frontface VEX function
If dot(I, Nref) is less than zero, N will be negated.
Contexts: image3d, chop, cop, pop, sop, surface, displace, fog, light, shadow, photon, cvex

vector frontface(vector N, vector I)
This form (which doesnâ€™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.