Accurately seeking to frames?

[Will McElderry]

Hi Nicolas,


Thanks for your fast & authoritative reply!


I've still got an issue with accurately seeking to a frame where seeking 
to the frame's (stream-time equivalent of) PTS does not give the target 
frame (or even the following frame - sometimes I get the preceding frame)

I believe the issue is caused by the data: a combination of variable 
frame rate with fixed frame duration (the duration is generally shorter 
than the inter-frame PTS intervals)


1. Could constant duration combined with variable inter-frame PTS 
explain 'weird seeking'? (I suspect it does, but could be guessing the 
wrong solution)  (if useful I can illustrate this with actual 
PTS/duration,seeks etc from a file)

2. If so, is there a minimal impact method to fix up a file with 
incorrect durations to match the frame PTS times?  (ideally not 
transcoding, just updating meta-data values)

     [container was mp4mux, video stream is h264 captured from an RTSP 
camera]

     (? Is that even possible, or is frame duration always just one 
number for the entire stream?)

3. What does frame duration mean?  (& what should it be for a file with 
variable frame rate?)

     3.1. What is a frame duration conceptually and how does it relate 
to the PTS?

             - the duration to show a frame, following the PTS

             - the duration to show a frame, before the PTS

             - the duration to show a frame, centered about the PTS

     3.2. should the first frame (frame 0) in the file have a duration 
equal to:

             - frame 0's PTS

             - the interval between frame 0 and frame 1 PTS

             - frame 1's PTS

     3.3. Subsequent frame (frame n) durations should be:

             - the interval between frame n-1 and frame n PTS

             - the interval between frame n and and n+1 PTS

     3.4. In a manner similar to PTS needing to be translated by the 
Segment to get the PTS time in the stream, does duration need any 
translation by the Segment?

Thanks again in advance!


Will.


R.E. Ken's comment: Thanks for your contribution - unfortunately that's 
not easy to answer as it all depends on your pipeline and seems like a 
totally different discussion.  If you want to discuss further, please 
start another thread and I'll tell you the limited amount I think I 
can.  All the best!