What's wrong with my use of timestamps/timebases for frame seeking/reading using libav (ffmpeg)?

Question

So I want to grab a frame from a video at a specific time using libav for the use as a thumbnail.

What I\'m using is t

Answer 1

I replaced

av_seek_frame(format_context, stream_index, request_timestamp, 0);

with

avformat_seek_file(format_context, stream_index, INT64_MIN, request_timestamp, INT64_MAX, 0);

and suddenly I get reasonable output. Great.
And it only took a day in almost complete documentation darkness. :/

Answer 2

It's mostly like this:

• the stream timebase is what you are really interested in. It's what the packet timestamps are in, and also pkt_pts on the output frame (since it's just copied from the corresponding packet).

• the codec timebase is (if set at all) just the inverse of the framerate that might be written in the codec-level headers. It can be useful in cases where there is no container timing information (e.g. when you're reading raw video), but otherwise can be safely ignored.

• AVFrame.pkt_pts is the timestamp of the packet that got decoded into this frame. As already said, it's just a straight copy from the packet, so it's in the stream timebase. This is the field you want to use (if the container has timestamps).

• AVFrame.pts is not ever set to anything useful when decoding, ignore it (it might replace pkt_pts in the future, to make the whole mess less confusing, but for now it's like this, for historical reasons mostly).

• the format context's duration is in AV_TIME_BASE (i.e. microseconds). It cannot be in any stream timebase, since you can have three bazillion streams, each with its own timebase.

• the problem you see with getting a different timestamp after seeking is simply that seeking is not accurate. In most cases you can only seek to closest keyframe, so it's common to be a couple seconds off. Decoding and discarding the frames you don't need must be done manually.