Subtractive blending with OpenGL?

Question

I would like to for example draw shapes A, B, C then set the blender, then draw shape D and everywhere where shape D is, the scene shows my background color of (1,1,1,0). Thanks

Answer 1

I'm not aware of that being possible directly. There are a few possibilities to get the same effect though. The cleanest would probably be to start by filling the stencil buffer with 1's, then drawing shape D into the stencil buffer with 0's, then draw everything else (with the stencil buffer enabled, of course).

Answer 2

glBlendEquation can do it.

Answer 3

Much simpler than other answers :

• Display shapes A, B and C the normal way
• glDisable(GL_DEPTH_TEST);
• glDisable(GL_ALPHA_TEST);
• glDisable(GL_BLEND);
• Display shape D with color (1,1,1,0)

and you're done.