https://bugs.kde.org/show_bug.cgi?id=380327
Bug ID: 380327
Summary: HSV blending modes do not correctly handle source
saturation
Product: krita
Version: 3.1.2
Platform: MS Windows
OS: MS Windows
Status: UNCONFIRMED
Severity: normal
Priority: NOR
Component: Color models
Assignee: krita-bugs-n...@kde.org
Reporter: sky...@quantentunnel.de
Target Milestone: ---
Created attachment 105766
--> https://bugs.kde.org/attachment.cgi?id=105766&action=edit
Overview of wrong (Krita) and correct (Gimp) HSV blending
The result of blending a color-layer and value-layer with HSV layer blending
modes "HSV Color" and "HSV value" appears to be not correctly calculated (maybe
similar issues with HSL, HSY...).
To be: In the Krita result of the attached overview (top right figure),
vertical y-axis slices (like the orange line) should represent gradients with
same hue+saturation and increasing value from top to bottom. Like it is
correctly implemented in Gimp (top center figure). Saturation and value in HSV
mode are independent of each other. Value refers to the MAX(r, g, b) while
saturation determins the ratio between MIN(r, g, b) and MAX(r, g, b). The value
affects all RGB channels proportionally (multiply).
As is: Instead of constant saturation over the whole vertical slice, the
saturation in the top part is all 100% and decreases from a certain point
towards the bottom from 100% to 0%. The extracted saturation channel (right
figure) illustrates the problem, where the whole top left half of the demo
setup composition is 100% saturated, which not at all corresponds to the
initial redscale color gradient.
Looking at the change in RGB values along the orange section of top right
figure from bottom (white) to top (black), the absolute values are reduced in
parallel, more or less by the same amount (subtract). I suppose here's a
problem in the formula, as the ratio between R, G and B should be constant.
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