Thanks Marissa, this works fine.
Thanks Alan for thinking with me, but that is not what I was after. I wanted to distort a volume and fill the surface of a volume with brush strokes, so that I could walk through a Van Gogh. That's out of my league, so I thought I could try a pointilistic picture first.
DomC's solution only works for top surfaces and I can't program in Python to see if I could improve on that, so I had to find another solution.
While I was waiting for an answer to my question, I realised that I didn't need the volume itself and I used the contours to fill the surface, making a 'pointcloud from volume', see attachment.
Next thing is: how to distort the contours to make it look more like a handdrawn thing. I think I can manage that.
I think it would be better if I didn't use spheres, but symbols (pebbles). But what I can't find out is how to query the direction of a certain point of a contour or volume. I would appreciate some help with that.
Please be carefull. There are about 10.000 spheres drawn by the wrapper, which maximises my 8Gb and sometimes crashes. The 'main distance'-node can be put on 200 for a first try.