach 2:
The patent introduces an intermediary color space (Yu'v') that mediates between the original Y'CbCr space and the final output space. This intermediary space allows for more efficient and less complex adjustments to be made, as the transformation matrix and processing operations are optimized for this intermediate representation, reducing the computational burden compared to direct adjustments in the original color space.
3Reliability
If chroma values are made smoother and more consistent to reduce artifacts, then visual quality is improved, but the ability to represent saturated colors accurately may be compromised
Solution Approach 1:
The patent applies local quality by differentiating the processing based on the local characteristics of the image. It identifies regions with high color saturation and applies stronger smoothing only to those regions, while preserving the original chroma values in regions where saturation is already low or where artifacts are not present. This selective approach maintains visual quality improvement while preserving color saturation accuracy where needed.
Solution Approach 2:
The patent introduces dynamic adjustment by adapting the degree of chroma smoothing based on the local luminance and chroma characteristics of each pixel or block. The processing strength varies dynamically according to the content, applying more aggressive smoothing to regions prone to artifacts while maintaining color accuracy in stable regions. This dynamic behavior allows the system to optimize between visual quality and color saturation accuracy on a per-region basis.
Data Source
AI summary
A processing for a first pixel in a picture comprises setting upper and/or lower limits of a first color component of the first pixel in a first color space to a respective fixed value if a maximum and/or minimum value of the first color component in the first color space results in a color component in a second color space that is below a first value of the color component in the second color space and above a second value of the color component in the second color space. A filtered value is obtained of the first color component and which is equal to or larger than the lower limit and equal to or lower than the upper limit. The processing results in filtered values that are cheaper to encode but that are visibly undistinguishable from the original colors of the pixels.