In Gimp I want to generate a gray scale image of a RGB image defined by this per-pixel formula:
value_xy = max(R_xy, G_xy, B_xy).
I didn't find a builtin feature so I wonder if there's some generic filter out there where I can operate on a per-pixel basis by just defining some formula.
Is there something like this already? Would I have to write my own filter for Gimp?