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I am writing a Lights/Shadows system for my game using Java alongside the LWJGL. For each one of the Light-Emitting Entities I generate such a texture:

Blue Light with Shadows Red Light with Shadows

I should warn you that these Textures are full of (0, 0, 1) or (1, 0, 0) pixels, and the gradient effect is achieved with the alpha channel. I interpret the Alpha channel as a gradient factor.

Afterwards, I wish to blend every light/shadow texture together on a single texture, each at it's respective correct position. For that, I use a Framebuffer. I tried to achieve the desired effect using the following blend equation/function combination:

glBlendEquationSeparateEXT(GL_FUNC_ADD, GL_MAX);
glBlendFuncSeparateEXT(GL_SRC_ALPHA, GL_DST_ALPHA, GL_ONE, GL_ONE);

I chose GL_ONE/GL_ONE for the Alpha Channel Blend Function arbitrarily, for GL_MAX will only do max(Sa, Da), as stated here, which means that the scaling factors are not used. The result of this combination is the following: Blended Lights/Shadows This image was obtained with Apple's OpenGL Driver Profiler, so I did not render it using my application (which could mess with the final result). The next step would be to render this texture over the actual game using multiply-blending, in order to darken the image, but the lights/shadows texture is obviously wrong, because we can see the edges of individual light/shadow textures over each other.

How should I proceed to achieve the desired result?

Edit: I forgot to explain my choices for the scaling factors: I think that it would be right to simply add the colors of each light (pondering each of them with their respective alpha values) and choose the alpha of the final fragment to be the biggest of each overlapping light.

Thiago
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1 Answers1

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Imagine that one of your texture rectangles was extended outside its current border with some arbitrary pattern, like pure green. Imagine further that we were somehow allowed to use two different blending functions, one inside the border, and one outside. You would get the same image you have here (none of the green showing) if outside the border you used the blend function

glBlendFuncSeparateEXT(GL_ZERO, GL_ONE, GL_ONE, GL_ONE)

We would then want whatever blending function we use inside to give us a continuous blending result. The blending function

glBlendFuncSeparateEXT(GL_SRC_ALPHA, GL_DST_ALPHA, GL_ONE, GL_ONE)

would not give us such a result. It is not so much because the first parameter which would mean ignoring the source near the border (small alpha on the border, if I read your image description correctly). So it must be the second parameter. We want the destination only when the source alpha is small. Change GL_DST_ALPHA to GL_ONE_MINUS_SRC_ALPHA. This would be more standard, but maybe I'm not understanding your objectives?

Little Endian
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