A simple question about glScissor().

Does the scissor operation such as glScissor() operate before the rasterization of the geometric elmenets or after it?

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Originally Posted by newbiecow

Does the scissor operation such as glScissor() operate before the rasterization of the geometric elmenets or after it?

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The OpenGL Specs say that the scissor test comes after rasterization, but the OpenGL Specs only define behaviour, not implementation. That means a driver may implement the scissor test in any way that yields the same results as the OpenGL Spec. Many drivers implement the scissor in the rasterizer, i.e. they don't generate fragments that fall outside the scissor rectangle.

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Originally Posted by mbentrup

The OpenGL Specs say that the scissor test comes after rasterization, but the OpenGL Specs only define behaviour, not implementation. That means a driver may implement the scissor test in any way that yields the same results as the OpenGL Spec. Many drivers implement the scissor in the rasterizer, i.e. they don't generate fragments that fall outside the scissor rectangle.

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Thanks a lot!
And can you tell me what graphics card you are using? Or do you know whether the graphics card as ATI or Nvidia do scissor test before rasterization or after it?

I don't know how specific cards implement scissor, but I think most implement the scissor test directly in the rasterizer. See http://fgiesen.wordpress.com/2011/07...e-2011-part-6/ for a detailed explanation how modern hardware rasterizers work.

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Originally Posted by mbentrup

I don't know how specific cards implement scissor, but I think most implement the scissor test directly in the rasterizer. See http://fgiesen.wordpress.com/2011/07...e-2011-part-6/ for a detailed explanation how modern hardware rasterizers work.

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Thanks a lot, mbentrup! You've really provided something valuable.


Best Regards,


newbiecow