Re: [FFmpeg-devel] [PATCH] hevcdec: skip slices with missing PPS instead of skipping the entire packet

[Hendrik Leppkes <h.leppkes@gmail.com>]

On Tue, Dec 28, 2021 at 3:26 PM James Almer <jamrial@gmail.com> wrote:
>
> On 12/28/2021 10:49 AM, Hendrik Leppkes wrote:
> > Aborting decoding of the entire packet on a missing PPS can result in
> > missing the actual PPS on streams with badly ordered NALs, where the
> > SPS/PPS/VPS are stitched to the back of the previous frame, instead of
> > the beginning of the next frame.
> >
> > Instead, skip the undecodable slice, and let the decoder process further
> > NALs in the same packet.
> > ---
> >   libavcodec/hevcdec.c | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/libavcodec/hevcdec.c b/libavcodec/hevcdec.c
> > index 3aa70e2245..c2451d682e 100644
> > --- a/libavcodec/hevcdec.c
> > +++ b/libavcodec/hevcdec.c
> > @@ -575,7 +575,7 @@ static int hls_slice_header(HEVCContext *s)
> >       sh->pps_id = get_ue_golomb_long(gb);
> >       if (sh->pps_id >= HEVC_MAX_PPS_COUNT || !s->ps.pps_list[sh->pps_id]) {
> >           av_log(s->avctx, AV_LOG_ERROR, "PPS id out of range: %d\n", sh->pps_id);
> > -        return AVERROR_INVALIDDATA;
> > +        return 1; // skip slice with missing PPS
>
> The decoder should IMO also set the output frame's decode_error_flags to
> FF_DECODE_ERROR_DECODE_SLICES in this scenario.
>

There is a variety of error conditions that can lead to a slice being
skipped, not just this one. I can potentially add it here, but it
won't be a very exhaustive flag.
If its also the first slice, the frame will fail to decode entirely as
well, although the point primarily here is to allow parsing of
non-image NALs.

- Hendrik
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