# Spherical Worlds

Recently I saw a description of spherical fractals in a blog post by Samuel Monnier.

These Julia-sets are constructed like ordinary Mandelbrots and Julias: first the argument is squared, but instead of adding a constant afterwards, a Möbius transformation is applied:

$$z = \frac{a z^2 + b}{c z^2 + d}$$

For the right choices of (complex) constants, plane-filling patterns appear.

There is an intimate connection between Möbius transformations and spherical geometry: if the plane is stereographically projected onto a sphere, a Möbius transformations corresponds to rotating and moving the sphere, and then project stereographically back to the plane (this is nicely visualized in this video).

This connection can be visualized graphically: if the plane-filling patterns are stereographically projected onto a sphere, they fit naturally on it. There are no discontinuities or voids, and no singularities near the poles.

Here I’ve used Fragmentarium to create some images of these plane-filling patterns, together with their stereographical projection onto a sphere. It was done by distance estimated ray marching, but in this case we could have used ordinary ray tracing, and calculated the exact intersections.

The Fragmentarium script can be found here.

## 2 thoughts on “Spherical Worlds”

1. knighty says:

Wow! These are really beautiful. In particular the second one. It should be possible to convert the colors into a height field on the sphere and explore those planets ;o).
And thank you for the shader.

2. Thanks Knighty! Actually the second one is a heightmap (although the effect is subtle). I experimented with it, and found it somewhat difficult to get results (and it is much slower!). But it can most likely be improved!