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I am compositing one image on another using glBlendFunc() trying to approximate photoshop's difference compositing feature. I want similarities in the two images to be black while anything different pops out.

As I understand the glBlendfunc() is associative for any images composited in the context. Is that correct?

kernelK
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I can't really think of any way to do this with glBlendFunc. You could use something like this:

glBlendEquation(GL_FUNC_SUBTRACT);
glBlendFunc(GL_ONE, GL_ONE);

But it would only work when the magnitude of the second texture was larger with the first (any subtraction result that is negative would be clamped to zero).

I'd recommend just sampling both textures at the same time with a multisampling shader, and outputting the result as the absolute difference.

gl_FragColor = abs(texture2D(tex1, uv) - texture2D(tex2,uv));
Tim
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  • is texture2D a DirectX function? What I am trying to do I can accomplish using CoreImage filters on OSX, but I am trying to get around using CoreImage for performance reasons. – kernelK Jun 24 '12 at 22:09
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    texture2D is a GLSL function. Are you at all familiar with shader programming? @kernelK – Tim Jun 24 '12 at 22:12
  • I'm not unfortunately. But now that I know that it is a part of GLSL, I'll look further there. – kernelK Jun 24 '12 at 23:14
  • The best resource for image manipulation in OpenGL is the GLSL Shader Orange Book. This book has ample examples of calculating various blending functions when compositing texture in a fragment shader. – kernelK Sep 14 '13 at 00:06