[Libav-user] Confusion about PTS in files and streams

Maziar Mehrabi maziar.mehrabi at gmail.com
Thu Nov 6 12:07:36 CET 2014


Is it possible that the PTS of a particular frame in a file is different
with the PTS of the same frame in the same file while it is being streamed?
When I read a frame using av_read_frame I store the video stream in a
AVStream. After I decode the frame with avcodec_decode_video2, I store the
time stamp of that frame in a int64_t using
av_frame_get_best_effort_timestamp. Now if the program is getting its input
from a file I get a different timestamp than if I stream the input (from
the same file) to the program.
To change the input type I simply change the argv argument from
"/path/to/file.mp4" to something like "udp://localhost:1234", then I stream
the file with ffmpeg in command line: "ffmpeg -re -i /path/to/file.mp4 -f
mpegts udp://localhost:1234".
Below is my code (simplified). By reading the previous mailing lists I
realized that the time_base that I'm looking for is in the AVStream and not
the AVCodecContext. Instead of using av_frame_get_best_effort_timestamp I
have also tried using the packet.pts but the results don't change.
I need the time stamps to have a notion of frame number in a streaming
I would really appreciate any sort of help.

// define AVFormatContext, AVFrame...
// register av, avcodec, avformat_network_init()...
avformat_open_input(&pFormatCtx, argv, NULL, NULL);
avformat_find_stream_info(pFormatCtx, NULL);
// find the video stream...
// pointer to the codec context...
// open codec...
while(av_read_frame(pFormatCtx, &packet)>=0) {
        AVStream *strem = pFormatCtx->streams[videoStream];
        if(packet.stream_index==videoStream) {
            avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished,
            if(frameFinished) {
                int64_t perts = av_frame_get_best_effort_timestamp(pFrame);
                if (isMyFrame(pFrame)){
                     cout << perts*av_q2d(strem->time_base) << "\n";
//free allocated space

Thanks a lot,
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