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

[FFmpeg]

#9872: WebP vs PNG alpha
-------------------------------------+-------------------------------------
             Reporter:  Jozef        |                     Type:  defect
  Chutka                             |
               Status:  new          |                 Priority:  normal
            Component:               |                  Version:
  undetermined                       |  unspecified
             Keywords:  scale , png  |               Blocked By:
  , webp , alpha                     |
             Blocking:               |  Reproduced by developer:  0
Analyzed by developer:  0            |
-------------------------------------+-------------------------------------
 using ffmpeg version 2022-07-31-git-1368b5a725-full_build-www.gyan.dev

 Attached x.png and x.webp (webp created from png) have transparent
 background covered by white X shape.

 For some reason FFmpeg scale filter treats the x.png and x.webp
 differently.

 Applying scale filter on the x.webp, causes the output video alpha around
 the X shape to be premultiplied and so renders with ugly edges.

 Following commands, esp. out-webp-scale.png output demonstrates the issue:

 ```
 ffmpeg -filter_complex "color=white:200x200[b];movie=x.png,[b]overlay" -y
 out-png.png

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

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

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

 I tried all possible scale filter flags, and also format changing (before
 and after scale) but no luck to achieve the same result. I am able to un-
 premultipy the problematic one with `unpremultiply` filter but I would
 like to avoid it.

 I wonder if this is a bug or an expected behaviour.
-- 
Ticket URL: <https://trac.ffmpeg.org/ticket/9872>
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