[FFmpeg-trac] #9872(undetermined:new): WebP vs PNG alpha

[FFmpeg]

#9872: WebP vs PNG alpha
-------------------------------------+-------------------------------------
             Reporter:  Jozef        |                    Owner:  (none)
  Chutka                             |
                 Type:  defect       |                   Status:  new
             Priority:  normal       |                Component:
                                     |  undetermined
              Version:  unspecified  |               Resolution:
             Keywords:  scale alpha  |               Blocked By:
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
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Comment (by Jozef Chutka):

 The workaround is to create yet another `color=#ffffff00` and `overlay`
 with the webp before the `scale` and the following filter graph. (`movie`
 replaced by `color+movie+overlay`):

 {{{
 ALPHA PROBLEM:
 ffmpeg -filter_complex
 "color=white:200x200[b];movie=x.webp,scale=200:200,[b]overlay" -y out-
 webp-scale.png

 ALPHA FIXED:
 ffmpeg -filter_complex
 "color=white:200x200[b];color=color=#ffffff00:size=100x100[c];movie=x.webp,[c]overlay,scale=200:200,[b]overlay"
 -y out-webp-scale-fix.png
 scale-fix.png
 }}}

 This makes me think that there might some strange format used for webp
 movie, which can not be enforced/changed by `format` filter, but can be
 changed/fixed by `overlay`-ing it to artificial `color`
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9872#comment:4>
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