If dot(I, Nref) is less than zero, N will be negated.

`vector frontface(vector N, vector I)`

This form (which doesn’t take a reference vector) is only available in the shading contexts, where the

`Ng`variable is used.`vector frontface(vector N, vector I, vector Nref)`

If dot(I, Nref) is less than zero, N will be negated.