[FFmpeg-trac] #4065(avfilter:new): support aspect-ratio changes in scale and pad filters

[FFmpeg]

#4065: support aspect-ratio changes in scale and pad filters
-------------------------------------+-------------------------------------
             Reporter:  mikaraento   |                     Type:
               Status:  new          |  enhancement
            Component:  avfilter     |                 Priority:  normal
             Keywords:               |                  Version:
             Blocking:               |  unspecified
Analyzed by developer:  0            |               Blocked By:
                                     |  Reproduced by developer:  0
-------------------------------------+-------------------------------------
 Currently it's reasonably easy to column (and letter) -box fixed-aspect-
 ratio input in ffmpeg, by using something like:

 {{{
 ffmpeg -filter_complex '[0:0]scale=iw*sar:ih,scale=-1:432,pad=768:432:(ow-
 iw)/2:(oh-ih)/2,setdar=16:9[outv]'
 }}}

 However, if the input has aspect-ratio changes (dimensions or sar) mid-
 stream, the above won't work.

 It would be nice to have per-frame -evaluated versions of the variables
 for scale and pad. (and the required other changes in these filters that
 assume that they don't change)

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Ticket URL: <https://trac.ffmpeg.org/ticket/4065>
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