CODEWITHSUNDEEP
openglopengl-4geometry-shader
Hal Gee
Hal Gee

Reputation: 105

OpenGL geometry shader, setting input size

I've written my first geometry shader with success. It takes in lines and outputs a little triangle at the center of each. I could do the same thing for triangles easily enough, but what about a cube? Is there a way to get a geometry shader to operate on an arbitrary number of points, or at the very least more than 3? I know I could compute the center myself and do another drawing operation, but I'd like to know if it's possible inside the shader.

Thanks.

Upvotes: 3

Views: 433

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 474436

Geometry shaders take as input a primitive, not a number of vertices. I mean yes, a specific primitive is made of a specific number of vertices. But GS's don't take vertex counts; they take primitives.

There are a number of special primitive types that allow GS's to access more vertices than those in the base primitive type. But these are for referencing vertices adjacent to the main primitive's vertices, and it's difficult to try to make them work as a general mechanism for consuming X vertices.

So you can only use a vertex count that matches a primitive's vertex count: 1, 2, 3, 4, or 6. Outside of these specific vertex counts, you can't make a GS do what you're trying to do.

You can attempt to employ tessellation, as patch vertex counts are user-specified (though limited by the implementation). But tessellation is more restrictive in terms of generating vertices.

Upvotes: 3

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