As I always forget how to effectively do this.

mouse-position.js

/**
 * @param eve an event object from function listening on the element, such as mousedown
 * @return object describing the position of the mouse on the canvas
 */
function mouse_pos(eve) {
   var rect = eve.target.getBoundingClientRect();
   return {
      y : eve.clientY - rect.top,
      x : eve.clientX - rect.left
   };
}