I've been having a lot of trouble trying to get a OpenGL blend function to work as I'd expect it to with like what I'd expect (or from any sensible image editing program). As an example, I'll use these two images to blend (bit difficult to see on white backgrounds so the colors are labeled):
Images to be blended
This is what I expect to happen (and what happens in paint.net):
Obviously opengl's default blend function makes it look like this (very wrong):
After a ton of testing, this is the closest I could get to creating a "good" blend function:
glBlendFuncSeparate(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA)
Looking back at the original expected result though, you'll notice that some of the colors are a bit dimmer than they should be (the middle left part). Specifically, they are premultiplied to half their color value (because of the .5 alpha), and I can't seem to make a function that does not do this (without causing odd blending issues with the barely visible red transparent part).
Does anyone know a solution to this issue? One that I had was to use premultiplied alpha in the sources (while I dont want to do this because it requires extra work to convert every color I use in my game to premultiplied or just write some stuff in each shader) and do it like that:
glBlendFuncSeparate(GL_ONE, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA) (No premultiplication)
Obviously thats wrong too, but this is actually the only correct result I've gotten so far:
glBlendFuncSeparate(GL_ONE, GL_ONE_MINUS_SRC_ALPHA, GL_ONE, GL_ONE_MINUS_SRC_ALPHA) (Premultiplied inputs)
Only problem is, how would I get rid of the premultiplication to display it on the screen? It would probably require an additional render cycle for each thing I blend and that seems way too complex for this issue, so I'm still looking for an answer (its interesting that I cant find anything on this, because OpenGL is so widely used that I'd image someone else ran into this problem).