You are likely working directly on gamma-corrected values instead of "linear light". See my answers in this thread: https://www.gimp-forum.net/Thread-Layer-...cial-guide.
Not included in the referenced post but something equivalent exists for layer masks. The gamma-corrected grayscale value of a 50% opacity layer mask is #BC, and a black layer over white or a white layer over black with a mask with #BC will produce a the middle gray which is also #BC.
Some more info here: https://en.wikipedia.org/wiki/SRGB#Transformation
The python functions that implement the sRGB transforms (and give identical result to Gimp as far as my tests go):
... but if you process whole images I hope you are using numpy or equivalent.
Not included in the referenced post but something equivalent exists for layer masks. The gamma-corrected grayscale value of a 50% opacity layer mask is #BC, and a black layer over white or a white layer over black with a mask with #BC will produce a the middle gray which is also #BC.
Some more info here: https://en.wikipedia.org/wiki/SRGB#Transformation
The python functions that implement the sRGB transforms (and give identical result to Gimp as far as my tests go):
Code:
import math
# Linear is [0.0 .. 1.0]
# sRGB is [0 .. 255]
def srgbToLinear(v255):
v=v255/255.
return v/12.92 if v <= 0.04045 else math.pow((v+0.055)/1.055,2.4)
def linearToSrgb(linear):
srgb=linear*12.92 if linear < 0.0031308 else 1.055*math.pow(linear,1/2.4)-0.055
return 255*srgb... but if you process whole images I hope you are using numpy or equivalent.