I need to have a fade effect on a texture mapped to a quad; in theory, it should increase the alpha value of the entire texture over an interval of time until the texture is invisible. I've enabled alpha blending and I can draw transparent textures (PNGs with an alpha channel), but I haven't figured out how to change the alpha of a texture in the code before I draw it. How can I do it?

Vertex colours will be modulated with the texture colour by default. Change your vertex colours' alpha to something other than 1 and you should fade the whole thing (eg. glColor4f(1.0f, 1.0f, 1.0f, 0.5f); ).

Thanks; that works pretty well. I've had to optimize how many times I use glColor4f though, it can slow things down in the game I'm coding (which uses a lot of two-dimensional textures).