Can the ffmpeg av libs return an accurate PTS?

Question

I\'m working with an mpeg stream that uses a IBBP... GOP sequence. The (DTS,PTS) values returned for the first 4 AVPackets are as follows: I=(0,3) B=(1,1)

Answer 1

I'm fairly certain you are getting accurate values. It might help if you thing of an MPEG stream as, well, a stream. In that case, prior to the IBBPBB that you see there would normally be another GOP. Maybe something like this (using same notation as original question):

P(-3,-2)  B(-2,-1)  B(-1,0)

Basically the B frames after the I frames are based on the I frame and the last P frame from the previous GOP.

While it makes logical sense for a video to start off with this:

Start GOP: IPBBPBBPBB...

Later on it must be

Start GOP: IBBPBBPBBPBB
Start GOP: IBBPBBPBBPBB
Start GOP: IBB... 

Remember that decoding any B frame requires a complete frame before it and after it. So each pair of B frames should be displayed before the I or P frame just prior to it in the file.

FFMPEG may just have forgone the "special case" of first GOP.

Since the first two B frames don't have a prior frame to manipulate, you should be able to safely discard them. Just rebase your timestamps off of the first I frame and adjust the audio stream the same amount.

Whether this will actually result in a loss of frames will depend on FFMPEG's implementation, but worse case scenario is that you lose 83 milliseconds (2 frames at 24 frames/sec).