[Libav-user] Fwd: Why is av_seek_frame() inaccurate in some cases ?

[Lucas Soltic]

Début du message réexpédié :

> De : Lucas Soltic <soltic.lucas at gmail.com>
> Objet : Rép : [Libav-user] Why is av_seek_frame() inaccurate in some cases ?
> Date : 14 avril 2012 19:24:01 HAEC
> À : "This list is about using libavcodec, libavformat, libavutil, libavdevice and libavfilter." <libav-user at ffmpeg.org>
> 
> 
> Le 14 avr. 2012 à 18:49, Michael Bradshaw a écrit :
> 
>> On Sat, Apr 14, 2012 at 10:40 AM, Lucas Soltic <soltic.lucas at gmail.com> wrote:
>> So I don't think I did anything fancy, but I still can't find out why the video seeking is inaccurate. I would be really grateful if someone had ideas about this issue.
>>  
>> There's two reasons I can see why you would be getting this result: 1) It could be seeking to the closest keyframe in the video, and 2) some demuxers seek by DTS and not PTS.
>> 
>> You say it works fine for H.264 videos though, so I'm not entirely sure what could be causing it.
> 
> Hello!
> 
> I tried to use AVSEEK_FLAG_ANY for the av_seek_frame() parameter, so that it does not only seek to keyframes, but it still gives important gaps.
> As for the DTS, in the movie I read, they are equal to the PTS (for any of the ~10 seeking I did). These results concern the MPEG4v2 video.
> I'm going to investigate further with other codecs but I wanted to tell these points.
> 
> Lucas SOLTIC


Actually I noticed than when seeking (even with AVSEEK_FLAG_ANY), the real sought position is 0 to 10s before the position I wanted. Then, if the video PTS is 9.90s before the position I wanted, and I seek a little bit further (say 0.1s), the video PTS becomes the exact position I wanted. So it looks like it's just seeking to keyframes, whatever seek flag I use.

Considering this, are there more accurate ways to directly reach the expected position or should I just do avcodec_decode_video2() until the PTS becomes the one I wanted ? Decoding 10s of video could take some time on high resolution movies...

Regards,
Lucas SOLTIC
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://ffmpeg.org/pipermail/libav-user/attachments/20120416/06bf2a69/attachment.html>