[FFmpeg-user] set image width only?

[Lou]

On Thu, 8 Sep 2011 01:23:01 +0200
Stefano Sabatini <stefano.sabatini-lala at poste.it> wrote:

> On date Wednesday 2011-09-07 21:29:02 +0100, Mark Himsley encoded:
> > On 07/09/2011 21:23, lovelove wrote:
> > >Hi. Sorry for being a newbie, but I have looked everywhere in
> > >http://ffmpeg.org/ffmpeg-doc.html and only found *-s WxH*, but how
> > >can I /set the width/ e.g. to 400 /*without specifying the
> > >height*/ (which FFmpeg should determine itself based on the aspect
> > >ratio)? (I'm outputting still images). Many thanks.
> > 
> > You missed this part of ffmpeg-doc.html
> > 
> > http://ffmpeg.org/ffmpeg-doc.html#SEC89
> 
> Better to read instead:
> http://ffmpeg.org/ffmpeg.html

More specifically see the scale filter information:
http://ffmpeg.org/ffmpeg.html#SEC155

Which currently states:
"If the value for width or height is -1, the scale filter will use, for
the respective output size, a value that maintains the aspect ratio of
the input image."