glReadPixels doesn't copy alpha values

OpenGL OpenGL: Advanced Coding
1

I’m trying to read RGBA data from the frame buffer using glReadPixels. The color data is read fine. However, the alpha doesn’t get read.

The value of GL_ALPHA_BITS is 0 with my setup. The values of GL_RED_BITS, GL_GREEN_BITS, and GL_BLUE_BITS are all 8. Is that the cause of the problem? If so, is there a way to get the frame buffer to have 8 bits of alpha? I’m running on a TNT card, could it be a hardware limitation?

I appreciate any help.

2

You’ll need to be in 32 bpp mode, and you will need to select a pixelformat that has an 8 bit alpha buffer. I’m certain that will work. However, I can’t remember if the TNT driver will fall back to software if you select an 8 bit alpha buffer in 16 bpp mode (if you even can), or if it just tosses the alpha into the bit bucket.

3

I set the pixelformat to an 8 bit alpha buffer and it worked like a charm.

Thanks for your help.