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we're trying to generate a 3d world using a 2d perlin noise (with a recorsive/fractal technique). We have generated mountains and valleys quite fine but now we are having problems with desert and dunes because we only worked on persistence and octaves and we aren't able to make the classic shape of the dune. Has anybody already experienced that? Any solution, possibly still using perlin noise, or also other algorithms which allow you to do this?

Kroj
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You could give the Musgrave ridged multifractal a try. It gives nice ridged structures and you can use your existing noise algorithms for it.

The C reference implementation for it is here

Bartek Banachewicz
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Dawnkeeper
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Dunes are lobsided: .='\ cross section... you may want to use an initial shape of that kind

They are regular, like waves in the sea. not completely noise

they are elongated towards the wind

I didnt use the first condition, but i have made great dunes by multiplying 2 1d perin noises together, or even 2 sin/parabol functions, where they are both lined to one axis. i.e. Z, and they have a small low frequency Sin or noise wobbling them along X axis, so they aren't alined.

try this:

dunes = sin ( X + 1dperlin(Z) *.2 ) * sin ( X + 1dperlin(Z+432) *.2 );

otherwise to test it:

dunes = sin ( X + sin(Z) *.2 ) (plus or times or devided by) sin ( X + sin(Z+432) *.2 );

0.2 makes dunes 10 times longer than wide, and it's like when two straight water waves meet at almost the same angle, plus an uncertainty variable using noise for the angle.

bandybabboon
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Maybe turbulence is yet enough for what you need... Try to play with turbulence using the absolute value of your octaves return values instead of the normal values. You can also evaluate separately and combine your noise and your turbulence to mix both effects in some areas.