I'm curious about the example with the disk. I'm under the impression that the vivid colors are a product of thin-film interference, but I'm not really sure where the anisotropic BRDF comes into play. Does the color vary how it does because the microfacets mostly only vary about the angle around the disk (and not the distance from the center), creating bands of color, but the color itself derives from thin-film interference? Or do the microfacets vary along the distance from the center? Or is the answer something else entirely?
While I understood that isotropic materials diffuse light uniformly and anisotropic materials have orientations (causing it to scatter light in an oriented way), I'm a little confused about what the equation in this slide is trying to convey?