Of itu-t sg16 wp3 and iso/iec jtc1/SC29/WG11

JCTVC-M0266 AHG9: On cross-layer alignments in HEVC 3DV and scalable extensions [Y.-K. Wang, A. K. Ramasubramonian, K. Rapaka, J. Chen, V. Seregin, Y. Chen (Qualcomm)]

This document discusses cross-layer alignment of IRAP pictures, IRAP picture types, leading pictures, relative POC values, "GOP structures", TSA pictures and STSA pictures, and proposes to add some related bitstream constraints or to relax some other related bitstream constraints.

JCT3V-D0046 contains basically the same content as this document.

M0096 and M0168 were noted to be related.

Currently specified to require IDR and BLA pictures to be aligned across layers, but not necessarily CRA pictures.

It was noted that there is an interaction with ultra-low-delay HRD, which is disallowed for CRA, such that changing an IDR to a CRA for purposes of escaping the alignment constraint would disable ultra-low-delay HRD operation.

The contribution proposes to remove that constraint for IDR pictures. Something similar is proposed in M0207 (using a different name for the "unconstrained IDR" pictures).

It was remarked that it seems important to enable "upswitching" with SPS activation in an enhancement layer without having an IDR in the base layer.

The contributor requested consideration of whether a bitstream could begin with a CRA or BLA in the base layer without an IRAP at the enhancement layer, and noted that this was discussed in M0206 and M0207. It was noted that the POC alignment constraint conflicts with this possibility, although it seems that we do not really need that constraint. See discussion of M0206 and M0207.

Generally, we plan to impose some significant constraints on alignment of cross-layer alignment of IRAP picture types, leading pictures, relative POC values, "GOP structures", and TSA pictures and STSA pictures. However, the specifics are deferred for further study.