depending on the application, random (white) noise may also still be usable, and is cheaper than perlin noise.
for example, in my case, I use Perlin noise for some large-scale features of the terrain, but use random-numbers for some small-scale / high-frequency features.
BTW: if simulating a planet-sized area, assuming a ground-level perspective, more likely I would just fake it with a large flat plane, and probably make it wrap around at some point. even if much of the surface of the planet were explored, it is unlikely that players would go much into the core.
if something like unbounded sky/depth is desired, possibly chunks/regions can be stacked in a 3D grid, say:
chunks are 16x16x16 meters;
regions are 16x16x16 chunks;
regions may be organized in a 3D grid space.
my engine is sort of like this, except using 32x32x8 regions, and still having a "bottom layer" generated by the terrain generator, although it is possible to build underneath the terrain, as the engine will simply generate a regions as-needed when voxels are placed there. related is that 128m is merely a default sky limit, as building above this point will simply result in region-stacking.
if it can be seen from space, it can instead be faked with a sphere and a texture (probably also generated using Perlin noise).
player probably wont notice...
maybe another trick:
for visited regions, build a sky-view texture;
these are then mipmapped;
from space, the single-pixel versions are then used for updating the world-texture (most of the rest is faked until visited, using large-scale features like biomes or similar to calculate pixel values).
this way, if the players go and build a massive platform or image on the ground, it can still be seen from space.
the polar regions would be a little fudged, since the actual surface would be a torus, which isn't exactly a sphere (one could have slight funkiness that the ground-view and space-view don't really match up spatially, and that going "north" of the north pole will teleport them to the south pole, ...). but, will anyone really care?...
Edited by cr88192, 21 February 2013 - 01:36 PM.