Right,
since x / y = x * (1 / y)
you can multiply two images, if you ‘invert’ the second.
Now invert does not mean just “channel -> invert” (255 – brightness)
but 255 / brightness
You can do this using a Curve
I suggest using Color Finesse, it has a better interface for finetuning and a more logical interpolation between curve-points
Just make a curve with these values:
input / output:
1 – 255
2 – 128
4 – 64
8 – 32
16 – 16
32 – 8
64 – 4
86 – 3
128 – 2
255 – 1
Is that what you mean ?
I’ve tried it and can’t seem to find a use for this: it returns a Lot of dark values