It should be possible to grab it without any fancy extras. You have to make sure you ask for a 32-bit backbuffer and must ask also for an alpha buffer. Then you get a destination alpha, so long as your graphics card does 32-bit mode not 24-bit. Then a simple glReadPixels() will read the contents, or you can grab the backbuffer into a texture. I know it works because I switch off the RGB channels and grab only the destination alpa channel into a GL_ALPHA texture and use that texture to do rendering, which I know works.