var canvas1 = document.createElement("canvas"),
canvas2 = document.createElement("canvas"),
ctx1 = canvas1.getContext("2d"),
ctx2 = canvas2.getContext("2d"),
img1 = new Image, img2 = new Image,
count = 2,
startTime1, startTime2, endTime1, endTime2, sum1, sum2;
performance = performance || Date;
img1.crossOrigin = img2.crossOrigin = "";
img1.onload = img2.onload = loader;
img1.src = "http://i.imgur.com/TJiD5GM.jpg";
img2.src = "http://i.imgur.com/s9ksOb1.jpg";
function loader() {if(!--count) test1()}
function test1(){
startTime1 = performance.now();
ctx1.drawImage(img1, 0, 0);
ctx2.drawImage(img2, 0, 0);
var data1 = ctx1.getImageData(0, 0, 500, 500).data,
data2 = ctx2.getImageData(0, 0, 500, 500).data,
i = 0, len = data1.length, sum = 0;
while(i < len) {
sum += Math.abs(data2[i] - data1[i++]) +
Math.abs(data2[i] - data1[i++]) +
Math.abs(data2[i] - data1[i++]);
i++
}
sum1 = sum;
endTime1 = performance.now();
test2();
}
function test2(){
startTime2 = performance.now();
ctx1.drawImage(img1, 0, 0);
ctx1.globalCompositeOperation = "difference";
if (ctx1.globalCompositeOperation !== "difference")
alert("Sorry, use Firefox or Chrome");
ctx1.drawImage(img2, 0, 0);
var data = ctx1.getImageData(0, 0, 500, 500).data,
i = 0, len = data.length, sum = 0;
while(i < len) {
sum += data[i++];
sum += data[i++];
sum += data[i++];
i++;
}
sum2 = sum;
endTime2 = performance.now();
result();
}
function result() {
var time1 = endTime1 - startTime1,
time2 = endTime2 - startTime2,
factor = time1 / time2,
res = "Manual method: " + time1.toFixed(3) + "ms<br>";
res += "Blending mode: " + time2.toFixed(3) + "ms<br>";
res += "Factor: " + factor.toFixed(2) + "x<br>";
res += "Sum 1 = " + sum1;
res += "<br>Sum 2 = " + sum2;
document.querySelector("output").innerHTML = res;
}
<output>Loading images and calculating...</output>