Vertex Shader/Defined Inputs

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Vertex Shaders have the following built-in input variables.

in int gl_VertexID;
in int gl_InstanceID;
gl_VertexID​
the index of the vertex currently being processed. When using non-indexed rendering, it is the effective index of the current vertex (the number of vertices processed + the first​ value). For indexed rendering, it is the index used to fetch this vertex from the buffer.
Note: gl_VertexID​ will have the base vertex applied to it.
gl_InstanceID​
the index of the current instance when doing some form of instanced rendering. The instance count always starts at 0, even when using base instance calls. When not using instanced rendering, this value will be 0.