[Libav-user] Why does av_seek_frame() not seek to a keyframe?

Michael Bradshaw mbradshaw at sorensonmedia.com
Thu Jul 19 21:47:29 CEST 2012

On Thu, Jul 19, 2012 at 12:42 PM, Carl Eugen Hoyos <cehoyos at ag.or.at> wrote:
> [...]
> Afair, Reimar believes the seek function should not try hard to find
> a keyframe because seeking faster is more important.

I think the current functionality is OK, provided it's documented
well. Though to be honest I'd like an a flag/function that provided
additional seeking ability, but like Hendrik said that's been an
unfinished GSOC project for awhile so I'm not expecting it anytime

> I honestly cannot comment on the flags-value, I can only confirm
> that FFmpeg only seeks to keyframes for your sample here.

What's the expected behavior of avcodec_decode_video2() and seeking? I
understand that things like P and B frames require lavc to do some
frame buffering, but if ffmpeg seeks to a keyframe, can I expect the
first call to avcodec_decode_video2() to return a fully decoded frame?
(and then after that possibly make several calls before getting a full
frame out if there are P or B frames after this keyframe)

The first 54 packets out of av_read_frame() have .flags=0 (checked
with -dump and -loglevel debug). Could they be wrong if ffmpeg is
indeed seeking to a keyframe?

Visually, things look ok (aside from the frame being ~1.5 seconds
after the requested time), but I think that's because ffmpeg just
keeps reading and decoding until it gets a full frame out, and it
won't do that until it hits a keyframe, and that it's disguising the
fact that it's not seeking to a keyframe (or maybe that a packet's
.flags value is lying). But I could be misunderstanding something.



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