ashikhmin vex function

1. bsdf ashikhmin(float exponentx, float exponenty, vector framex, vector framey)

2. bsdf ashikhmin(vector nml, float exponentx, float exponenty, vector framex, vector framey)

An anisotropic bsdf similar to phong() but with independent controls for the highlight size along 2 tangent vectors.

• exponentx – phong exponent along the framex vector.

• exponenty – phong exponent along the framey vector.

• framex – highlight X direction.

• framey – highlight Y direction.