Intra Block Copy Prediction Overlap Handling
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Solution Overview
Problem
The current intra block copy prediction mode in HEVC extensions faces inefficiencies in encoder-side decision-making and inefficient signaling of block vector values, leading to suboptimal performance in terms of rate-distortion and computational efficiency.
Innovation Solution
The proposed solution involves innovations in block vector prediction and encoder-side operations for intra block copy prediction, including estimating reconstructed sample values within overlap areas and using ping-pong approaches for BV prediction, which constrain the search range and improve coding efficiency.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If intra block copy prediction is performed at CU level with overlap between current CU and reference CU, then prediction can be performed using previously reconstructed sample values, but direct block copy fails due to unavailable sample values in overlap area
Solution Approach 1:
The current CU is divided into multiple TUs (transform units), and intra BC prediction is performed at the TU level rather than CU level. This segmentation allows the prediction process to work on smaller, manageable units where overlap handling becomes feasible. Each TU can be processed independently with its own prediction and reconstruction, resolving the conflict between using overlap areas and maintaining prediction reliability.
Solution Approach 2:
The patent performs preliminary reconstruction of sample values for TUs that precede the current TU in decoding order, even within the same CU. By the time the current TU is processed, the overlap area sample values from previously decoded TUs are already reconstructed and available in the buffer, enabling direct block copy to work correctly in overlap regions without failure.
2Ease of manufacture
If block vector values are signaled without prediction, then encoding is simple, but signaling efficiency is poor and bit rate increases
Solution Approach 1:
The patent implements a feedback mechanism where previously decoded BV values are stored in a buffer and used to predict current BV values. The encoder signals only the difference between actual and predicted BV values, significantly reducing the number of bits required. This feedback loop maintains encoding simplicity while dramatically improving signaling efficiency and reducing bit rate.
Solution Approach 2:
Instead of signaling absolute BV values directly, the patent transforms the parameter representation by signaling BV differences relative to previously decoded values. This parameter transformation reduces the magnitude and variability of signaled values, enabling more efficient entropy coding and reducing overall bit rate while maintaining the ability to reconstruct exact BV values at the decoder.
3Adaptability or versatility
If encoder makes decisions about intra BC prediction usage, then prediction mode selection is possible, but encoder-side decision-making is ineffective
Solution Approach 1:
The patent implements dynamic decision-making where the encoder evaluates multiple candidate BV values and selects the optimal one based on rate-distortion criteria. The decision process adapts to local content characteristics, switching between intra BC prediction and other prediction modes as appropriate. This dynamic approach improves both the effectiveness of prediction mode selection and the overall encoding efficiency.
Solution Approach 2:
The encoder performs self-service by automatically evaluating candidate BV values and making optimal prediction mode decisions without external intervention. The rate-distortion optimization process autonomously selects the best prediction mode and BV values based on the current block's characteristics, improving decision-making efficiency while maintaining adaptability to different content types.
Data Source
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AI summary
Innovations in encoder-side options for intra block copy ("BC") prediction mode facilitate intra BC prediction that is more effective in terms of rate-distortion performance and/or computational efficiency of encoding. For example, some of the innovations relate to estimation of sample values within an overlap area of a current block during block vector estimation. Other innovations relate to prediction of block vector ("BV") values during encoding or decoding using "ping-pong" approaches.