[Libav-user] Output video data to file and pipe: concurrently

Hillel Rosensweig hillelro at yahoo.com
Thu Mar 31 10:30:05 EEST 2022

Hello all, 
I have been trying to output video (from my webcam) simultaneously to both a file ('out.mkv') and pipe: The file gets filtered frames packets, and the pipe: gets ufiltered rawvideo.My frame rate is 30 fps. However, I am getting a far lower framerate in my file output.
Attached is the while loop which reads packets and writes them to output:
while (1) {    av_read_frame(ifmt_ctx, packet);    stream_index = packet->stream_index;        StreamContext *stream = &file_stream_ctx[stream_index];        av_packet_rescale_ts(packet,                         ifmt_ctx->streams[stream_index]->time_base,                         stream->dec_ctx->time_base);    avcodec_send_packet(stream->dec_ctx, packet);        while (ret >= 0) {        avcodec_receive_frame(stream->dec_ctx, stream->dec_frame);            stream->dec_frame->pts = stream->dec_frame->best_effort_timestamp;        ret = filter_encode_write_frame(stream->dec_frame, stream_index, file_stream_ctx,                                                file_filter_ctx, file_ofmt_ctx);        ret = av_interleaved_write_frame(pipe_ofmt_ctx, packet);    }}
'ifmt_ctx' is the AVFormatContext for the webcam.'file_ofmt_ctx', is the AVFormatContext for the pipe for the output file, and pipe_ofmt_ctx is the AVFormatContext.'file_stream_ctx' and 'file_filter_ctx' are the stream and filter contexts used for filtering and encoding the file output.

My guess is that writing to the pipe is taking too long and not allowing the next packet to be read on time - causing a lower frame rate. Does that make sense? If so - any suggestions on how to fix it? (I tried using AVFMT_FLAG_NONBLOCK but it doesn't seem to help).
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