How can I make parts of my wall (2 quads -> it has thickness) transparent. The problem is that I don''t know at coding time where to place and what size of window(s) on my wall. So the user gives the data to the program at runtime, so I think alpha channel can''t be used, I can''t store the place of the windows in the wall texture files. Another solution would be to subtract a prism (the window) from a bigger prism (wall) but I don''t know if this is supported by OpenGL. Please help me! If you can help, please write a short (at least) pseudo-code what the order of operations should be and what functions should I use or if you know a place on the Internet when should I get more info on the topic, tell me! Vic

From what i can understand from you description you want to place a transparent window somewhere on a quad.

use a stencil map.

draw the window(or rather the cliping polygons) to the stencil buffer.
Draw the window inside that area
and draw the wall outside that area