<!DOCTYPE html>
<html>
<body>
<canvas id="myCanvas" width="300" height="150" style="border:1px solid #d3d3d3;">
Your browser does not support the HTML5 canvas tag.</canvas>
<script>
var c = document.getElementById("myCanvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "red";
ctx.fillRect(10, 10, 50, 50);
var imgData = ctx.getImageData(30, 30, 50, 50);
red = imgData.data[0];
green = imgData.data[1];
blue = imgData.data[2];
alpha = imgData.data[3];
alert(red + " " + green + " " + blue + " " + alpha);
</script>
</body>
</html>