Re: [FFmpeg-devel] [PATCH v3 1/3] avcodec/nvenc: Enable recovery point SEI for intra refresh mode

[Kieran Kunhya via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

On Fri, Jan 24, 2025 at 3:59 AM Zhao Zhili
<quinkblack-at-foxmail.com@ffmpeg.org> wrote:
>
>
>
> > On Jan 24, 2025, at 02:02, Timo Rothenpieler <timo@rothenpieler.org> wrote:
> >
> > On 23.01.2025 15:17, Zhao Zhili wrote:
> >> From: Zhao Zhili <zhilizhao@tencent.com>
> >> Otherwise all frames can be dropped after seek without the
> >> output_corrupt/showall flags.
> >> ---
> >>  libavcodec/nvenc.c | 2 ++
> >>  1 file changed, 2 insertions(+)
> >> diff --git a/libavcodec/nvenc.c b/libavcodec/nvenc.c
> >> index 06e3fb81a4..68b1b661e4 100644
> >> --- a/libavcodec/nvenc.c
> >> +++ b/libavcodec/nvenc.c
> >> @@ -1247,6 +1247,7 @@ static av_cold int nvenc_setup_h264_config(AVCodecContext *avctx)
> >>          h264->intraRefreshPeriod = cc->gopLength;
> >>          h264->intraRefreshCnt = cc->gopLength - 1;
> >>          cc->gopLength = NVENC_INFINITE_GOPLENGTH;
> >> +        h264->outputRecoveryPointSEI = 1;
> >>  #ifdef NVENC_HAVE_SINGLE_SLICE_INTRA_REFRESH
> >>          h264->singleSliceIntraRefresh = ctx->single_slice_intra_refresh;
> >>  #endif
> >> @@ -1380,6 +1381,7 @@ static av_cold int nvenc_setup_hevc_config(AVCodecContext *avctx)
> >>          hevc->intraRefreshPeriod = cc->gopLength;
> >>          hevc->intraRefreshCnt = cc->gopLength - 1;
> >>          cc->gopLength = NVENC_INFINITE_GOPLENGTH;
> >> +        hevc->outputRecoveryPointSEI = 1;
> >>  #ifdef NVENC_HAVE_SINGLE_SLICE_INTRA_REFRESH
> >>          hevc->singleSliceIntraRefresh = ctx->single_slice_intra_refresh;
> >>  #endif
> >
> > Is there any reason someone might not want this?
> > Or could it break certain scenarios?
> > Reads to me like it'd only make the bitstream slightly larger, or are there other implications?
>
> For H.264 and H.265, GDR is intra refresh encoding together with recovery point SEI. SEI is optional for
> decoding, but it’s critical for GDR decoding. For H.266, GDR is a NALU type, which makes this
> technique complete.
>
> Without recovery point SEI, we can try decoding at any point blindly by specify -flags output_corrupt,
> which isn’t the default behavior of our decoder. Otherwise all frames can be dropped and
> never recovery.
>
> x264 don’t make option to disable recovery point SEI, which I think is the right design.
> On the other hand, both x265 and hevc_nvenc don’t output recovery point SEI or output
> recovery point SEI with useless fields, which makes user who want to use intra refresh
> encoding must specify -flags output_corrupt for decoding. That is, without recovery point
> SEI, user should have prior knowledge on encoding configuration before decoding.
>
> Anyway, even incorrect recovery point SEI is helpful so we don’t need to specify -flags
> output_corrupt manually. Follow x264’s design, I don’t think save a few bytes worth the
> trouble of disable recovery point SEI.

I don't know anything about nvenc specifically, but I think this
decision about recovery point SEI is correct.

Kieran
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