which is easy when everything is so steep that you have at least one height unit difference to the surroundings
you get the "solid" position and a higher up "empty" position (next to the surrounding voxels) and interpolate the values between those coordinates.

it gets hard when you try to smooth a very flat slope, because then you can't just look at your neighbors, you need to do way more.

and if your heightmap isn't smooth, then this method won't work either.

Ok, yes, I see what you mean.



I can see that a gentle slope is indeed difficult here (e.g. if it's flat for 10 voxels, then goes up one voxel, then is flat for another ten, and so on). You'd need a lot of blurring and this would destroy other details in you map.

Perhaps a better approach is to consider converting the heightmap into a mesh (at least conceptually), and then voxelising that mesh. Then you can consider exactly where that mesh intersects your volume and set the voxel densities to best represent this.

Just an idea... maybe it's worth research voxelising a mesh into a density field (rather than a 'binary' field)?

I wonder if you had a higher resolution heightmap, whether that could be used to create smooth meshes over large flat-ish areas (slowly sloping ground).

By higher resolution, I don't mean on the standard 2D XY plane, I mean maybe using a colour heightmap instead of a greyscale one. You'd have at least 65535 "steps" rather than 255, and I wonder if you could use them to set better density values....just a thought.

lets combine those two ideas:
1. convert the 8bit-heightmap to a 16bit-heightmap (simply left-bitshifting all values by 8)
2. create a boolean heightmap that has true for all pixels that have the same value as their neigbors
3. keep on smoothing all pixels marked in the bool heightmap until sufficiently satisfied
4. convert to heightmap
4.1 remember to check for very steep slopes (more than 1 voxel) and do some magic there (see my earlier post)

Thank you guys for all the help, here's my progress so far

_________________
Dream Big Or Go Home.
ENet - http://enet.bespin.org
Recast - https://github.com/memononen/recastnavigation
Irrlicht - http://irrlicht.sourceforge.net/forum
PolyVox - http://www.volumesoffun.com/phpBB3/
Help Me Help You.

Right, sorry for the slow reply, had a busy weekend!

I think there's some interesting ideas here, and maybe it's also worth converting the input heightmap to a floating-point format for better interpolation and smoothing. I find this all quite hard to visualise though, so I look forward to trying to implement it myself.

@kklouzal - Looking good!