JVET-Y0190 AHG2/AHG8: Suggestions for the operation range extensions GCI [J. Gan, Y. Yu, H. Yu (OPPO)] [late]
It is asserted that the current text for general constraints information (GCI) in VVC version 2 contains two bugs. For the first bug, it is asserted that incorrect semantics and handling of gci_num_additional_bits lead to GCI decoding behaviour in VVC version 2 incompatible with VVC version 1 decoders. For the second bug, it is asserted that setting gci_present_flag to 1 and gci_num_additional_bits to 0 leads to undefined constraint behaviour for the 6 coding tools which are governed by the 6 additional GCI flags introduced in VVC version 2.
It is additionally asserted that VTM 15.0 contains a bug related to decoding of GCI as reported by JVET-Y0005.
The same issue is raised as in JVET-Y0049 (aspect 2).
Beyond that, it is pointed out (aspect 1) that a problem might occur when the gci_num_additional_bits has another value than 0 or 6 in version 2. The solution proposed appears appropriate.
Aspect 3 points out a software bug. Aspect 1 and 3 together are likely causing the problem with the conformance bitstreams.
New conformance streams need to be generated with all 6 GCI flags.
In the discussion, it is also mentioned that it might be desirable to rename the syntax element gci_reserved_zero_bit, as it could in have values of 0 or 1. This may however not be necessary.
Side activity with relevant experts, developing an integrated text to fix the bugs.
Follow-up discussion in session 9 Fri 14 Jan. No consensus yet on the question if a decoder should expect that only values 0 or 6 would appear in a bitstream, i.e. if a decoder action for values 1…5 should be defined. If such action would not be defined, values 1…5 would need be reserved forever.
It is mentioned that the suggested solution “aspect 1a” implies values 1..5 to be ignored.
Both methods should not be difficult to describe.
One argument is to retain decoder conformance of main profiles consistent across versions. Minhua Zhou will provide a text that accommodates this and further discuss with other experts.
JVET-Y0190 AHG2/AHG8: Suggestions for the operation range extensions GCI [J. Gan, Y. Yu, H. Yu (OPPO)] [late]
See section 4.2.