How can I generate Perlin noise on a spherical surface?
I am trying to generate terrain using Perlin noise. I understand how to generate it using Cartesian coordinates, but can't quite wrap my head around how it would work on a sphere. I know that you can project 2D surfaces onto spheres, but wouldn't the distortion mess up the noise distribution? The best idea I can come up with for generating uniform noise on the surface of a sphere is to map the point on the sphere to a 3D Cartesian coordinate and use a 3D noise function. (Basically, to generate a cube of noise and "shave away" the corners to make it round, as it were.) Is there a better method I'm missing?
Answer
I believe the approach is to actually use a 3 dimensional noise field (every point in a 3D space has a scalar noise value) as opposed to a 2 dimensional field (every point on a 2D plane has a noise value).
When using a 2D noise function to generate a height map, you offset the z value according to the noise value.
When using a 3D field, you sample the noise at points on the surface of a sphere, then use the noise value to offset each point radially away from or towards the center of the sphere.
3D noise is harder and slower to produce obviously, but you don't have to deal with the complications of wrapping a surface around the sphere, and because the noise function is continuous there are no seams.
This can obviously be applied to any arbitrary shape.