https://bugs.kde.org/show_bug.cgi?id=505420
Bug ID: 505420
Summary: Unexpected alpha blending with linear color space
Classification: Applications
Product: krita
Version First 5.2.9
Reported In:
Platform: Microsoft Windows
OS: Microsoft Windows
Status: REPORTED
Severity: normal
Priority: NOR
Component: * Unknown
Assignee: krita-bugs-n...@kde.org
Reporter: m...@huegon.com
Target Milestone: ---
Created attachment 182142
--> https://bugs.kde.org/attachment.cgi?id=182142&action=edit
Image of the described problem. The two sides uses different color spaces for
alpha blending (blend mode "Nornal").
SUMMARY
When using a linear color space, the alpha blending with “Transparency
Checkerboard” and a background layer of the same color will give different
results. See attached image.
STEPS TO REPRODUCE
1. Set image to a linear color space.
2. Set the “Transparency Checkerboard” to some color (for example black).
3. Create a gradient from some color (for example white) to fully transparent.
4. Create a layer behind the gradient layer. Fill half of this layer with the
same color as the “Transparency Checkerboard” color and the other half as fully
transparent.
OBSERVED RESULT
Even though the “Transparency Checkerboard” and the background are the same
color, the halfs look different. The half that is filled black is the expected
result, while the transparent side seemingly use sRGB for the blending even
though the image should use a linear color space. The attached image shows this
better than words.
EXPECTED RESULT
That the two sides look the same.
SOFTWARE/OS VERSIONS
Windows: 10
Link to Krita forum post about this issue that has some additional information:
https://krita-artists.org/t/unexpected-alpha-blending-with-linear-color-space/126236
I am not familiar with the Krita source code, but I would assume this issue
arises because Krita internally uses the correct color space when rendering the
layers, but not when the resulting image is blit to the canvas. This is
problematic when you want to work in a linear color space and also have an
accurate image of how the resulting semi-transparency will look when exporting.
The work-around for now is to create a background-layer while working on the
image so it looks the same as when the image is exported.
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