[Libav-user] How to get precise frame timestamp

[Alex Cohn]

On Wed, Oct 30, 2013 at 5:51 AM, YIRAN LI <mrfun.china at gmail.com> wrote:
>
> Hi friends,
>
> Here I have a question about AVFrame pts needs your help.
>
> My application uses av_read_frame to get packets and then calls  avcodec_decode_video2 to get video frames. But recently I found one specific video file generates strange pts/dts.
>
> Below is a part of the log:
>
> 00:00:17.301  MAIN  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 0
> 00:00:17.301  MAIN  timestamp_debug:            frame.pkt_pts = -9223372036854775808, frame.pkt_dts = -9223372036854775808, frame.pts = -9223372036854775808
> 00:00:17.301  1108  FFMPEG: looks like this file was encoded with (divx4/(old)xvid/opendivx) -> forcing low_delay flag
>
> 00:00:17.301  MAIN  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 1
> 00:00:17.301  MAIN  timestamp_debug:            frame.pkt_pts = -9223372036854775808, frame.pkt_dts = -9223372036854775808, frame.pts = -9223372036854775808
> 00:00:17.301  MAIN  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 2
> 00:00:17.301  MAIN  timestamp_debug:            frame.pkt_pts = -9223372036854775808, frame.pkt_dts = -9223372036854775808, frame.pts = -9223372036854775808
> 00:00:17.301  MAIN  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 3
> 00:00:17.301  MAIN  timestamp_debug:            frame.pkt_pts = -9223372036854775808, frame.pkt_dts = -9223372036854775808, frame.pts = -9223372036854775808
> 00:00:17.301  MAIN  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 4
> 00:00:17.301  MAIN  timestamp_debug:  [a full frame : frame.pkt_pts = -9223372036854775808, frame.pkt_dts = 0], frame.pts = 0
> 00:00:18.393  LVid236  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 5
> 00:00:18.408  LVid236  timestamp_debug:  [a full frame : frame.pkt_pts = -9223372036854775808, frame.pkt_dts = 1], frame.pts = 0
> 00:00:18.439  LVid236  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 6
> 00:00:18.439  LVid236  timestamp_debug:  [a full frame : frame.pkt_pts = -9223372036854775808, frame.pkt_dts = 2], frame.pts = 0
> 00:00:18.439  LSou259  FFMPEG: first_dts 0 not matching first dts 113424 in the queue
> 00:00:18.486  LVid236  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 7
> 00:00:18.486  LVid236  timestamp_debug:  [a full frame : frame.pkt_pts = -9223372036854775808, frame.pkt_dts = 3], frame.pts = 0
> 00:00:18.517  LVid236  timestamp_debug:  pkt.pts = -9223372036854775808, dts = 8
> 00:00:18.517  LVid236  timestamp_debug:  [a full frame : frame.pkt_pts = -9223372036854775808, frame.pkt_dts = 4], frame.pts = 0
> 00:00:18.549  LSou259  FFMPEG: first_dts 0 not matching first dts 114675 in the queue
>
> We can see that, it was until 4th packet is sent to decode function that we got the first full frame. and all frames had pts = 0
>
> So here I want to know, frame.pts, frame.pkt_dts, packet,dts. which is best to represent the time stamp of a video frame (I mean pts, but in this case, no pts is valid).
>
> For example, in this case, for the 1st frame, should I use frame.pkt_dts (that's 0), or pkt.pts (the last packet that generates this frame, in this case 4) as the time  position
> of the frame?
>
> The file I used to test is here https://dl.dropboxusercontent.com/u/89678527/av_divx_24_yuv420p_mp3_44100_2_1.avi
>
> Thanks


That's what best_effort_timestamp is for. See
http://dranger.com/ffmpeg/tutorial05.html about different values of
pts/dts.

BR,
Alex Cohn