JVET-U0073 AHG2: Errata on referencing of parameter sets [K. Sühring, R. Skupin, Y. Sanchez, T. Schierl (Fraunhofer HHI)]
In the discussion of merge request #1971 it was noted that the specification text was unclear regarding the availability of a suffix APS to the decoding process. It seems the specification text does not clearly express that a picture cannot refer to a suffix APS in the same PU. Ticket #1443 was filed for this issue. During further investigation it was found that there may be some unclarity about references to parameter sets in general. This contribution proposes text improvements to express previous meeting decisions more clearly.
It is proposed to add the following sentence to the semantics of each syntax element that is a reference to a parameter set ID (adapted to the appropriate syntax elements). For example, for sh_alf_cc_cr_aps_id:
If multiple APS with the same value of aps_adaptation_parameter_set_id exist in the bitstream, the PH refers to the APS with aps_adaptation_parameter_set_id equal to sh_alf_cc_cr_aps_id, that most closely precedes the VCL NAL unit in decoding order, which refers to the PH.
It was generally agreed that the specification needs to clarify this item. A problem may be that the spec could be interpreted such that first all related NAL units are parsed before reference is made. It might not be clear that the second parameter set with same ID which is not used yet should not be parsed before the decding of the current picture is done.
It was pointed out that currently a note exists which clarifies that the suffix APS NAL unit is only for usage with later pictures. Such a note, however, is not normative.
The proposed text was further improved (offline action item between proponents and editors, all relevant people were involved). The result is available in v3.
Decision: Adopt JVET-U0073 text in v3, to be put into JVET-U1004.