## Mandelbrot set tutorial

A fractal image is an image in which a pattern is repeated when the image is magnified. In this article we’ll see how to draw a Mandelbrot set, one of the most well known fractals.

### Definition

The Mandelbrot set is the set of all the complex numbers \(c\) for which the iteration

(1) \(z_{n+1}=z_n^2+c\)

starting with \(z_0=0\), never escapes to infinity, but stays **bounded**, no matter how many iterations we do. For example:

- If \(c=1+2i\) we have \(z_0=0\), \(z_1=1+2i\), \(z_2=-2+6i\), \(z_3=-31-22i\)… as we go on \(z_n\) grows without stopping,
**it escapes to infinity**. - If \(c=-1\) we have \(z_0=0\), \(z_1=-1\), \(z_2=0\), \(z_3=-1\)… in this case \(z_n\) is
**bounded**, so \(c\) is in the Mandelbrot set.

### Plotting

On a Cartesian plane the Mandelbrot set intersects with the \(x\) axis at \((-2.5, 1)\) and with the \(y\) axis at \((-1,1)\). To plot the set on a \(WxH\) surface we scale each \((x,y)\) point of the surface in the \((-2.5,1)\) range for \(x\) and \((-1,1)\) range for \(y\). The scaled \(x\) becomes the real part of \(c\) and the scaled \(y\) becomes the imaginary part. Once we have \(c\) how do we know if \(z_n\) is bounded or not? We can’t count to infinity so we introduce limits. One is the upper bound on the number of iterations (for example 50). The other is a bailout condition: if \(|z_n| > 2\) then \(z_n\) has grown too much and we know that it’ll grow indefinitely so \(c\) is **not** in the set, if at the end of the loop we have \(|z_n| < 2\) then \(c\) is **in** the set.

### All black

black.go is a basic implementation. If a pixel is inside the set we draw it black, otherwise transparent. It uses the image Golang package.

To run do:

`$ go run black.go > black.png`

### Escape time algorithm

The way the set is drawn is up to the programmer. One common technique (known as *Escape time algorithm*) is to use a different color based on how many iterations takes \(z_n\) to escape to infinity. escape.go shows an implementation. On http://colourlovers.com you may find some inspiration for your colors.

### Smooth coloring

The problem with the above algorithm is that it creates bands of color. An improved algorithm is the “Normalized Iteration Count”. We use a **fractional iteration count** to better smooth the colors. The integral part is used to choose two colors, the fractional part is used to blend them togheter. To get this count do:

\(f = i - log_2(log_2(z_n))\)

where \(i\) is the number of iterations done. For the derivation of the formula see Renormalizing the Mandelbrot Escape.

This time we’ll use lucasb-eyer/go-colorful which provides some color manipulation functions. The implementation is in smooth.go.

### Binary decomposition

We can draw the Mandelbrot set however we want. For example we can use one palette of colors if the imaginary part of \(z_n\) is \(<0\) and another if it’s \(>=0\). See decomposition.go.

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