Chroma Intra Prediction Signaling Using Luma Context Selection
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Solution Overview
Problem
In AV1 video coding format, the semi decoupled partitioning (SDP) scheme results in luma and chroma blocks having different partitions, leading to suboptimal chroma intra prediction mode signaling, as the top-left position of the chroma block is not always the best location for corresponding luma mode, and luma modes within the super block are not effectively exploited for better codewords.
Innovation Solution
Identify contexts for entropy coding chroma intra prediction modes based on a co-located luma block at predefined positions, using quantized luma modes and flags to enhance signaling efficiency, and construct a list of previously encoded luma modes for improved chroma intra prediction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If the top-left position of the chroma block is used for corresponding luma mode in SDP scheme, then the signaling process is simplified, but the chroma intra prediction mode signaling becomes suboptimal because the top-left position is not always the best location
Solution Approach 1:
The patent applies local quality by selecting different reference positions within the luma block based on the specific chroma block location and prediction mode. Instead of always using the top-left position, the system identifies the most appropriate local position (e.g., top-left, top-right, bottom-left, or center) that best corresponds to the chroma block, thereby optimizing prediction accuracy for each local case while maintaining overall system efficiency.
2Device complexity
If luma modes within the super block are not exploited, then the encoding process is simpler, but the chroma intra prediction mode signaling efficiency is reduced
Solution Approach 1:
The patent applies preliminary action by pre-constructing a list of candidate luma modes from previously encoded luma blocks within the super block. This list is prepared in advance and made available for chroma prediction mode selection, allowing the decoder to efficiently choose from pre-processed options without performing complex real-time analysis, thereby improving signaling efficiency while controlling complexity.
Solution Approach 2:
The patent introduces an intermediary structure (the list of previously encoded luma modes) that mediates between the luma blocking structure and chroma prediction. This intermediary list serves as a bridge, allowing chroma blocks to leverage luma mode information from multiple positions within the super block without requiring direct complex interactions, thus improving efficiency while managing system complexity.
3Adaptability or versatility
If semi decoupled partitioning is used, then luma and chroma blocks can have different partitions for better adaptability, but the correspondence between chroma block and luma mode becomes less optimal
Solution Approach 1:
The patent resolves the SDP correspondence issue by applying local quality - selecting reference positions locally within the luma block based on the specific chroma block configuration. This allows the system to maintain the adaptive partitioning benefits of SDP while locally optimizing the chroma-luma mode correspondence for each block pair, thereby preserving reliability despite the decoupled structure.
Data Source
AI summary
An example method of video encoding includes receiving a video data that includes a current chroma block, and selecting an entropy encoding context for entropy encoding one or more parameters of the current chroma block based on a number of neighboring blocks of the current block that use a chroma-from-luma (CfL) mode. Selecting the context includes, when the number of neighboring blocks that use the CfL mode is less than a threshold number, selecting a first context as the entropy encoding context; and, when the number of neighboring blocks that use the CfL mode is greater than the threshold number, selecting a second context as the entropy encoding context, wherein the second context is different than the first context. The method further includes entropy encoding a first flag using the entropy encoding context, where the first flag indicates whether the CfL mode is enabled for the current chroma block.