A small script that gets the relative mouse position inside a Canvas element.

canvasMouse.js

function mousePosition(e_) {
  //accepts a mouseEvent
  var canvas = document.getElementById("myCanvas");
  var canvas_bound_rect = canvas.getBoundingClientRect();
  
	mousex = Math.round(((e_.clientX - canvas_bound_rect.left)/(canvas_bound_rect.right - canvas_bound_rect.left)) * canvas.width);
	mousey = Math.round(((e_.clientY - canvas_bound_rect.top)/(canvas_bound_rect.bottom - canvas_bound_rect.top)) * canvas.height);	  
  
  return {x : mousex, y : mousey}
}