https://bugs.kde.org/show_bug.cgi?id=424797
Bug ID: 424797
Summary: Alpha calculations on alpha masked layers with opacity
is 'wrong'?
Product: krita
Version: nightly build (please specify the git hash!)
Platform: Microsoft Windows
OS: Microsoft Windows
Status: REPORTED
Severity: normal
Priority: NOR
Component: General
Assignee: krita-bugs-n...@kde.org
Reporter: i...@ralek.art
Target Milestone: ---
Created attachment 130486
--> https://bugs.kde.org/attachment.cgi?id=130486&action=edit
Example of the bug
This one's going to be a hard one.
Krita seems to miscalculate how semi-opaque pixels merge with other semi-opaque
pixels when combined as an alpha mask. This can create a 'halo' effect around
lineart or completely mess up gradients with alpha masks applied to them.
To better explain the issue, I've included a Krita file with 3 examples. Take a
look at the red square to see what I'm talking about.
The top group [group 1] is the setup when I first noticed this behavior.
Something was wrong, off. Even if the calculations on this layer are somehow
mathematically 'correct', this definitely is not what artists want since it can
make lineart look like it has a white 'gap' between the lineart and the color
behind it. For example: https://i.imgur.com/96114DI.png
There's a fix for it, technically speaking.
In group 2, It's the exact same setup, but the 50% opacity layer is set to the
'addition' layer blend mode. This should theoretically change nothing, but it
fixes the alpha issue entirely.
The final group [group 3] is the reference, this is how the line should look if
there was no alpha mask entirely and the line was just straight up colored what
you wanted it to be. Notice that it more closely matches group 2 than group 1,
which is affected by the bug.
Krita
Version: 4.3.1-alpha (git 08ecea2)
OS Information
Kernel Version: 10.0.18363
Pretty Productname: Windows 10 (10.0)
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