Wow, those results look really nice! I especially like the one with the highest power.

I was wondering if it'd be too much to ask if you could expand on the algorithm you're using? I wouldn't mind being able to implement some erosion in my terrain, but I totally understand if you want to keep it under wraps.

It's basically a hack. I spawn a bunch of 'rain drops' all over the map (which are just x/y pairs), then iterate through the list of raindrops, moving each one down by choosing it's lowest neighbor, until it can't move down anymore. At each step, I calculate new heights for the current location and the lowest neighbor by:

ht=height at current location
lowh=height of lowest neighbor
new_lowh = (ht + (1.0-power)*(lowh-ht)); // New height for lowest neighbor
new_ht = (ht + power*(lowh-ht)); // New height for current location

This has a tendency to average or smooth out these adjacent values, raising the low neighbor and lowering the current cell. At full-power (power=0.5) they meet in the middle. I clamp the power to the range [0,0.5] since values outside this range can be bad. (In particular, power > 0.5 can result in an infinite loop where the raindrop bounces back and forth between 2 locations)

Sometimes you have to run a single pass of a small Gaussian blur kernel across the final result to smooth out some jaggies, since it really is a hack of an algorithm. For some terrain types it works very well, but others not so much. I'd still like to work out a suitable hydrology-based erosion filter for more realistic results.

In my algorithm, you can specify the number of raindrops to spawn. If the number is greater than 0, then it spawns that many drops randomly located across the map. If the number is <=0 then it spawns one drop at every heightmap location, then randomly shuffles that list.

I also randomize the order in which I visit the neighbors of a raindrop each time, so that one direction isn't constantly favored. This adds another random_shuffle per step, so it is a source of slowdown. Thinking of changing it to only shuffle per raindrop rather than step.

EDIT: Here is a rendering of some mesa-style desert terrain showing the artifacts introduced by this erosion method. A blur usually takes care of these kinds of jaggies.

Wow, thanks for that! I might end up implementing something similar for my terrain generation. Currently I only use user-seeded diamond-square, but some erosion settings might provide some better realism.

Rate up! [smile]