Separately, I was trying to understand how he was special-casing the ambient occlusion computations given that he was working with a heightmap rather than a more general scene, and in the course of looking around for stuff accidentally found this other page that is like "LOL JUST DO A HIGH PASS".
And, dang, it actually looks pretty decent! Here's a before-and-after:
The former is just a heightmap of north america from over here with gimp's "emboss" applied to it, and the latter is that same emboss multiplied with a level-adjusted version of heightmap - gaussian_blur(heightmap). Really makes the lakes and rivers pop out nicely.
Found some entertaining things at ponies and lights where they convert the endianness of a giant image file by using... the audio processing tool sox.