Trying to compare two Canvas elements

I am using below code to compare two canvas elements

function createImage(html, can) {
     var canvas = $( "#" + can );
     var ctx = canvas[0].getContext("2d");
     var data = "<svg xmlns='http://www.w3.org/2000/svg' width='1000' height='1000'>" +
                    "<foreignObject width='100%' height='100%'>" +
                        "<div xmlns='http://www.w3.org/1999/xhtml'>" +
                            html +
                        "</div>" +
                    "</foreignObject>" +
                "</svg>";
    var DOMURL = self.URL || self.webkitURL || self;
    var img = new Image();
    img.crossOrigin = '';
    var svg = new Blob([data], { type: "image/svg+xml;charset=utf-8" });
    var url = DOMURL.createObjectURL(svg);
    img.onload = function () {
        ctx.drawImage(img, 0, 0);
        DOMURL.revokeObjectURL(url);
    };
    img.src = url;
    //return img.src;
    return canvas[0];
}
var a1 = createImage("<span style='font-size:34px'><i><b>Hello</b></i></span>","can1");
var a2 = createImage("<span style='font-size:34px'><i><b>Hello</b></i></span>", "can2");
setTimeout(function() {
    var ctx1 = a1.getContext('2d');
    var imageData = ctx1.getImageData(0, 0, a1.width, a1.height);
    var pixels = imageData.data;
    var ctx2 = a2.getContext('2d');
    var imageData2 = ctx2.getImageData(0, 0, a2.width, a2.height);
    var pixels2 = imageData2.data, count;
    for(var i = 0, il = pixels.length; i < il; i++) {
        if(pixels[i] == pixels2[i]){
            count++;
        }
    }
    if(count === pixels.length && count === pixels2.length){
        alert("Match");
    }
},5000);

But it is returning me error like below

Unable to get image data from canvas because the canvas has been tainted by cross-origin data.

How can I get rid of this error?

Answers


The reason you get a cross-origin error is because of the use of <svg> with namespace declarations located at http://www.w3.org/, which is of a different origin:

var data = "<svg xmlns='http://www.w3.org/2000/svg' width='1000' height='1000'>" +
             "<foreignObject width='100%' height='100%'>" +
               "<div xmlns='http://www.w3.org/1999/xhtml'>" +
                    html +
               "</div>" +
             "</foreignObject>" +
           "</svg>";

I can tell this method is the one from Drawing DOM objects into a canvas on MDN.

When you re-access the data this way,

var ctx1 = a1.getContext('2d');
var imageData = ctx1.getImageData(0, 0, a1.width, a1.height);

you will hit the error:

Unable to get image data from canvas because the canvas has been tainted by cross-origin data.

You can test this on Chrome:

You can only return the data from the function to avoid getting this error. But because of the asynchronous nature of img.onload,

img.onload = function () {
    ctx.drawImage(img, 0, 0);
    DOMURL.revokeObjectURL(url);
};

you have to defer the retrieval of data, forcing you to re-access the data out of the function and causing the error.

Thus, you should use an alternate method of building the canvas with DOM objects that does not rely on <svg>, like html2canvas.

function createImage(html) {
    var dfd = new $.Deferred();
    var el = document.createElement("div");
        el.innerHTML = html;
        el.style.display = 'inline-block';
        document.body.appendChild(el);
    html2canvas(el, {
      onrendered: function(canvas) {
          document.body.appendChild(canvas);
          document.body.removeChild(el);
          dfd.resolve(canvas.toDataURL());
      }
    });
    return dfd;
}
$.when(
    createImage("<span style='font-size:34px'><i><b>Hello</b></i></span>"), 
    createImage("<span style='font-size:34px'><i><b>Hello</b></i></span>")
).done(function(a1, a2){
   if (a1 === a2) {
      alert("Match"); 
   }
});

See DEMO.


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