HEVC/H.265 supports a lightweight form of slicing – entropy slice or dependent slice segments. Unlike to regular slices, entropy slices are not self-contained:
- Intra and MV prediction is not broken across slice boundaries.
- CABAC context models are not reset to defaults at the start of each entropy slice (context models of the previous slice are taken). However, CABAC engine per se is flushed and reset in order to make the slice to end at byte boundary.
Notes:
- entropy slice carries a shortened slice header, missing parameters are taken from the preceding regular slice header (a regular slice must present, the first slice of a frame must be regular).
- entropy slice is specified by the flag dependent_slice_segment_flag in slice header.
Thus, due to spatial predictions and CABAC dependencies between successive entropy slices, the entropy slicing is not tailored for enhancement of error resilience. The purpose of the entropy slicing is to enable a generalized wavefront parallelization and/or ultra-low latency.
Use case of the entropy slicing: How realize wavefront processing with entropy slices?
Each frame is divided into entropy slices (slice = CTU row). Due to dependendies in CABAC context models between slices, CABAC should be decoupled from the main processing.
1. The first thread starts processing (syntax elements after transform and quantization are sent directly to CABAC).
2. Upon the first thread completed several CTUs, the second thread starts the second CTU row (data for prediction is available). Syntax elements generated by the second thread are buffered since CABAC can’t start processing until all syntax elements of the first slice coded (context models are not reset).
3. Upon the second thread completed several CTUs, the third thread starts the third CTU row. Syntax elements generated by the third thread are buffered (CABAC will take them after encoding the second slice)
Ultra-low latency is achieved if CABAC immediately sends just completed slice.
23+ years’ programming and theoretical experience in the computer science fields such as video compression, media streaming and artificial intelligence (co-author of several papers and patents).
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