If it's looked purely from educational point of view it's quite hard to find graphics code that is this clear and concise.

The author modifies this short code to implement various shading techniques with code that is as pithy and understandable as any.

You have to recall that 40 years ago even texture mapping was a scientific publication quality material.

This code makes several non-obvious things obvious - and simple! I don't think that's a light achievement.