Chroma Prediction Direction Correction for 4:2:2 Image Decoding
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Solution Overview
Problem
In image decoding, the luma prediction mode reuse is inefficient when the color format of the input image is 4:2:2, as it fails to accurately reflect the size relationship between the luma and chroma domains, leading to inaccuracies in prediction image generation.
Innovation Solution
An image decoding apparatus that corrects the prediction direction for chroma prediction images by using a prediction mode corresponding to the gradient with respect to the main direction in the intra-prediction method, accounting for the difference in pixel shape between luma and chroma pixels, and adjusts the prediction mode for chroma pixels based on the size ratio to improve accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If luma prediction mode reuse is applied to chroma prediction in 4:2:2 color format, then coding efficiency is improved through mode reuse, but prediction accuracy deteriorates due to mismatched prediction directions between luma and chroma domains
Solution Approach 1:
The patent applies local quality by making the chroma prediction process adaptive to local characteristics of the color format. When 4:2:2 format is detected, the system locally adjusts the prediction direction calculation to account for the different sampling structure, rather than applying a uniform prediction approach across all color formats. This localized adaptation resolves the contradiction by maintaining mode reuse efficiency while correcting prediction accuracy for the specific 4:2:2 chroma subsampling characteristics.
Solution Approach 2:
The patent changes the prediction direction parameter based on the color format type. For 4:2:2 format, it modifies the prediction direction calculation by applying a scaling factor or adjustment to account for the different chroma pixel density and arrangement. This parameter change allows the same luma prediction mode to be reused effectively while adapting the prediction geometry to match the chroma domain characteristics, thereby maintaining both coding efficiency and prediction accuracy.
2Device complexity
If the same prediction direction is used for both luma and chroma domains, then implementation complexity is reduced, but prediction precision deteriorates in 4:2:2 format due to different pixel shape relationships
Solution Approach 1:
The patent introduces dynamics by making the prediction direction calculation adaptive rather than static. The system dynamically adjusts the prediction direction based on the detected color format, using different calculation methods for 4:2:0 versus 4:2:2 formats. This dynamic approach maintains low implementation complexity through automated format detection and conditional processing, while achieving high prediction precision by selecting the appropriate prediction direction calculation for each format type.
Solution Approach 2:
The patent changes the prediction direction parameter based on the color format type. For 4:2:2 format, it modifies the prediction direction calculation by applying a scaling factor or adjustment to account for the different chroma pixel density and arrangement. This parameter change allows the same luma prediction mode to be reused effectively while adapting the prediction geometry to match the chroma domain characteristics, thereby maintaining both coding efficiency and prediction accuracy.
Data Source
AI summary
In a case that a pixel shape in a luma domain is not similar to a pixel shape in a chroma domain in an input image, a prediction image that is generated in the reuse of a luma prediction mode is made more accurate.A moving image decoding apparatus 1 includes a gradient deriving unit 1453B that references a chroma gradient definition DEFANG1C, and derives a chroma prediction direction from a prediction mode based on a luma prediction direction.