I have been trying to implement some of the layer blending modes of GIMP (GEGL) to Python. Currently, I am stuck in Subtract Blending mode. As per documentation, Subtract = max(Background - Foreground, 0)
. However, doing a simple test in GIMP, with Background image = (205,36,50)
and Foreground image = (125,38,85)
, the resultant composite image/colour comes to be (170, 234, 0)
which doesn't quite follow the math above.
As per understanding, Subtract does not use Alpha Blending. So, could this be a compositing issue? Or Subtract follows different math? More details and background can be find in a separate SO question.
EDIT [14/10/2021]:
I tried with this image as my Source. Performed following steps on images normalised in range [0, 1]
:
sRGB -> linear RGB
was done) and obtained this from my implementation which matches with GIMP result.sRGB -> linear RGB
conversion on Colour Dodge and Source image. [Reference]Background = Colour Dodge
and Foreground = Source Image
linear RGB-> sRGB
I obtain this from POC. Left RGB triplet: (69,60,34); Right RGB triplet: (3,0,192). And the GIMP result. Left RGB triplet: (69,60,35); Right RGB triplet: (4,255,255)
If you are looking at channel values in the 0 ➞ 255
range they are likely gamma-corrected. The operation is possibly done like this:
0.0 ➞ 1.0
range using something likeL = ((V/255) ** gamma) (*)
V = (255 * (Diff ** (1/gamma)))
With gamma=2.2
you obtain 170 for the Red channel, but I don't see why you get 234 on the Green channel.
(*) The actual formula has a special case for the very low values IIRC.