| JCTVC-J0255 AHG9: Slice prefix for sub-picture and slice level HLS signalling [T. Schierl, V. George, R. Skupin, D. Marpe (HHI)]
(This was initially reviewed in the AHG9 meeting, where discussion of this was chaired by Y.-K. Wang and G. Sullivan.)
In this document it was proposed to apply a functionality like SEI messages as well as additional high level syntax items, beyond the ones included in the NAL unit header, on a per slice level basis. Such messages were proposed as a "slice prefix NAL unit". Syntax and semantics of the slice prefix and slice-level/sub-picture SEI messages, use cases for low delay/sub-picture CPB operations, tile signalling and region of interest (ROI) signalling were described.
Y.-K. Wang expressed a basic understanding of the proposal prior to the availability of a more formal indication of a cross check.
Several potential sub-picture-level SEI messages were described.
It was commented that during the design of AVC the possibility was discussed to have slice-specific SEI messages or otherwise not require SEI messages to be at the beginning of the picture, but this was not done (for simplicity) because there was no clear need for it at the time.
It was commented that even today we still have reserved NUTs in AVC that could be used for such a purpose.
It was commented that we could just allow SEI NUT to follow the first VCL NALU of the picture, rather than using a different NUT for the slice level SEI than for the picture-level SEI.
Some participants commented that ultra-low-delay decode could benefit from definition of a timing SEI specification.
A participant suggested to also enable some SEI type of NALU that could follow after the relevant VCL NALU or the AU rather than precede it – e.g., for checksum data.
The general concept was supported by most participants in the AHG, and there was further discussion in the JCT-VC meeting.
Possible concepts:
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Allow ordinary SEI NUT to follow the first VCL NAL unit and precede the last VCL NAL unit.
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Define a prefix SEI NUT (or more than one of these, or combined with above concept).
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Define a suffix SEI NUT.
For items 2 & 3 above, we already have NUTs defined that have these properties – all that remains would be to "unreserve" some – which does not necessarily need to be done in the first edition of the standard.
For approach 1, we would need to affect edition 1.
This proposal had some specific proposed slice-level SEI messages.
The first of these seemed to have the most interest expressed by participants. Some related proposals are discussed in section 5.12.8.1. Further study of the others was encouraged.
Proper and complete text was needed for the sub-picture timing topic.
This was further discussed. Text was provided (uploaded as v2 of the contribution) that relaxed the order constraint on SEI message to allow them to appear between slices, and defined a sub-picture timing SEI message. Decision: Adopted.
It was remarked that existing SEI messages that have whole-picture scope should be constrained to appear before the first slice in the picture (e.g. in their semantics). Decision: Agreed.
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