Adding terrain features to a perlin noise heightmap

Question

I'm currently using Perlin Noise to generate heightmaps and model moisture levels as well as tree density to create terrain (in the form of 2D tilemaps - think dwarf fortress).

How can I generate things like caves, or other landmarks like massive boulders or buildings?

I don't want to model these features (i.e. I don't care about the inside of the cave), I just want the game to be able to say 'there is a cave here' when you're at the x,y position of the cave, and importantly, follow rules about where caves might be (e.g. side of a mountain / rocky areas).

I also want to be able to generate this in a procedural way, similar to perlin noise, so that with any 'seed', the same landmarks will always appear.

I'm struggling to think of a way to create these features, and have them always reappear with the same seed. Does anyone have any ideas? :)

Answer 1

Depending on your perlin noise implementation you may have some kind of pseudo random 0..1 float generation . In my implementation I use a gradient array and a permutation table and given two int coords i get a value :

private float pseudoRandomFromPerm(int x, int y)
{
    return grad[(x + p[y & 255]+Seed) % grad.GetLength(0)];
}

I also have a 3d version of that and it looks like this:

private float pseudoRandomFromPerm(int x, int y, int z)
{
    return grad[(x + p[y + p[z & 255] & 255]+Seed) % grad.GetLength(0)];
}

So assuming that in your perlin algorithm you call a 2d generator at x,y , try using a 3d generator at x,y ; call it with z=0 in terrain generation call it with z=1000 (take some distance from zero) to 'decide' if in x,y there's a cave or a building or anything else. When you get the value for v=pseudorandom3d(x,y,1000) manage the results (assuming it is in 0..1 range)

if v<=0.05 and (current terrain conditions) then
 cave entrance
else if v>0.05 and v<=0.07 and (current terrain conditions) then
 building
...