adapt xcompmgr to enable a color-transparency

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.

  1. 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.

  2. 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 !