Get mouse position on the canvas

getCanvasMousePosition.js

function getCanvasMousePosition(e, canvas) {
	var isRetina = true;
	var offsetX = 0, offsetY = 0, mx, my;

	if (canvas.offsetParent !== undefined) {
		do {
			offsetX += canvas.offsetLeft - canvas.scrollLeft;
			offsetY += canvas.offsetTop - canvas.scrollTop;
		} while ((canvas = canvas.offsetParent));
	}

	if( e.pageX || e.pageY ){

		mx = e.pageX - offsetX - document.body.scrollLeft;
		my = e.pageY - offsetY - document.body.scrollTop;
	}
	else if( e.touches[0].clientX || e.touches[0].clientY ){

		mx = e.touches[0].clientX - offsetX;
		my = e.touches[0].clientY - offsetY;
	}

	if( isRetina ){
		mx *= 2;
		my *= 2;
	}

	return {x: mx, y: my};
}