https://bugs.kde.org/show_bug.cgi?id=413125
--- Comment #9 from Estecka ---
Hi,
I'm a long time user of Gimp, who just started trying out Krita 5.2.2.
Krita's colour to alpha filter indeed works nothing like Gimp's, and no
changing the threshold value can fix that:
[Red to Yellow gradient, Orange
removed](https://bugs.kde.org/attachment.cgi?id=170471)
At a threshold of 43/255, Krita is both overly agressive on the red side of the
gradient, andoverly lenient on the yellow side.
The red side never reaches 100% opacity, while the yellow leaves a large strip
that is 100% opaque. Both sides of the gradient still have traces of orange in
them, when they should be pure Red or Yellow.
> I don't know how is implemented in gimp 2.10 but it gives me the same results.
As was mentionned, Gimp can be very finicky with its blending, so in order to
get the expected result in Gimp you need to either:
- With the Color to Alpha filter, switch the layer's composite space to
"Perceptual", or switch the Blend mode to Legacy Normal.
- Or, do not use the Color to Alpha filter, instead use the Bucket Fill tool
with the non-legacy "Color Erase" blend mode.
My expectation for this filter is that it should be the opposite operation to
the "Behind" blend mode. Removing a colour and then backfilling with the same
colour should result in a visually indistinguishable picture. (Safe for
arithmetic imprecision.)
I haven't checked the code for Gimp, but intuitively: if the Normal/Behind
blend modes are linear interpolations between two colours, then Colour to
Alpha/Colour Erase would be a form of linear *extrapolation*. Amongst the line
of possible results, it would pick the first colour where any of the RGB
channel has reached an extreme (either 0 or 255).
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