Back at the dawn of procedural texture terrain generation in Decade I decided to skip slope lighting. My reason for this was that I have always planned on adding real lighting effects with shadow maps or shadow volumes.
Reciently my long time friend and arch nemisis Jelly Edwards, who is also now my blog enemy, has started a DX10 terrain engine and his slope lighting effect was impressive especially considering the ease at which it can be added and the free cost of this effect.
Slope lighting is calculated during texture generation therefore has no cost during runtime. Each vertex in the height map is checked against its neighbour in the direction from which the light is coming. If the neighbour is higher than the current vertex shade is applied based on the difference in height (how much the terrain is facing away from the light).
I am hoping that this effect will not be used in the majority of cases, however it is a nice addition to enhance the terrain on computers which do not have the power to do real time shadows.