In this example, the subroutine is defined within the vertex shader. The first step involves declaring the subroutine type, as follows:
subroutine vec3 shadeModelType( vec3 position, vec3 normal);
This defines a new subroutine type with the name shadeModelType. The syntax is very similar to a function prototype, in that it defines a name, a parameter list, and a return type. As with function prototypes, the parameter names are optional.
After creating the new subroutine type, we declare a uniform variable of that type named shadeModel:
subroutine uniform shadeModelType shadeModel;
This variable serves as our function pointer and will be assigned to one of the two possible functions in the OpenGL application.
We declare two ...
