Chroma Intra Mode Derivation for Non-Intra Luma Blocks
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Solution Overview
Problem
In HEVC, when the derived-mode (DM) is used for a chroma block, the luma Intra mode used by the upper-left minimum CU of the corresponding luma CU area may not be Intra mode coded, leaving the chroma DM Intra prediction undefined.
Innovation Solution
A method to determine a corresponding luma block area for a chroma block, identify candidate luma coding modes from sub-blocks of this area, and derive a chroma Intra mode based on these modes, ensuring valid Intra prediction even when the corresponding luma block is non-Intra coded.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If the upper-left minimum CU of the corresponding luma CU area is used to determine chroma DM Intra mode, then the chroma prediction can be derived from luma mode, but the luma mode may not be Intra mode coded, leaving the chroma DM Intra prediction undefined
Solution Approach 1:
The patent checks the luma coding mode of the upper-left minimum CU in advance before using it for chroma DM prediction. If the luma mode is not Intra mode coded, the patent performs preliminary actions to select an alternative Intra prediction mode for chroma, ensuring the prediction is defined and reliable before proceeding with encoding.
Solution Approach 2:
The patent introduces an intermediary checking mechanism that verifies whether the luma mode is suitable for chroma DM prediction. When the luma mode is not appropriate (not Intra mode coded), the intermediary mechanism selects an alternative chroma Intra prediction mode, acting as a mediator between the luma mode and chroma prediction to ensure compatibility and reliability.
2Device complexity
If the luma Intra mode is always used for chroma DM prediction, then the coding process is simple, but it fails when the luma block is non-Intra coded
Solution Approach 1:
The patent implements a dynamic prediction mode selection mechanism. Instead of always using the luma Intra mode, the system dynamically checks the luma coding mode and adapts by selecting an appropriate chroma Intra prediction mode based on the actual luma mode, allowing the system to handle different luma coding scenarios effectively.
Solution Approach 2:
The patent changes the prediction mode parameter based on the luma coding mode. When the luma mode is not Intra mode coded, the system changes the chroma prediction approach from direct luma mode copying to alternative Intra prediction mode selection, adapting the prediction parameters to match the actual coding conditions.
3Reliability
If alternative methods are used to handle non-Intra coded luma blocks, then chroma prediction reliability is improved, but the coding complexity increases
Solution Approach 1:
The patent applies partial checking action by only verifying the luma mode of the upper-left minimum CU, which is the most critical case. This partial approach provides sufficient reliability improvement for the most important scenario without performing exhaustive checks on all CUs, thereby limiting the increase in coding complexity to only what is necessary.
Data Source
AI summary
A method and apparatus for video coding of chroma coded in derived-mode Intra prediction are disclosed. If derived-mode (DM) Intra prediction is selected for a current chroma block, a corresponding luma block area is determined by locating a first upper-left pixel address of the corresponding luma block area corresponding to that of the current chroma block. Further, one or more candidate luma coding modes are determined from a sub-block set consisting of one or more sub-blocks of the corresponding luma block area, where if the sub-block set consists of only one sub-block, the sub-block set excludes a corner sub-block located at a top-left corner of the corresponding luma block area. Additionally, a current chroma Intra mode for the current chroma block is derived based on information comprising the candidate luma coding modes, and Intra-prediction encoding or decoding is applied to the current chroma block using the current chroma Intra mode derived.