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OpenGL How to do additive blending between two polygons excluding the clear color?

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I'm trying to blend the colors of two rectangles I've drawn in OpenGL without including the background color (clear color). How do I go about doing this? I have blending enabled/glBlendFunc(GL_ONE,GL_ONE); and all that but when I draw my two transparent rectangles they not only blend their colors together but they also blend the background color. I dont want this. I only want the two rectangles to blend.l

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So we understand you correctly:


- You have two rectangles

- Where they don't overlap, you want them to be opaque so the background doesn't show through

- Where they do overlap, you want them to be the sum of their colors (background also not showing through)


Is that correct? Or is there more to it? You say they are "transparent rectangles", yet you don't want the background to show through. That's what's confusing me.


Anyway, assuming my assumptions are correct, I think you can do the following (I don't use OpenGL, so I'm not familiar with the exact names for the blending functions, but it should be clear enough):


- use separate blending for the alpha and color channels

- ensure the render target has an alpha channel, and that it's been cleared to zero. And that anything you drew previously on it didn't write to the alpha channel (Alpha = SourceAlpha * Zero + DestAlpha * Zero).

- when drawing your rectangles, use:

    Alpha = SourceAlpha * One + DestAlpha * Zero

    Color = SourceColor * One + DestColor * DestAlpha


So after drawing one rectangle, the render target should have alpha = 1.0 where the rectangle is. And that basically becomes an indication that you *do* want to blend that part of the background (which has the first rectangle on it) with the next rectangle.


I haven't tried it, but I think that should work.

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