The Joint Collaborative Team on Video Coding (jct-vc) of itu-t wp3/16 and iso/iec jtc 1/sc 29/wg 11 held its second meeting during 21-28 July, 2010 at the itu-t premises in Geneva, ch

JCTVC-B113 [A. Segall, Y. Su (Sharp Labs)] Codeword restrictions for improved coding efficiency

It was proposed to modify the clipping points that follow the motion compensation and adaptive loop filtering processes. In terms of performance, improvements were reported for image sequences with high contrast but with sample values that do not cover the full input dynamic range. For example, it was reported that the BQSquare sequence coding performance was improved by 3% and 1.9% for Hierarchical-B and IPPP coding, respectively, with the modified clip points. It was suggested that this be considered a bug fix for the HEVC design. The proponent recommended adoption of the technique into the TMuC.

This was part of proposal JCTVC-A105.

It was asked whether adding clipping as a post-processing stage would give the same performance, and the proponent indicated that although that provides some significant portion of the benefit, it helps to include the clipping within the loop.

It was remarked that similar clipping is included in the JCTVC-B077 scheme.

The average gain was not reported, but was estimated by the authors of JCTVC-B077 to be about 0.5%.

Proposed syntax is to put the clipping in the picture parameter set level.

It was remarked that having the special flag for the [16..235] range may not be advisable. The name "codeword" may also be somewhat confusing. The position of the codeword_restrict_sameC_data_flag should perhaps be shifted to after the codeword_restrict_maxCr element.

It was suggested that this has somewhat similar functionality to the JCTVC-A124 approach for content adaptive dynamic range (CADR).

Encoder specifies the value limits (low/high) for each frame. Clipping is performed based on this at the output of the MC as well as the output of the loop filter. Gain reported as roughly 3% BR reduction for BQSquare.

Note: To avoid latency, it may be necessary to use the range data of the last frame.

Relationship with JCTVC-A124 dynamic range expansion and clipping – JCTVC-B113 seems to be simpler.

Some concern raised about complexity even of the clipping which requires 2 compare operations per sample.

Explore in CE: Performance over entire set; comparison against only output clipping (outside loop), and clipping only at MC output (not at the loop filter). Things to study include average gain, output (post-decoding) clipping, clipping only in one of the two places versus in both places.