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-J0239 AHG10 - Selective inter-layer prediction signaling for scalable extension [J. Xu (Microsoft)]

This document proposes a flag that is equivalent to the discardable_flag in SVC to be included in the NAL unit header of the HEVC base specification, and proposes to change the 5-bit reserved_one_5bits to be 4-bit reserved_one_4bits.

It was commented that the addition of enhancement layers later may need change of the value of the flag in the base layer. It was remarked by another expert that the use case of adding enhancement layers later on does not make much sense.

It was suggested to use NAL unit types instead of using a bit in the NAL unit header, as using of NAL unit types is equivalent to using of a fraction of a bit.

It was remarked that the proposed flag can be useful and that this is why the discardable_flag was included in the SVC NAL unit header. However, during the SVC development, there were 24 additional bits to consider what fields could be included in the NAL unit header extension, while now there are only 8 additional bits. In any case, the bits used by reserved_one_5bits should not be reduced anymore, as 4 bits would be too few to represent layer IDs in future extensions. Adding one more byte in the NAL unit header is an option, but that would make sense only if there are sufficient useful information piece to be included in the NAL unit header to justify yet one more byte. Getting rid of another bit in the current NAL unit header is yet another option, but it was questioned whether there any other bit currently in the NAL unit header less important than the proposed flag? It seems nothing besides nal_ref_flag, which is being proposed to be removed by multiple proposals. However, that bit could be used for multi-standard extension support, for which the requirement has been specified in MPEG, or to have 6 bits for the layer ID space for future extensions, both seem to be more important than having a discardable flag. Moreover, entire layer discardability (e.g. for simulcast) can be better indicated by layer dependency information, and individual layer representation discardability can be indicated by non-required layer representation SEI message in SVC.

Further study was encouraged, to study whether there are any more information pieces that should be put into the NAL unit header, whether equivalent information as present in SVC and MVC NAL unit headers should be present in future HEVC exensions, and consider whether we should have one more byte for the NAL unit header.