GLSL vertex shader cancel render

opengl glsl terminate vertex-shader
user346443 picture user346443 · Aug 13, 2013 · Viewed 9.6k times · Source

Can the rendering for a pixel be terminated in a vertex shader. For example if a vertex does not meet a certain requirement cancel the rendering of that vertex?

Answer

Nicol Bolas picture Nicol Bolas · Aug 13, 2013

I'll assuming you said "rendering for a vertex be terminated". And no, you can't; OpenGL is very strict about the 1:1 ratio of input vertices to outputs for a VS. Also, it wouldn't really mean what you want it to, since vertices don't get rendered. Primitives do, and a primitive can be composed of more than one vertex. What would it mean to discard a vertex in the middle of a triangle strip, for example.

This is why Geometry Shaders have the ability to "cull" primitives; they deal specifically with a primitive, not merely a single vertex. This is done by simply not emitting any vertices; GS's must explicitly emit the vertices that it wants to output.

Vertex shaders now have the ability to cull primitives. This is done using the "cull distance" feature of OpenGL 4.5. It's like gl_ClipDistance, only instead of clipping, it culls the entire primitive if one of the vertices crosses the threshold.

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