[LWJGL] Combining 2 images but *not* combining their alpha values for output.

Newbie & Debugging Questions
#1

I’m trying to workout a shadow system that “builds” static shadows in my game, but in some instances the shadow images overlap, and when that happens I want the shadows to “combine”. I feel like there’s a GL11 setting I can use in LWJGL, but I can’t seem to figure it out. All my results either end up like the first example, or totally black/while/incorrect.

Technical details:

  • All shadows are rendered at the same time
  • All shadows have the exact same color/alpha values
  • Only difference in the various shadows is their shape.

Top is what I get, the bottom is what I want.

http://sixtygig.com/junk/transparencyoverlap.png

Any advice? I’m totally stumped, and a bit of a newbie when it comes to playing with OpenGL rendering settings.

#2

Render your shadows without alpha to a texture, then blend the texture with your scene.

#3

(FBO)

#4

That effect is not possible to achieve in OpenGL using only blending modes. Riven is correct (obviously).