Based on the AOMedia Film Grain Synthesis 1 (AFGS1) spec:
https://aomediacodec.github.io/afgs1-spec/
The parsing has been changed substantially relative to the AV1 film
grain OBU. In particular:
1. There is the possibility of maintaining multiple independent film
grain parameter sets, and decoders/players are recommended to pick
the one most appropriate for the intended display resolution. This
could also be used to e.g. switch between different grain profiles
without having to re-signal the appropriate coefficients.
2. Supporting this, it's possible to *predict* the grain coefficients
from previously signalled parameter sets, transmitting only the
residual.
3. When not predicting, the parameter sets are now stored as a series of
increments, rather than being directly transmitted.
4. There are several new AFGS1-exclusive fields.
I placed this parser in its own file, rather than h2645_sei.c, since
nothing in the generic AFGS1 film grain payload is specific to T.35, and
to compartmentalize the code base.
Implementation copied wholesale from dav1d, sans SIMD, under permissive
license. This implementation was extensively verified to be bit-exact,
so it serves as a much better starting point than trying to re-engineer
this from scratch for no reason. (I also authored the original
implementation in dav1d, so any "clean room" implementation would end up
looking much the same, anyway)
The notable changes I had to make while adapting this from the dav1d
code-base to the FFmpeg codebase include:
- reordering variable declarations to avoid triggering warnings
- replacing several inline helpers by avutil equivalents
- changing code that accesses frame metadata
- replacing raw plane copying logic by av_image_copy_plane
Apart from this, the implementation is basically unmodified.
