I would like to adapt xcompmgr (for an old project, with fvwm window manager !!!).
Aim is to change a window with a darkgrey background to have the background transparent, but leave all other colors not transparent.
I have seen this feature working with a compiz plugin; mainly: it makes a matrix of boolean: where colors was the same as the “background color”, then use this matrix as the alpha color.
xcompmgr is written in C, using XRender functions. I don’t know if it is easy to manipulate then the XPixmap into opengl.
Point 1. and 2. are difficult for me… i someone have an idea, it could helps !