They are talking about a bidirectional reflectance distribution function, a now common modeling technique for practical representation of complex surface properties when illuminated.

That said, a full BRDF is not the only way to approach this problem, especially at reduced resolution where the artifacts are more apparent.

If you're an ACM member, there is a wealth of information in SIGGRAPH publications. Having been in graphics since the 90s, I started with Foley and Van Dam (Computer Graphics: Principles and Practice) and Watt and Watt (Animation and Rendering Techniques) and then stayed on top of SIGRAPH (attending regularly, though not annually) since then.

For a more whimsical survey from the pen of a straight up genius, Jim Blinn's books (e.g. Dirty Pixels) are fantastic reads.