Tessellation Evaluation Shader/Defined Inputs

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Tessellation Evaluation Shaders have the following built-in inputs.

in vec3 gl_TessCoord;
in int gl_PatchVerticesIn;
in int gl_PrimitiveID;
the location within the tessellated abstract patch for this particular vertex. Every input parameter other than this one will be identical for all TES invocations within a patch.
Which components of this vec3 that have valid values depends on the abstract patch type. For isolines and quads, only the XY components have valid values. For triangles, all three components have valid values. All valid values are normalized floats (on the range [0, 1]).
the vertex count for the patch being processed. This is either the output vertex count specified by the TCS, or the patch vertex size specified by glPatchParameter if no TCS is active. Attempts to index per-vertex inputs by a value greater than or equal to gl_PatchVerticesIn results in undefined behavior.
the index of the current patch in the series of patches being processed for this draw call. Primitive restart, if used, has no effect on the primitive ID.
Note: The tessellation primitive generator will cull patches that have a zero for one of the active outer tessellation levels. The intent of the specification seems to be that gl_PrimitiveID will still be incremented for culled patches. So the primitive ID for the TES is equivalent to the ID for the TCS invocations that generated that patch. But this is not entirely clear from the spec itself.

The TES also has access to the tessellation levels provided for the patch by the TCS or by OpenGL:

patch in float gl_TessLevelOuter[4];
patch in float gl_TessLevelInner[2];

Only the outer and inner levels actually used by the abstract patch are valid. For example, if this TES uses isolines, only gl_TessLevelOuter[0] and gl_TessLevelOuter[1] will have valid values.

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

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.