JVET-P0407 AHG15: cu_qp_delta_subdiv and cu_chroma_qp_offset_subdiv syntax dependency removal [J. Chen, R.-L. Liao, J. Luo, Y. Ye (Alibaba)]
In VVC draft 6, cu_qp_delta_subdiv and cu_chroma_qp_offset_subdiv are signaled in PPS, while the range of these two syntax elements depends both on SPS and slice level information, which causes a syntax dependency issue. This contribution proposes to remove the syntax dependency by moving cu_qp_delta_subdiv and cu_chroma_qp_offset_subdiv to slice header.
It is asserted that keeping the syntax at PPS is not really necessary. In case when locally adaptive QP is used, the necessary overhead at block level would be much larger than the signalling of few parameters for each slice.
For the case of a picture containing both intra and inter slices, it would be desirable controlling the parameters independently. On the other hand, if there are multiple slices per picture, the overhead might again grow large.
With the new design of the picture header, it is suggested to better put the parameters there. Two independent sets of parameters should be defined for inter and intra slices, which would then be used for each slice of the respective type in the given picture. The dependency on mtt depth would also be resolved, as it is reported that this has already been decided to be moved from slice to picture header.
Decision: Adopt JVET-P0407, but as a subsequent action related to HLS design moving the parameters from slice header to picture header as stated above.