canvas坐标拾取器

<script type="text/javascript">
window.onload = function () {
var canvas = document.getElementById("myCanvas");
canvas.onclick = function (event) {
var loc = windowTocanvas(canvas, event.clientX, event.clientY)
var x = parseInt(loc.x);
var y = parseInt(loc.y);
alert('clientX:'+event.clientX+'，clientY:'+event.clientY+'\n\r'+'x:'+x+'，y:'+y);
}
}

function windowTocanvas(canvas, x, y) {
var bbox = canvas.getBoundingClientRect();
return {
x: x - bbox.left * (canvas.width / bbox.width),
y: y - bbox.top * (canvas.height / bbox.height)
};
}
</script>