Hi guys - I'm using render to texture in one of my apps and I'm experiencing alpha blending issues. Here's what I have: simple RTT (render to texture) with two quads. Each quad has as a background an opaque black background (24-bit BMP). In front of that is a small (doesn't cover the entire background) 32-bit TGA with varying transparency. Here's the issue: on the portion of the quad that is covered by 32-bit TGA I can see right through. It appears as though OpenGL is doing what it's suppose to do - blending not only the RGB but also the alpha. For my blending function I'm using (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA) - So, when this 32-bit TGA is blended against the opaque black background it's blending the alpha components as well which results in an overall decreased alpha value. Hence, that opaque black background is now semi-transparent.
What I did to (hopefully) solve this is to premultiply the incoming fragments via a shader attached to each incoming node that is drawn into the RTT and use (GL_ONE, GL_ONE_MINUS_SRC_ALPHA) as my blending function for that node. Seems to solve it. At least now everything is blending the way I'd expect. The opaque parts are opaque and the semi-transparent are blending as expected against the opaque background, etc. My question is this: what other method might I employ to deal with these alpha blending issues? How do you guys recommend handling this? What's the recommend route to take?
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