Re: [FFmpeg-devel] [PATCH 4/6] ffmpeg: don't skip packets before a keyframe was seen if a bsf with delay is used

From: Anton Khirnov <anton@khirnov.net>
To: FFmpeg development discussions and patches <ffmpeg-devel@ffmpeg.org>
Subject: Re: [FFmpeg-devel] [PATCH 4/6] ffmpeg: don't skip packets before a keyframe was seen if a bsf with delay is used
Date: Tue, 15 Feb 2022 13:03:29 +0100
Message-ID: <164492660970.19727.6128970737155226820@lain.red.khirnov.net> (raw)
In-Reply-To: <7d8d3b14-7fbf-f7fd-878a-c5f37a839a37@gmail.com>

Quoting James Almer (2022-02-15 12:48:09)
> 
> 
> On 2/15/2022 8:41 AM, Anton Khirnov wrote:
> > Quoting James Almer (2022-02-14 23:41:54)
> >> A keyframe could be buffered in the bsf and not be output until more packets
> >> had been fed to it.
> >>
> >> Signed-off-by: James Almer <jamrial@gmail.com>
> >> ---
> >>   fftools/ffmpeg.c | 3 ++-
> >>   1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/fftools/ffmpeg.c b/fftools/ffmpeg.c
> >> index 6aa0986f02..48d9016b4c 100644
> >> --- a/fftools/ffmpeg.c
> >> +++ b/fftools/ffmpeg.c
> >> @@ -2026,7 +2026,8 @@ static void do_streamcopy(InputStream *ist, OutputStream *ost, const AVPacket *p
> >>       }
> >>   
> >>       if ((!ost->frame_number && !(pkt->flags & AV_PKT_FLAG_KEY)) &&
> >> -        !ost->copy_initial_nonkeyframes)
> >> +        !ost->copy_initial_nonkeyframes &&
> >> +        !(ost->bsf_ctx && ost->bsf_ctx->filter->capabilities & AV_BSF_CAP_DELAY))
> >>           return;
> > 
> > Wouldn't it be simpler to add an OutputStream field that tracks whether
> > we've seen a keyframe packet yet? No new API required.
> 
> Probably. It would also only trigger when a keyframe was seen instead of 
> unconditionally for all delay flagged bsfs.
> 
> I still think this new API is a good addition, either way. Only a 
> handful of bsfs buffer packets and require the caller to flush them 
> after sending NULL (av1_frame_merge, vp9_superframe, and setts after 
> this set) so library users could have all this time never signaled EOF 
> and never noticed anything wrong, much like it happened here.
> The presence of this flag might help library users know they really need 
> to signal EOF.

I don't see where the advantage would be. The callers still need to have
the flushing code, so might as well always call it.

The disadvantage for us is more complixity and we have to maintain the
list of delay BSFs.

-- 
Anton Khirnov
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