JVET-AC0131 AHG9: On NNPFC SEI message repetition [Hendry, J. Nam, H. Jang, S. Kim, J. Lim (LGE)]

In this contribution, it is asserted that the current semantics of NNPFC SEI message has the following problems:

1. It is not clear what it means when the signalling of formatting, purpose, and complexity is not present as there can be two possibilities:
   1. For NNPFC SEI message in which formatting, purpose, and complexity information is not present, those information is inferred from the NNFPC SEI message that contains the base neural-network post-processing filter, or,
   2. For NNPFC SEI message in which formatting, purpose, and complexity information is not present, those information is not known.
2. NNPFC SEI message that contains base filter cannot be repeated since only the first SEI message with a particular nnpfc_id can have the value of that flag equal to 1. Any SEI message with same nnpfc_id may be perceived as referencing to the first SEI message (i.e., an update).

To address the above asserted problems, the following modifications are proposed:

1. Specify that the signalling of formatting, purpose, and complexity information is present only in NNPFC SEI message that contains base filter.
   1. This means that when NNPFC SEI message contains base neural-network post-processing filter, the value of nnpfc_formatting_and_purpose_flag is equal to 1 and likewise, when the value of nnpfc_formatting_and_purpose_flag is equal to 1, the NNPFC SEI message contains base neural-network post-processing filter.
2. Specify that for NNPFC SEI message that contains the update filter, the formatting and purpose information is inferred to be the same as the information signalled in associated NNPFC SEI message (i.e., NNPFC SEI message that has same value of nnpfc_id) that contains the base neural-network post-processing filter.
3. NNPFC SEI message that contains base neural-network post-processing filter may be repeated. When repeated, the repeated SEI message contains the same information as the first NNPFC SEI message (i.e., the first one with the particular value of nnpfc_id).