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 tenth meeting during 11-20 July 2012 at the City Conference Centre (ccc), a k. a

JCTVC-J0314 Cross-verification of "An experimental comparison of memory bandwidth between P and B-slice coding" (JCTVC-J0166) [S. Fukushima, H. Nakamura (JVC Kenwood)]
JCTVC-J0175 AHG7: Memory bandwidth limitation for higher level [K. Kondo, T. Suzuki (Sony), U. Motoharu, S. Fukushima (JVC Kenwood Corporation)]

This contribution proposes to restrict the bi-prediction 8x8 with the control flag in SPS. In current HM 7.0 design, bi-prediction 8x4 and 4x8 is replaced to uni-prediction for all levels. The limitation for memory bandwidth is no problem for HD sequences, but it may not be enough for higher level in the future. So this contribution presents that the encoder send a flag whether bi-prediction 8x8 is replaced or not, and the decoder replaces the inter direction for also 8x8 as according to the flag. The flag is sent in sequence parameter sets (SPS). In this experiment, it is shown that with this technique the coding efficiency loss is decreased.

A flag is proposed to prohibit 8x8 bipred by inferring list 0 prediction, conditioned on an additional SPS flag, as done for 4x8/8x4. Having the flag improves coding efficiency relative to encoder-only avoidance of bi-pred at the 8x8 level. It was suggested that the use of this constraint be considered for large picture formats (e.g. in profile/level conformance requirements).

The author indicated that the loss of compression for Class A sequences, relative to allowing 8x8 bi-prediction, can be characterized as follows:

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  Prohibiting 8x8 as an encoder-only constraint results in a loss of 1–2%.
  
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  Changing the entropy coder to remove the ability to signal 8x8 bipred results in a loss of about 0.5%.
  

Some concern was expressed about the burden of being able to support two ways of entropy coding the mode selection in order to support 8x8 in lower-level bitstreams while not allowing it in higher-level bitstreams.

The topic was further discussed.

No action taken. If a level-based constraint is desirable in some high level, to avoid the memory bandwidth, it can be imposed as a bitstream-only constraint without effect on the decoding process.