With a little bit more thinking, I could find the right operation for
bending an object (along the x-axis), so that a cylinder is bent to a
torus:
xnew = atan(z/x)*radius
ynew = y
znew = sqrt(x^2+z^2)-radius
--> function(atan(z/x)*radius, y, sqrt(x^2+z^2)-radius)
The rendering speed is ok, so I can use it.
