JVET-AC0132 AHG9: On design for region-based neural-network post-filter SEI message [Hendry, J. Nam, H. Jang, S. Kim, J. Lim (LGE)]

This contribution proposes signalling to support region-based application of neural-network post-filters. It is asserted that such capability is desirable for use-cases such as application of the filter for region of interest. The proposed signalling is based on the signalling proposed in JVET-AB0134 with the following modifications:

1. Specify a flag (i.e., nnpfc_region_based_flag) in NNPFC SEI message whether or not the neural-network post-filter (NNPF) may be used for region(s) within picture(s).
2. When nnpfc_region_based_flag is equal to 1, a list of candidate region size may be signalled in the NNPFC SEI message. This list of region size may be referred to from NNPFA SEI message by index.
3. Specify a two-bit indication (i.e., nnpfa_region_based_idc) in NNPFA SEI message with the following semantics:
   1. nnpfa_region_based_idc equals to 0 indicates that the activated NNPF applies to the whole picture.
   2. nnpfa_region_based_idc equals to 1 indicates that the activated NNPF applies to region(s) of the picture. The size of the region is derived from the list of candidate region size signalled in NNPFC SEI message and the location of the region is signalled in NNPFA SEI message.
   3. nnpfa_region_based_idc equals to 2 indicates that the activated NNPF applies to region(s) of the picture. For this mode, the location of the region and its size are signalled in NNPFA SEI message.
   4. nnpfa_region_based_idc equals to 3 is reserved.
4. The following constraints may need to be defined:
   1. When the value of nnpfc_region_based_flag is equal to 0, the value of nnpfa_region_based_idc shall be equal to 0.
   2. When there is no candidate region size signalled in NNPFC SEI message, the value of nnpfa_region_based_idc shall not be equal to 1.