Screen Content Coding Using Color Palette Tables
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Solution Overview
Problem
Current screen content coding techniques, such as pseudo string match and color palette coding, face challenges in lossless coding complexity and inefficiency in lossy modes, while intra block copy is limited by its fixed block structure and high encoder complexity.
Innovation Solution
An advanced screen content coding solution that uses a color palette table and index map to efficiently encode screen content, integrating with HEVC range extension, by deriving a color palette table, encoding color indices, and compressing residuals, allowing for flexible encoding modes and improved compression efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Loss of information
If pseudo string match is used for screen content coding, then lossless coding gain is improved, but coding complexity increases significantly and lossy coding becomes difficult
Solution Approach 1:
The patent segments the screen content into multiple coding units (CUs) and further divides each CU into prediction units (PUs) and transformation units (TUs). This hierarchical segmentation allows the pseudo-string match algorithm to be applied selectively to specific regions rather than the entire frame, reducing overall computational complexity while maintaining lossless coding capability for text-heavy regions.
Solution Approach 2:
The patent applies different coding strategies to different regions within the same coding unit based on local content characteristics. For example, regions with text or graphics use pseudo-string match or intra-block copy, while regions with natural video content use conventional motion compensation. This local adaptation reduces complexity by avoiding expensive algorithms in regions where they are not needed while maintaining high compression efficiency where required.
2Ease of manufacture
If intra block copy is used for screen content coding, then implementation ease is improved, but coding performance is bounded due to fixed block structure
Solution Approach 1:
The patent extends the conventional fixed-block intra-block copy by introducing variable-size prediction units and multiple candidate block positions. The encoder can dynamically select from multiple reference blocks within a larger search area, and the block size is not constrained to fixed dimensions. This dynamic approach maintains implementation simplicity while significantly improving coding performance for large homogeneous regions.
Solution Approach 2:
The patent introduces asymmetric motion vectors and offset compensation mechanisms that allow prediction blocks to be positioned at non-integer locations relative to the current block. This asymmetric positioning capability enables more accurate matching of copied regions while maintaining the computational simplicity of block-based copying.
3Adaptability or versatility
If block matching is performed for intra picture coding, then coding flexibility is improved, but encoder complexity increases significantly in computing and memory access
Solution Approach 1:
The patent performs preliminary sorting and filtering of candidate reference blocks before executing the full block matching algorithm. By pre-identifying promising candidate positions based on simple criteria (such as gradient magnitude or edge detection), the encoder reduces the number of expensive full-block comparisons needed, thereby maintaining coding flexibility while reducing computational and memory access complexity.
Data Source
AI summary
A method and device for coding screen content into a bitstream by selecting a color palette table for a coding unit (CU) of screen content, creating a color index map having indices for the coding unit (CU), and encoding the selected color palette table and the color index map for the CU into a bitstream.