[FFmpeg-devel] [PATCH v2] avformat/hls Implement support for using AVSEEK_FLAG_BACKWARD when seeking
Gustav Grusell
gustav.grusell at gmail.com
Fri Jun 18 23:16:25 EEST 2021
On Sun, Jun 13, 2021 at 5:14 PM Lingjiang Fang <vacingfang at hotmail.com>
wrote:
> On Sat, 12 Jun 2021 23:14:16 +0200
> Gustav Grusell <gustav.grusell at gmail.com> wrote:
>
> > On Sat, Jun 12, 2021 at 6:51 PM Lingjiang Fang
> > <vacingfang at foxmail.com> wrote:
> >
> > > On Sat, 12 Jun 2021 14:16:22 +0200
> > > Gustav Grusell <gustav.grusell at gmail.com> wrote:
> > >
> > > > On Fri, Jun 11, 2021 at 6:29 PM Lingjiang Fang
> > > > <vacingfang at foxmail.com> wrote:
> > > >
> > > > > On Sat, 5 Jun 2021 22:42:26 +0200
> > > > > Gustav Grusell <gustav.grusell at gmail.com> wrote:
> > > > >
> > > > > > Before, seeking in hls streams would always seek to the next
> > > > > > keyframe after the given timestamp. With this fix, if
> > > > > > AVSEEK_FLAG_BACKWARD is set, seeking will be to the first
> > > > > > keyframe of the segment containing the given timestamp. This
> > > > > > fixes #7485.
> > > > > >
> > > > > > Signed-off-by: Gustav Grusell <gustav.grusell at gmail.com>
> > > > > > ---
> > > > > > libavformat/hls.c | 9 +++++++++
> > > > > > 1 file changed, 9 insertions(+)
> > > > > >
> > > > > > diff --git a/libavformat/hls.c b/libavformat/hls.c
> > > > > > index 8fc6924c90..fb8c9b071a 100644
> > > > > > --- a/libavformat/hls.c
> > > > > > +++ b/libavformat/hls.c
> > > > > > @@ -2197,6 +2197,15 @@ static int
> > > > > > hls_read_packet(AVFormatContext *s, AVPacket *pkt) break;
> > > > > > }
> > > > > >
> > > > > > + /* If AVSEEK_FLAG_BACKWARD set and not
> > > > > > AVSEEK_FLAG_ANY,
> > > > > > + * we return the first keyframe
> > > > > > encountered */
> > > > > > + if (pls->seek_flags &
> > > > > > AVSEEK_FLAG_BACKWARD &&
> > > > > > + !(pls->seek_flags & AVSEEK_FLAG_ANY)
> > > > > > &&
> > > > > > + pls->pkt->flags & AV_PKT_FLAG_KEY) {
> > > > > >
> > > > > taking account of the case of using flag backward and flag any
> > > > > together, a logic like this is better:
> > > > >
> > > > > if ( pls->seek_flags & AVSEEK_FLAG_BACKWARD &&
> > > > > (pls->seek_flags & AVSEEK_FLAG_ANY ||
> > > > > pls->pkt->flags & AV_PKT_FLAG_KEY)) {
> > > > > ....
> > > > > }
> > > > >
> > > > >
> > > > Thanks for your feedback!
> > > > I'm not sure about this, I think it makes more sense if seeking
> > > > with AVSEEK_FLAG_BACKWARD and AVSEEK_FLAG_ANY behaves the same as
> > > > when using only AVSEEK_FLAG_ANY, that is to seek to as close as
> > > > possible as the exact timestamp, regardless if that is a keyframe
> > > > or not. If
> > > what do you mean "AVSEEK_FLAG_BACKWARD and AVSEEK_FLAGAV_ANY
> > > behaves the same as when using only AVSEEK_FLAG_ANY"?
> > > I think the default behavior of seek is seek to key frame, where
> > > AVSEEK_FLAGAV_ANY means we can accept seeking to non-key frame.
> > >
> > >
> > Sorry I think you can disregard what I wrote. I was thinking of the
> > case where the user would set both BACKWARDS and ANY flags, and
> > forgot that if the user sets the BACKWARDS flag, the ANY flag will be
> > added for other playlists than the one containing the stream
> > searched in. I think this made me miss your point.
> > So what I meant was that if seeking with both AVSEEK_FLAG_BACKWARD and
> > AVSEEK_FLAG_ANY, optimally, I would expect the result to
> > be to search to the nearest frame before or exactly at the seek
> > timestamp. Note that this is not how i behaves with my patch, it will
> > seek to the first frame with a timestamp equal to or greater to the
> > seek timestamp. In my opinion this is probably good enough, and is
> > in any case the same behaviour as before the patch.
