[FFmpeg-user] Image extraction broken with ffmpeg, but not with mencoder. How can I fix this?

James Board jpboard2 at yahoo.com
Thu Sep 5 16:23:09 CEST 2013

>From: Tom Evans <tevans.uk at googlemail.com>
>To: FFmpeg user questions <ffmpeg-user at ffmpeg.org> 
>So why do you get the dupes. It would seem like your input video is
>VFR, and you are outputting to image2, which is a CFR muxer, so ffmpeg
>is duplicating frames to get your requested output.
>BTW, this seemed quite clear to me from the docs for vsync. Could you
>suggest any improvements that would have made it clearer for you?

The documentation doesn't mention that image2 is a CFR muxer and that
it will duplicate and/or drop images from the input video in order to achieve
some CFR.  I'd suggest the documention should mention that since it happens
'behind the scenes' and there's no way for a new user to know or predict that
will happen.
It will also be nice if the image2 muxer and the select filter handled frame numbers
consistently.  I understand that different people want the default behavior to do different
things.  That's okay with me.  But if the image2 muxer is telling me that frame
100 is one thing, and the select filter interprets frame 100 as another thing, then
that is a problem for me, and I'd consider that a bug.
If most people want ffmpeg to behave that way, then that's okay.  It's your product. 
Then I think my best option is to produce a sequence of image files, edit that sequence directly, 
then produce the output video directly from the individual files: no image2 muxer,
and no select filter.
Thanks for explaining everything to me.

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