[FFmpeg-devel] [PATCH] libavcodec/videotoolbox: fix decoding of h264 streams with minor SPS changes

[Hendrik Leppkes]

On Wed, Nov 15, 2017 at 10:15 PM, Aman Gupta <ffmpeg at tmm1.net> wrote:
> From: Aman Gupta <aman at tmm1.net>
>
> Previously the codec kept an entire copy of the SPS, and restarted the VT decoder
> session whenever it changed. This fixed decoding errors in [1], as
> described in 9519983c. On further inspection, that sample features an SPS change
> from High/4.0 to High/3.2 while moving from one scene to another.
>
> Yesterday I received [2], which contains minor SPS changes where the
> profile and level do not change. These occur frequently and are not associated with
> scene changes. After 9519983c, the VT decoder session is recreated unnecessarily when
> these are encountered causing visual glitches.
>
> This commit simplifies the state kept in the VTContext to include just the first three
> bytes of the SPS, containing the profile and level details. This is populated initially
> when the VT decoder session is created, and used to detect changes and force a restart.
>
> This means minor SPS changes are fed directly into the existing decoder, whereas
> profile/level changes force the decoder session to be recreated with the new parameters.
>

The profile and level are not exactly the only things that can change
to force a decoder to be re-created.
How about the frame dimensions, within the same level?

- Hendrik