as brushed metal, satin, and hair. Parameters Input parameters: D = Thread direction ( In original papers this appears as T ) s = Shininess exponent. Values are between 0 and infinity N = Real surface normal L = Vector from point to light V = Vector from point to viewer T = Thread direction based on real surface normal. P = Projection of vector L onto plane with normal T ( in original paper this appears as N ). R = Reflected incoming light ray against T. Incoming light ray is equal to negative L. All vectors are unit. Conditions If some of the conditions are not satisfied from the list then the color is zero Note: This list is not optimized. Formula First we need to correct original direction of fiber D to be perpendicular to real surface normal N. This can be done by projection fiber direction on to plane with normal N: It is expected that fiber is cylindrical. Note the fact that normal of fiber depends on light position. Normal of fiber at given point is: Reflected ray needed for specular calculation: Final calculation Optimization Calculation of R and P are expensive operation. To avoid their calculation original formula can be rewritten in next form: Diffuse Specular Comments T can be observed as bump normal and after that it is possible to apply other BRDF than Phong. The anisotropic should be used in conjunction with an isotropic distribution like a Phong distribution to produce the correct specular highlight Ward anisotropic distribution The Ward anisotropic distribution uses two user-controllable parameters αx and αy to control the anisotropy. If the two parameters are equal, then an isotropic highlight results. The specular term in the distribution is: The specular term is zero if N·L < 0 or N·V < 0. All vectors are unit vectors. The vector V is the viewing direction, L is the direction from the surface point to the light, H is the half-angle direction between V and L, N is the surface normal, and X and Y are two orthogonal vectors in the normal plane which specify the anisotropic directions. Cook–Torrance model The Cook–Torrance model uses a specular term of the form . Here D is the Beckmann distribution factor as above and F is the Fresnel term. For performance reasons, in real-time 3D graphics Schlick's approximation is often used to approximate the Fresnel