Cellular Automaton

This little cellular automaton was discovered by me in about 1995 but I'm sure someone else invented it before. If anyone knows its proper name I'd love to hear it. It can't be used to do anything clever but it does produce a surprising variety of patterns from a simple rule.
Each cell holds a value from 0-255 and has four neighbours. On each iteration, if the sum of all neighbours equals a given number ('the rule') the cell's value becomes 255. Otherwise it is decreased by 1 if it is greater than zero. If it's zero, then it stays as zero.
I've implemented it using HTML5 canvas, doing pixel-level access with ImageData.

See it here. It works on the 'modern' browsers that I've tested; Firefox, Safari and Chrome. It ought to work on IE9 and above and maybe Opera.
A quick note on the implementation:
I created a 2d context for the canvas element

var canvas = document.getElementById("ccc");
var ctx = canvas.getContext("2d");

Then an ImageData to cover the whole canvas.

var imd = ctx.createImageData(canvas.width,canvas.height);

I drew the image with

ctx.putImageData(imd, 0, 0);

I didn't work directly with the ImageData's data, which is always in 32-bit RGBA format. Instead I created a Uint8Array with 8 bits per pixel (I only needed to store 0-255), then converted it to image format as follows

function convi(imd, data) {
 var i;
 var out = imd.data;
 for (i=0; i<data.length; ++i) {
  out[i*4] = out[i*4+1] = out[i*4+2] = data[i];
  out[i*4+3] = 255;
 }
}

I just converted the 0-255 to a greyscale but here would be the place to implement a colour look-up table and get a nice palette.