[Ffmpeg-devel] question: problem by seeking and counting video frames

[Fred Rothganger]

cybercake wrote:
> So the question is, is there any general way or what people usually do to set up a track bar and an error-free seeking function?
>   

I wrote code to seek to exact frames in MPEG files for my PhD research 
(which you are welcome to if you want it).  The method you describe is 
generally correct.  I wouldn't bother with trying to note which frames 
are I-frames, or with using LOW_DELAY, hurry_up, or any frame_number 
value returned by FFMPEG.  (I also wouldn't bother building an index if 
you video has a constant frame rate.)  Basically, just seek to a 
timestamp sufficiently ahead of the desired frame to catch the I-frame 
before it, then decode through the frames until the one you want comes 
up.  I believe that the DTS of the frame you pass into the decoder is 
the PTS of the picture that comes out.  There is generally a few frames 
of latency in the decoder.  If your seek isn't sufficiently early, then 
seek earlier. :)  ffplay.c should have an example of basic slider-bar 
style seeking, though it doesn't actually present a slider bar but 
rather uses the position of the mouse pointer on the window.  Also, 
ffplay.c isn't written to be frame-exact like we need for research.

-- Fred