Geometry Shader/Defined Inputs

Geometry Shaders provide the following built-in input variables:

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

These variables have only the meaning the prior shader stage(s) that passed them gave them.

There are some GS input values that are based on primitives, not vertices. These are not aggregated into arrays. These are:

in int gl_PrimitiveIDIn;
in int gl_InvocationID; // Requires GLSL 4.0 or ARB_gpu_shader5

gl_PrimitiveIDIn

the current input primitive's ID, based on the number of primitives processed by the GS since the current drawing command started.

gl_InvocationID

the current instance, as defined when instancing geometry shaders.