[Ffmpeg-devel] Seeking and keyframes

[Michel Bardiaux]

M?ns Rullg?rd wrote:
> Steve Willis <steve at stevewillis.net> writes:
> 
> 
>>Greetings,
>>
>>Please pardon my novice question. I'm working on a project where the
>>user needs to analyze video frame by frame, and possibly cut video at
>>any arbitrary frame (not necessarily a keyframe.) This is not the same
>>as a typical video player...video does not have to be "played" in
>>real-time, just stepped through frame by frame. More importantly, I
>>need a way to get a frame for a particular time accurate to the frame
>>(not to the closest keyframe.)
>>
>>I've been looking at the documentation for av_seek_frame(), and I see
>>that it seeks to the first keyframe before the given timestamp by
>>default. Is there a simple way to change this behavior so it finds the
>>previous keyframe and then regenerates the forward intra-frame to
>>exactly match the timestamp?
> 
> 
> No.  Libavformat doesn't have access to the decoder to perform the
> decoding of the frames after the keyframe up to the requested frame.
> 
> 
>>Alternatively, is there a simple way to use ffmpeg to transcode the
>>video such that every frame is a keyframe?
> 
> 
> You could use mjpeg.
> 
> 
>>A final (very beginner) question: why is seeking only to keyframes the
>>default? Is it very inefficient to actually generate the exact frame
>>for a given timestamp each time a user seeks?
> 
> 
> It's somewhat inefficient, but also impossible.
> 
Would it be possible to have av_seek_frame also return a set of 
instructions to follow to obtain the specified frame? E.g. for a GOP of 
structure IBBP in display order, if you specified seek to the 2nd B 
frame, seek_frame would tell you back "read_frame+decode, 
read_frame+skip, read_frame+decode, read_frame+decode" (Correct?)


-- 
Michel Bardiaux
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Tel : +32 2 790.29.41