#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>
FFmpeg <https://ffmpeg.org>
FFmpeg issue tracker
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