# 11a: What is an iterated function system (IFS)?

## Description

This article is from the Fractal FAQ, by Ermel Stepp stepp@muvms6.mu.wvnet.edu with numerous contributions by
others.

# 11a: What is an iterated function system (IFS)?

If a fractal is self-similar, you can specify mappings that map the

whole onto the parts. Iteration of these mappings will result in convergence

to the fractal attractor. An IFS consists of a collection of these (usually

affine) mappings. If a fractal can be described by a small number of

mappings, the IFS is a very compact description of the fractal. An iterated

function system is By taking a point and repeatedly applying these mappings

you end up with a collection of points on the fractal. In other words,

instead of a single mapping x -> F(x), there is a collection of (usually

affine) mappings, and random selection chooses which mapping is used.

For instance, the Sierpinski triangle can be decomposed into three self-

similar subtriangles. The three contractive mappings from the full triangle

onto the subtriangles forms an IFS. These mappings will be of the form

"shrink by half and move to the top, left, or right".

Iterated function systems can be used to make things such as fractal ferns

and trees and are also used in fractal image compression. _Fractals

Everywhere_ by Barnsley is mostly about iterated function systems.

The simplest algorithm to display an IFS is to pick a starting point,

randomly select one of the mappings, apply it to generate a new point, plot

the new point, and repeat with the new point. The displayed points will

rapidly converge to the attractor of the IFS.

An IFS fractal fern can be viewed on the WWW at

gopher://life.anu.edu.au:70/I9/.WWW/complex_systems/fern.gif .

Frank Rousell's hyperindex of clickable/retrievable IFS images:

ftp://ftp.cnam.fr/pub/Fractals/ifs/Index.gif

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