[glibby]

Use the partial derivatives of the topo height to calculate a normal. dot that normal against your fake sun angle to shade your terrain. it seems though that the magic in this is in throwing away specific details and changing the sun angle when it is aesthetically pleasing which makes sense. on the surface it looks like you should be able to use the heightfield and standard filters to get a reasonable result but I'll take their word for it that real world data will prove more difficult.