Day 19: Twinkle, Twinkle Little Star
If you're tracing rays or transforming coordinates, you could be using PDL!
A little knowledge is a dangerous thing. Having seen how to do matrix multiplication on 16 December, the Elf R&D department, headed by Rudolf and Dancer, decided to calculate the sparkle from their baubles. In optics, tracing light rays through bits of glass that act as mirrors and lenses can be done by working through a series of equations or by multiplying matrices and vectors!