[FFmpeg-devel] [PATCH] lavc/vvc: Correct sps_num_subpics_minus1 minimum

[Nuo Mi]

On Mon, Feb 26, 2024 at 5:20 AM Frank Plowman <post at frankplowman.com> wrote:

> On 25/02/2024 19:50, James Almer wrote:
> > On 2/25/2024 2:51 PM, Frank Plowman wrote:
> >> The spec says "the value of sps_num_subpics_minus1 shall be in the
> >> range of 0 to MaxSlicesPerAu − 1, inclusive."
> >>
> >> Signed-off-by: Frank Plowman <post at frankplowman.com>
> >> ---
> >>   libavcodec/cbs_h266_syntax_template.c | 2 +-
> >>   1 file changed, 1 insertion(+), 1 deletion(-)
> >>
> >> diff --git a/libavcodec/cbs_h266_syntax_template.c
> >> b/libavcodec/cbs_h266_syntax_template.c
> >> index 26ee7a420b..1d4d0c796f 100644
> >> --- a/libavcodec/cbs_h266_syntax_template.c
> >> +++ b/libavcodec/cbs_h266_syntax_template.c
> >> @@ -1130,7 +1130,7 @@ static int FUNC(sps)(CodedBitstreamContext *ctx,
> >> RWContext *rw,
> >>         flag(sps_subpic_info_present_flag);
> >>       if (current->sps_subpic_info_present_flag) {
> >> -        ue(sps_num_subpics_minus1, 1, VVC_MAX_SLICES - 1);
> >> +        ue(sps_num_subpics_minus1, 0, VVC_MAX_SLICES - 1);
> >
> > Strictly speaking, MaxSlicesPerAu varies depending on the stream's
> > general_level_idc value, listed in table A.2 from Annex-A (in ITU-T
> > H.266 V3).
> >
> > Right now, VVC_MAX_SLICES is defined as 600 in vvc.h, but the max value
> > in the spec is 1000, from level 6.3. I assume it was added in V3?
>
> Good catch.  It looks like level 6.3 has been in the spec since the
> first version, but all the VVC_MAX_* definitions derived from Annex A
> did not account for it.  Here is a patch which corrects this:
> https://ffmpeg.org//pipermail/ffmpeg-devel/2024-February/322140.html

No, level 6.3 was not present in v1 (08/2020); it was added in v2 (04/2022).


>
> >>           if (current->sps_num_subpics_minus1 > 0) {
> >>               flag(sps_independent_subpics_flag);
> >>               flag(sps_subpic_same_size_flag);
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