Bug 505420

Created attachment 182142 [details]
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.