Tessellation Control Shader/Defined Inputs

Tessellation Control Shaders provide the following built-in input variables:

 in int gl_PatchVerticesIn;
 in int gl_PrimitiveID;
 in int gl_InvocationID;

gl_PatchVerticesIn is the number of vertices in the input patch. gl_PrimitiveID is the index of the current patch within this rendering command. gl_InvocationID is the index of the TCS invocation within this patch. A TCS invocation writes to per-vertex output variables by using this to index them.

The TCS also takes the built-in variables output by the vertex shader:

in gl_PerVertex
{
  vec4 gl_Position;
  float gl_PointSize;
  float gl_ClipDistance[];
} gl_in[gl_MaxPatchVertices];

Note that just because gl_in is defined to have gl_MaxPatchVertices entries does not mean that you can access beyond gl_PatchVerticesIn and get reasonable values.

Note that every TCS invocation for an input patch has access to the same data (except for gl_InvocationID, which will be different for each). So a TCS invocation can look at the input vertex data for the entire input patch