I wrote this post as a question but then stumbled on the answer. So here you go. I’m trying to figure out how to control the anisotropy (stretched highlights) effect with a texture map. What I really want to know is: what color value in a texture map will result in zero distortion? Why? So I can have a texture map with isotropic and anisotropic areas, naturally! Also, to understand this cool but traditionally under-documented feature.
Red channel controls the amount of anisotropic distortion, and Green channel controls the rotation of the distortion. So far so good (if a bit vague). What value of red equals perfectly isotropic highlights, or zero anisotropy?
When Anisotropy Factor is at 0, the texture map has no effect. Highlights and reflections are always isotropic. When Anisotropy Factor is non-zero, the map has an effect, creating anisotropic highlights. But what value of Red returns us to isotropic?
Turns out, both Red and Green must be 50% gray in the texture map. This really busted my brain until I stumbled upon it. Now I can create brushed metals, satin highlights with control.