Fractal Colorings

Last time, we saw some fairly primitive images of the Mandelbrot set. In particular, the coloring scheme was somewhat gaudy — there were noticeable discrete bands in all the images. The reason for this was in the code, where I had set the value of each pixel to be whichever integer we escaped at, creating a discrete range of possibilities. Luckily, there are fancier methods of coloring which smooth out these discontinuities. While there are a ton of different implementations out there, I’m just going to mention two: Hubbard-Douady Potential, and the Normalized Iteration Count.

In the Hubbard-Douady method, we treat the Mandelbrot set as though it is an electric field and consider the corresponding electric potential that it induces. This function turns out to be \( \phi(z) = \lim_{n \rightarrow \infty} \frac{\log|z_n|}{2^n} \), where \( z_n \) is the value of \(z\) after \(n \) iterations. So one option for smoother coloring is to assign to pixel \(z\) the value of \( \phi(z) \), or an approximation for a large value of \( n\). Here’s an example.

 

In the Normalized Iteration Count, we try to come up with a corresponding continuum to the different integer range that we had specified before. There are a few variants of this formula, but the simplest is given by mapping each point \( z \) to the real number \( r = n \ – \log_2\left(\frac{\log|z_n|}{\log R}\right) \), where \(R\) is the bailout radius and \( z_n \) is the first iteration value that escapes the disk of radius \(R\) centered at the origin. (One way to understand this is to see that this is equivalent to choosing a real value \(r\) which satisfies \( \frac{\log R}{2^r} = \frac{\log|z_n|}{2^n} \).) This formula will give us real numbers \(r\) in the interval \( [0, n) \) so that upon division by \(n\) we get a standard color mapping. Here are a couple examples.

 

Finally, here’s another spinning Julia video, but with a continuously modified smooth color scheme. I’ve mostly used my own variant of the Hubbard-Douady method in this particular piece.