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 an AVStream. After I decode the frame with avcodec_decode_video2, I store the time stamp of that frame in an 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 from when 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". Can it be because the "-f mpegts" arguments change some characteristics of the media?
Below is my code (simplified). By reading the ffmpeg mailing list archives 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 video that is being received. I would really appreciate any sort of help.
//..
//argv[1]="/file.mp4";
argv[1]="udp://localhost:7777";
// define AVFormatContext, AVFrame, etc.
// register av, avcodec, avformat_network_init(), etc.
avformat_open_input(&pFormatCtx, argv, NULL, NULL);
avformat_find_stream_info(pFormatCtx, NULL);
// find the video stream...
// pointer to the codec context...
// open codec...
pFrame=av_frame_alloc();
while(av_read_frame(pFormatCtx, &packet)>=0) {
AVStream *strem = pFormatCtx->streams[videoStream];
if(packet.stream_index==videoStream) {
avcodec_decode_video2(pCodecCtx, pFrame, &frameFinished, &packet);
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
}
//..
Timestamps are stored at the container level, so changing the container can change the timestamps. In addition, TS stores a timestamp for every frame (based on a 90kHz clock). MP4 only stores the frame durations with an assumed start time of 0 (this gets more complicated with bframes since the first PTS is zero, and the first DTS is < 0). So to get the time stamp all the frame durations are added. Mp4 also allows the clock rate be set. It is often 1001/3000 ticks per second for 29.97FPS, but it can be set to anything. so av_frame_get_best_effort_timestamp returns you ticks in codec->stream_base units. For TS codec->stream_base is always 1/90000