var canvas = $("#canvas"), 
  ctx = canvas.getContext("2d"),
  width = 512,
  height = 480;

canvas.width = width;
canvas.height = height;

var canvas = document.getElementById("canvas"), 
  ctx = canvas.getContext("2d"),
  width = 512,
  height = 480;

canvas.width = width;
canvas.height = height;

Update canvas.js