Use only alpha channel of texture in OpenGL?

Question

Hey, I'm trying to draw a constant color to the framebuffer and blend it using the alpha channel from an RGBA texture. I've been looking at glBlendFunc and glBlendColor, but can't seem to figure out a way to ignore the RGB values from the texture. I'm thinking I'll have to pull out the alpha values myself and make a second texture with GL_ALPHA. Is there a better way to do this?

Thanks!

Answer 1

When using glBlendFunc, GL_SRC_ALPHA and GL_DST_ALPHA can be used to select the alpha channel in your texture (which to choose depends on your blend operation and textures). For more detailed blending tutorials see:

• nehe tutorial #32
• tutorial #3

The relevant OpenGL documentation:

• http://www.opengl.org/sdk/docs/man/xhtml/glBlendFunc.xml