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What Is A Geometry Shader?

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Can someone highlight the principles of its effect to rendering. I'm continuing to try and find a decent non diagramatical description of its purpose and im struggling. Dave

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I personally find the diagrams to be more useful than the textual descriptions [smile] We need to get Dustin on the case - he seems to know stuff about the GS

It's often also referred to as a "Topology Shader" - a term that seems to pop-up more often in general graphics-programming literature.

Simply put it operates on the triangles and their 1-ring adjacent vertices and provides more information to the rasterizer (and optionally to the stream-out). With more information about the primitive being rendered its possible to come up with some better pixel-shaders as well as to optimize the pipeline (single pass cubemap rendering) a bit more.

Any more specific questions?

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Geometry shaders are going to be really cool. From what I understand. Geometry shaders take as input a set of primitives. This means you no longer have the 1 vertex in, 1 vertex out limit. What comes out of the processing that a Geometry shader gets in is a set of processed primitives. Primitives being the points, lines and triangles. In addition to Geometry shaders, Direct3D 10 will introduce Stream outputs that will ease the way we extract data from a certain point in the pipeline. After the Geometry shader has done it's processing, you are able to extract the processed primitives via the Stream output.

To put it plainly like a vertex shader handles processing of primitives on a per-vertex basis. The Geometry shader will handle the processing of primitives on a per-primitive basis.

Basically the pipeline will look like this
1) Command Input
2) Input Assembler
3) Vertex Shader
4) Geometry Shader
5) Rasterizer /Interpolator
6) Pixel Shader
7) Output Merger

I hope this helps.
Take care.

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