Colours with a saturation of zero (Greys) have no hue value (stored as
RGB). When increasing the saturation of such colours, "saturation" layer
blend mode assumes they have a hue of zero, i.e. red.
I'm assuming using a HSV or HSL colour profile might avoid this (by
having a redundant hue value even when saturation is zero), but I
haven't found any such profiles to test.
Note that increasing saturation using the hue-saturation tool does not
give the same result: white remains white, grey remains grey.
I would regard the saturation layer blend mode's result as unexpected
behaviour for the following reasons:
- Making assumption of non-existent hue data.
- Not useful to end user. Controlling the saturation of parts of an
image via a saturation layer doesn't work if the image contains any
white (or grey) pixels. For example, an image of a green apple with
white highlights will end up as a green apple with red or pink
highlights. (With discontinuity being introduced as very pale green
suddenly reaches pure white)
- Behaviour different from tool with same name.
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