[Ffmpeg-devel] Trouble with av_seek_frame() and av_read_frame()

[Steve Lhomme]

Devin Lane wrote:
> ffmpeg-devel:
> 
> I'm having some trouble using av_seek_frame(). I want to use it to seek 
> both forwards and backwards, both by frame and keyframe.
> 
> Right now, my strategy is to seek to a position ahead of my current one, 
> flush the codec buffers, then read the next frame using av_read_frame(). 
> I can't say I'm having much luck in getting that to work.
> 
> Here's where my confusion lies:
> 
> 1. I'm getting the current timestamp from the last displayed packet's 
> pts value. I figure if I add, say, 10 to that and seek to that position 
> I should get to the next keyframe, but it appears to go to just the next 
> frame. I'm using the following call:
>     av_seek_frame(formatContext, -1, (cur_pts+10), 0);
> 
> What am I doing wrong? I suspect it has a lot to do with the timestamp 
> units -- I can't seem to figure out which units they should be in. I 
> don't understand what units the packet's pts value is in, so I don't 
> know how to convert it to something acceptable for av_seek_frame()

I attach our QT VideoPreviewWidget code that mostly works to seek in 
video files. The timecode operations can be found there. (better than 
putting many lines of code here, out of context)

Have a look in SetSource_internal() and on_buttonPrevFrame_clicked().

> 2. I can't seem to seek backward in the video at all. Even when I seek 
> to a timestamp of 0, I end up going forward.

You might try to seek by bytes, not time. It works better for some 
containers (MPEG and MP4).

> 3. I can't find a way to get the total number of frames in the movie. I 
> call formatContext->streams[videoStream]->nb_frames; but get a value of 0.
> 
> Thank you very much,
> Devin Lane
> 
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