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

Paul B Mahol onemda at gmail.com
Thu Sep 5 14:23:13 CEST 2013

On 9/5/13, Robert Krueger <krueger at lesspain.de> wrote:
> On Thu, Sep 5, 2013 at 1:11 PM, Tom Evans <tevans.uk at googlemail.com> wrote:
>> On Wed, Sep 4, 2013 at 9:46 PM, James Board <jpboard2 at yahoo.com> wrote:
>>> I consulted the documentation.  It doesn't explain why how '-vsync 0'
>>> stops
>>> ffmpeg from artificially inserting duplicate frames into your output
>>> image files.
>>> If you think the documentation does explain this, them point out which
>>> sections
>>> explain.
>> The vsync section in the manual describes the behaviour of the
>> different vsync options.
>> "0" passes through each frame with its timestamp to the muxer. This
>> means if your source is VFR then you will not have a CFR output. If
>> you were transcoding video+audio, this would probably result in audio
>> desynch.
>> "1" will duplicate and/or drop frames as needed in order to have the
>> required output fps. Eg, if a section of the VFR video is 10fps, and
>> the requested ofps is 25fps, then ffmpeg will duplicate frames to get
>> the desired fps, and avoid audio desynch. This produces CFR files.
>> "2" can drop frames and/or pass them through with their timestamps to
>> the muxer. This enables the muxer to generate VFR files.
>> The default is "-1", which intelligently chooses between "1" and "2"
>> based upon the capabilities of the muxer you are feeding. So if you
>> don't want duplicates or dropped frames, you must not use "-1", "1" or
>> "2".
>> 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 linked ticket explains the reasoning for this. This is effectively
>> your position:
>>> Yeah I agree that for most people, their default thought is
>>> "I'm converting to images? It will output one output image per
>>> input frame" so +1 from me for making -vsync vfr default in this case...
>>> :)
> Just as one sampling point: For me it is the expected/intuitive
> behavior for the image2 demuxer (just to emit the images as they are)

image2 DEmuxer really does that.

<fill in> muxer is another story.

> as well and I can understand his astonishment as I have run into that
> myself in the past. Having said that, with any default you decide for,
> you will normally have people disagreeing and you either vote or let
> someone who feels responsible (maintainer) decide and trust them.
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