   direct texture data acess
  Hello, Here is the context of the problem: I generate a greyscale texture with a frame buffer object (FBO). This texture is meant to be applied as a an alpha mask on another texture. In order to achieve this two ideas went (vaguely) through my mind: - Just write a color to alpha function and apply it on the greyscale texture to make an alpha mask out if it. Leading to the question: How can one directly access texture data maintained by OpenGL's machinery? - Use OpenGL mechanism of texture combiner functions. Idea's feasibility  which disappeared after some reading. but I might be wrong!Hence my question: Can one directly access texture data maintained by OpenGL? (and the subsidiary question: Is there an official way(tm) to do the global operation I'm aiming at?)   