The geometry shader (GS) is designed to execute once for each primitive. It has access to all of the vertices of the primitive, as well as the values of any input variables associated with each vertex. In other words, if a previous stage (such as the vertex shader) provides an output variable, the geometry shader has access to the value of that variable for all vertices in the primitive. As a result, the input variables within the geometry shader are always arrays.
The geometry shader can output zero, one, or more primitives. Those primitives need not be of the same kind that were received by the geometry shader.
However, the GS can only output one primitive type. For example, a GS could receive a triangle, and output ...
