[Libav-user] Looping a stream?

[Strahinja Radman]

On Thu, Feb 10, 2022 at 10:01 PM John Regan via Libav-user <
libav-user at ffmpeg.org> wrote:

> On Thu, 10 Feb 2022 06:19:47 +0100
> Strahinja Radman <dr.strashni at gmail.com> wrote:
>
> > On Thu, Feb 10, 2022 at 1:14 AM John Regan via Libav-user <
> > libav-user at ffmpeg.org> wrote:
> >
> > > Hi there, I was hoping somebody could offer advice on how to loop a
> > > video with libavformat/libavcodec.
> > >
> > > I have a filter graph created, so my core decoding code is
> > > basically:
> > >
> > > if( (err = av_read_frame(inputCtx, packet)) < 0) {
> > >     if(err != AVERROR_EOF) {
> > >         /* handle errors, return */
> > >     }
> > >     avformat_seek_file(inputCtx,videoStreamIndex, 0, 0, 0, 0));
> > >     avcodec_flush_buffers(codecCtx);
> > > }
> > >
> > > if(packet.stream_index == videoStreamIndex) {
> > >     avcodec_send_packet(codecCtx,packet);
> > >     avcodec_receive_frame(codecCtx,tmpFrame);
> > >     av_buffersrc_write_frame(bufferSrcCtx,tmpFrame);
> > >     if(av_buffersink_get_frame(bufferSinkCrx,frame) >= 0) {
> > >         /* we have a frame */
> > >     }
> > > }
> > >
> > > I'm not sure if there's steps that need to be taken to reset the
> > > filter graph? Like is there a timestamp discontinuity?
> > >
> > > It seems like this should work - but I'm missing something, it seems
> > > like the buffersink_get_frame stops producing frames after I reset.
> > >
> > > Thank you in advance,
> > > -John
> > > _______________________________________________
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> > >
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> > >
> >
> > Hello,
> >
> > I dont think you need to reset the filter graph, in the end it simply
> > takes in AVFrame* and does some processing. Timestamps in filters are
> > only changed if you specify a different timebase for the filters (as
> > far as I am aware). But timestamp discontinuity is something you have
> > to take care of. For example you could generate them by using this
> > formula
> >
> > frame_timestamp = iteration_count * max_timestamp +
> > current_frame_timestamp
> >
> > max_timestamp is the last timestamp of the video stream.
> > This should enable you to get a looped sequence of frames that can be
> > properly played. On the other hand, check if you are actually getting
> > a frame from your decoder by saving it to a PGM file. Here is the code
> >
> > static void pgm_save(unsigned char* buf, int wrap, int xsize, int
> > ysize, const char* filename)
> > {
> >
> > FILE* f;
> > int i;
> >
> >
> > f = fopen(filename, "w");
> > fprintf(f, "P5\n%d %d\n%d\n", xsize, ysize, 255);
> > for (i = 0; i < ysize; i++)
> >
> > fwrite(buf + i * wrap, 1, xsize, f);
> >
> > fclose(f);
> >
> > }
> >
>
> That tip about timestamps was a huge help, thank you!
>
> I figured out the issue was non-sequential timestamps with the "fps"
> filter.
>
> What I'm doing now is saving the "pts offset" variable and a "last
> pts" variable, and "pts offset" is initialized to zero. When I receive
> a packet, I save its original PTS to the "last pts" variable, and add
> the offset to the packet's PTS. Then, when av_read_frame returns EOF, I
> add that last_pts to the offset.
>
> Question, is it more appropriate to meddle with the packet PTS I get
> from the demuxer, or the frame PTS I get from the decoder? Or does it
> not really matter?
>
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>


Well as AVPacket has pkt_pts and pkt_dts which the decoder uses, I would
rather change AVFrame's PTS value.
-- 

Regards
Strahinja Radman
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