Yes, another non-music digression. I can't stand not to write about this though, considering the fact that I've had a serious breakthrough today with a new algorithm.
Here's the problem: erosion algorithms tend to produce a sort of homogeneous soup of terrain without recreating some of the more true-to-life effects such as fractal coastlines and such. The solution? A terrain density map. As far as I know, this is a completely original algorithm. I've seen nothing like it in the literature, so I'm quite excited about having invented it. The initial test runs also produced exciting results!
Here's the basic concept: real terrains are not homogeneous. Not all dirt is created equal. So, along with a 2-dimensional array representing the heightmap of a terrain, we should also create an equally-sized 2-dimensional array representing the density (or integrity, if the term is preferred) of the terrain at each given point. The density array can be filled with a low-octave perlin noise function (I found two to three octaves to give optimal results, with a persistence of between 1.5 and 2).
Now, we perform an erosion algorithm as usual, except that we use the density of the terrain at each point as the threshold value for erosion. That is, if the amount of potential erosion at a point is less than the density of the terrain at that point, then the point will resist erosion. Ideally, this algorithm erodes terrain in a more coherent, less uniform way than typical thermal erosion algorithms. For example, coasts display some fractal dimension now since some areas of the terrain erode more easily than others.
A sample heightmap using the new algorithm:
The difference is most notable around the rivers, where the coasts clearly display some fractal coherence absent in thermal erosion algorithms. Notice that "noise" of the coastlines is clearly more coherent than single-octave noise, thanks to the perlin-based density map.
I am still trying to work out how to make the features appear larger (that is, make the coast even more jagged), since the heightmap, although nice, isn't drastically different from previous ones. I am quite confident, however, that there's a lot of potential in this new density erosion algorithm. Who knows, maybe this will be the future choice algorithm for procedural terrain erosion!