What are some systematic ways for determining reflection functions for different classes of materials? I'd imagine that one possible way of doing this would be purely based on brute forcing parameters until the appearance matches with human subjective experience, but is it also possible to do this from a pure physics/materials science perspective?
In one of the past semesters, Professor Ng did talk about some materials having a constant value for their reflectivity. It is based on the type (plus composition for mixed materials) of material used for a certain object.
It's not much different how small differences can change the constant of gravity of 9.81 m/s.
Some material (and color actually) absorb all light and reflect light more than other materials.
There are machines that are used to determine this function for different materials.