zondag 23 maart 2014

Paterson's Worms in Scalable Vector Graphics

Paterson's worms are cellular automata devised in 1971 by Mike Paterson and John Horton Conway.

They model the behavior and feeding patterns of certain prehistoric worms.  These worms fed upon sediment at the bottom of ponds and avoided retracing paths they had already travelled because food would be scarce there.

This behaviour was mathematically modelled by Mike Paterson as described by Michael Beeler (MIT) in 1973.

Ever since I read Martin Gardners feature in Scientific American on Paterson's worms in November 1973, they have fascinated me. For decades I procrastinated writing a turtle graphics program to draw them. I remember making an attempt in Logo long ago... Now I made an implementation in Scalable Vector Graphics (SVG).

The worms crawl a regular pattern along an isometric grid avoiding areas already visited. The grid consists of 'nodes' connected by 'segments', where I use coordinates as follows:



The direction the worm crawls is numbered as follows:



The encoding for worm type I used for the worm type is described by Ben Chaffin, he also created a list of all known types of worms which I incorporated in a drop down menu to select a type.

Nodes I store in an object called field which is actually an associative array (of lines) holding associative arrays (of points on the 'perpendicular' lines). A construct I learned from an article by Peter-Paul Koch. The worm crawls from node to node and line segments are appended to the SVG.

I added colouring: the Worms 'head' will always appear yellow, the end of its 'tail' red.



Some things remain to be done, my wish list:
  • Make a version with possibly multiple worms
  • Increase code efficiency (use typed arrays?)
  • It would be great if you could 'pan' also on the negative side of axes(why doesn't SVG support that...)
  • Add color like here (pworms.wordpress.com)
  • Simple things like adjustable speed and pause
  • ...
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