> > So if the user has etAVSEEK_FLAG_ANY we would not then want to search
> > to the first frame of the segment, even if we have the
> > AVSEEK_FLAG_BACKWARD set. But of course this is different if
> > AVSEEK_FLAG_ANY has been set because this is not the seek playlist.
> >
> >
> > > > I'm not mistaken, with your proposed change, using both
> > > > AVSEEK_FLAG_BACKWARD and AVSEEK_FLAG_ANY would always seek to the
> > > > first frame of the segment which is always a keyframe, so
> > > > behaviour would be the same as using only AVSEEK_FLAG_BACKWARD.
> > > when the first packet of a segment is not key-frame(rarely seen, but
> > > is possible), the behavior will be different.
> > > - AVSEEK_FLAG_BACKWARD will go on seeking until find a key frame
> > > - AVSEEK_FLAG_BACKWARD + AVSEEK_FLAG_BACKWARD should return
> > > immediatly
> > >
> > >
> > Ah yes you are correct. I was thinking that the HLS specification
> > requires an IDR frame at the start of every (video) segment, but now
> > that I looked it up that is not strictly the case.
> > Apples HLS authorign specification does require it though.
> >
> >
> > there is a similar logic at the end of this if{} block.
> > >
> > > >
> > > >
> > > > > > + pls->seek_timestamp = AV_NOPTS_VALUE;
> > > > > > + break;
> > > > > > + }
> > > > > > +
> > > > > > tb = get_timebase(pls);
> > > > > > ts_diff = av_rescale_rnd(pls->pkt->dts,
> > > > > > AV_TIME_BASE, tb.den, AV_ROUND_DOWN) -
> > > > >
> > > > > BTW, when I used seek backward before, what I want is to get
> > > > > exactly packets of the specified seek time, not only the
> > > > > specified stream but also all streams I need. To achieve this,
> > > > > I will put this patch lines before this if block, like this:
> > > > >
> > > > > if (pls->seek_timestamp == AV_NOPTS_VALUE)
> > > > > break;
> > > > >
> > > > > if ( pls->seek_flags & AVSEEK_FLAG_BACKWARD &&
> > > > > (pls->seek_flags & AVSEEK_FLAG_ANY ||
> > > > > pls->pkt->flags & AV_PKT_FLAG_KEY)) {
> > > > > ....
> > > > > }
> > > > >
> > > > > if (pls->seek_stream_index < 0 ||
> > > > > ...
> > > > >
> > > > >
> > > > Not sure I'm following. If I'm not mistaken, the hls spec does not
> > > > require audio and video segments to be aligned. Given this, it
> > > > seems to me that the above proposed change could lead to
> > > > incorrect seek for some streams when using AVSEEK_FLAG_BACKWARDS,
> > > > since we would get the first frame of the segments for all
> > > > streams, and for audio streams this might not match the timestamp
> > > > of the first segment of the video stream.
> > > it's hard to seek precisely when we want to seek backward, we just
> > > guarantee we seek to a place before the given time, like what this
> > > patch does.
> > > the way I proposed is to follow the flag as much as possible, and
> > > behave similarly no matter audio and video are in different
> > > playlists or in one, because it only affects the stream in the same
> > > playlist.
> >
> > Ah, it took me a while, but now I just realized you have a very valid
> > point. If audio is in different playlist than video and we seek in
> > video stream, the audio playlist will seek
> > with AVSEEK_FLAG_ANY and with my patch may seek to a timestamp that
> > comes quite some time (a few seconds or so) after the timestamp we
> > seeked to in the video stream. This seems undesirable, I will rethink
> > and rewrite the patch.
> sorry, I didn't catch your point, can you give an example :)
>
>
Suppose you have a hls manifest with separate playlists for video and
audio. Video and audio are not aligned, suppose video segments have a
duration of 4s and audio segments have a duration of 3.97s. Then say you
want to seek to 163s from the start of the video. For the video, this would
be in the 41th segment, that starts at 160s. For audio, the seek point
would be in the 42nd segment, that starts at 162.77. With my patch for
seeking (with or without your amendments) both video and audio would seek
to the beginning of the segment containing the timestamp 163s from start.
This would mean that after seeking the video stream would be at 160s, and
the audio stream at 162.77, so we would miss some audio packages.
> BTW, summary of my view:
> if I have more data, I can drop it; but if I missed something, I can't
> make up it.
>
> Ah yes, that makes sense.
Cheers,
Gustav Grusell
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