These kinds of knots are easy to describe by a parametrized curve, but making a distance estimator for them is impressive – I wouldn’t have guessed it was possible at all.
It is also possible to create several variations:
In the same thread, Knighty came up with an impressive figure-8 knot distance estimator:
In another thread at Fractal Forums, Knighty also published an interesting technique (“Fold and Cuts”) for creating a large variety of distance estimated polyhedra:
(If you wonder about the materials, I’ve added some 3D Perlin noise to the distance estimate – this is simple way to creature a structural texture, and it creates true displacements, not just surface normal perturbations).
The threads linked to above contains Fragmentarium scripts with the relevant distance estimators.