Useful function to obtain canvas relative mouse coordinates

canvasRelativeCoords.js

var canvasX, canvasY;
theCanvas.onmousemove = transformIntoCanvasRelative;
function transformIntoCanvasRelative(evt) {
  var canvas = evt.target;
  var rect = canvas.getBoundingClientRect();
  canvasX = evt.clientX - rect.left;
  canvasY = evt.clientY - rect.top;
}