# Is anisotropic BSDF different to principled BSDF w anisotropy?

I have a simple sphere. Half of it has an anisotropic BSDF material, the other half has a Principled BSDF material with anisotropy. I believe I have the same settings for the two materials, but they render differently. How do I get the materials to match?

It looks like if you play a bit with the Principled settings you get about the same result. I can't tell you more as I don't know anything about anisotropy ;)

Yes, there is a difference in the formulae for calculating the U and V roughnesses. The Principled BSDF copies the anisotropy controls of the Disney BSDF.

Here is the code that calculates the roughnesses for the Anisotropic BSDF from the source code.

if (aniso < 0.0) {
roughness_u = roughness / (1.0 + aniso);
roughness_v = roughness * (1.0 + aniso);
}
else {
roughness_u = roughness * (1.0 - aniso);
roughness_v = roughness / (1.0 - aniso);
}


With the Pricipled BSDF the code looks a bit different.

float aspect = sqrt(1.0 - Anisotropic * 0.9);
float r2 = Roughness * Roughness;

float alpha_x = r2 / aspect;
float alpha_y = r2 * aspect;


alpha_x and alpha_y are the U and V roughnesses.

A while back, I made a node group that can convert U and V roughnesses to Roughness and Anisotropy (for Anisotropic BSDF) or Roughness and Anisotropic (for Principled BSDF)

This node group will output the Anisotropy and Anisotropic differently so that the roughnesses are consistent across both nodes.

Compared to the Anisotropic BSDF node, the direction of highlight elongation is rotated by 90°. Add 0.25 to the value to correct.

• There is still a bit of a difference. Can you tell why? screencast.com/t/7t0HoxB7xt Nov 29, 2018 at 12:04
• Part of the difference there is the specular component of the Principled BSDF node. Mar 13, 2021 at 21:08