[Libav-user] Looping a stream?

[Strahinja Radman]

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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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);

}

-- 

Regards
Strahinja Radman
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