Re: [FFmpeg-devel] [PATCH v2] lavc/vvc: Error if SPS ID is duplicated within CVS

[Nuo Mi <nuomi2021@gmail.com>]

On Sun, Apr 7, 2024 at 8:16 PM Anton Khirnov <anton@khirnov.net> wrote:

> Quoting Nuo Mi (2024-04-07 14:13:58)
> > On Sun, Apr 7, 2024 at 2:15 PM Anton Khirnov <anton@khirnov.net> wrote:
> >
> > > Quoting Frank Plowman (2024-04-06 15:46:09)
> > > > Key line from the spec is:
> > > >
> > > > "All SPS NAL units with a particular value of
> sps_seq_parameter_set_id
> > > > in a CVS shall have the same content."
> > > >
> > > > Prior to this patch, the VVC decoder's behaviour on encountering a
> > > > duplicated SPS ID (within the entire bitstream, not restricted to
> > > > a CVS) was simply to replace the entry in the SPS lookup table with
> the
> > > > new data.  Illegal bitstreams with multiple SPSs in the same CVS
> sharing
> > > > an ID but differing elsewhere could cause all manner of issues.
> > > >
> > > > The patch tracks which SPS IDs have been used in the given CVS using
> the
> > > > new sps_id_used field of VVCParamSets.  If it encounters an SPS with
> an
> > > > ID already in use and whose content differs from the previous SPS, it
> > > > throws an AVERROR_INVALIDDATA.
> > >
> > > I wonder if it wouldn't be better to do what H264/HEVC do, which is
> > > replace the SPS, invalidate the PPSes that depend on the old one, and
> > > start a new CVS.
> > >
> > Consider two scenarios:
> > If the first SPS is incorrect, the entire CVS is undecodable because the
> > key frame is wrong, no matter what we do.
> > If the second SPS is incorrect, H.264/HEVC can't recover until the next
> CVS
> > because it replaces the correct SPS with the wrong one. However, the
> > current VVC logic allows for recovery in such cases.
> > Therefore, in the second case, the current logic may have benefits.
>
> Could the new SPS not signal the start of a new CVS?
>
Yes.
Take AUD_A_Broadcom_3.bit as an example, the nal order is

SPS
PPS
IDR_N_LP
TRAIL
MORE TRAILS...
SPS
PPS
CRA
TRAIL
MORE TRAILS...
SPS
PPS
IDR_N_LP

The SPS before CRA will not start a new CVS, unless you seek to the CRA.

>
> --
> Anton Khirnov
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