globals.js
var g_ctx;
var g_side=0;
var g_step;
my.js
function resize() {
  var side=90*vmin();
  if (side!=g_side) {
    setup();
  }
}
function setup() {
  g_side=90*vmin();
  var canvas=document.getElementById('c');
  canvas.width=g_side; canvas.height=g_side;
  g_ctx=canvas.getContext('2d');
  g_step=g_side/800;
}
function vmin() {
  var w=innerWidth; var h=innerHeight;
  var v=w;
  if (h<w) v=h;
  return v/100;
}

Now we're ready to go "turtling" again!