How to set crossOrigin attribute when using canvas.toDataURL?

Question

So, I'm trying to create a print map function for the OpenLayers 3 application that I am creating. I know their example , but whenever I try to use it, I encounter the problem of a terrible spoiled canvas. I read the entire internet and came across people speaking first to install CORS correctly (done and done), but also:

          var img = new Image();
          img.setAttribute('crossOrigin', 'anonymous');
          img.src = url;

Above described here .

My question is that I have never used toDataURL () before, and I'm not quite sure how I am sure that the created image has the crossOrigin attribute set correctly before it slams into:

Error: Failed to execute 'toDataURL' on 'HTMLCanvasElement': Tainted canvases may not be exported.

Any thoughts?

I saw this one . My question is how they incorporate this into a function that works. Sort of:

    var printMap = function(){
     var img = new Image();
     img.setAttribute('crossOrigin', 'anonymous');
     img.src = url;
     img.onload = function() {
      var canvas = document.getElementsByTagName('canvas');
      var dataURL = canvas.toDataURL("image/png");
      console.log(dataURL);
     };
   };

Answer 1

If the property / attribute is crossOriginsupported by the browser (now it is in FF, Chrome, the latest Safari and Edge), but the server does not respond with the corresponding headers ( Access-Control-Allow-Origin: *), the img event is triggered onerror.

, , .
, , , , - toDataURL catch try.

:

var urls = 
    ["http://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png", 
     "http://lorempixel.com/200/200"];

	var tainted = false;

	var img = new Image();
	img.crossOrigin = 'anonymous';

	var canvas = document.createElement('canvas');
	var ctx = canvas.getContext('2d');
	document.body.appendChild(canvas);

	var load_handler = function() {
	  canvas.width = 200;
	  canvas.height = 200;

	  ctx.fillStyle = 'white';
	  ctx.font = '15px sans-serif';

	  ctx.drawImage(this, 0, 0, 200, 200*(this.height/this.width));

	  // for browsers supporting the crossOrigin attribute
	  if (tainted) {
	    ctx.strokeText('canvas tainted', 20, 100);
	    ctx.fillText('canvas tainted', 20, 100);
	  } else {
	    // for others
	    try {
	      canvas.toDataURL();
	    } catch (e) {
	      tainted = true;
	      ctx.strokeText('canvas tainted after try catch', 20, 100);
	      ctx.fillText('canvas tainted after try catch', 20, 100);
	    }
	  }
	};

	var error_handler = function() {
	  // remove this onerror listener to avoid an infinite loop
	  this.onerror = function() {
	    return false
	  };
	  // certainly that the canvas was tainted
	  tainted = true;

	  // we need to removeAttribute() since chrome doesn't like the property=undefined way...
	  this.removeAttribute('crossorigin');
	  this.src = this.src;
	};

	img.onload = load_handler;
	img.onerror = error_handler;

	img.src = urls[0];

	btn.onclick = function() {
	  // reset the flag
	  tainted = false;

	  // we need to create a new canvas, or it will keep its marked as tainted flag
	  // try to comment the 3 next lines and switch multiple times the src to see what I mean
	  ctx = canvas.cloneNode(true).getContext('2d');
	  canvas.parentNode.replaceChild(ctx.canvas, canvas);
	  canvas = ctx.canvas;

	  // reset the attributes and error handler
	  img.crossOrigin = 'anonymous';
	  img.onerror = error_handler;
	  img.src = urls[+!urls.indexOf(img.src)];
	};

<button id="btn"> change image src </button><br>

toDataURL , try catch , 1px * 1px , toDataURL try-catch:

var urls = ["http://upload.wikimedia.org/wikipedia/commons/4/47/PNG_transparency_demonstration_1.png", "http://lorempixel.com/200/200"];

	var img = new Image();
	img.crossOrigin = 'anonymous';

	var canvas = document.createElement('canvas');
	var ctx = canvas.getContext('2d');
	document.body.appendChild(canvas);

	 //create a canvas only for testing if our images will taint our canvas or not;
	var taintTester = document.createElement('canvas').getContext('2d');
	taintTester.width = 1;
	taintTester.height = 1;

	var load_handler = function() {
	  // our image flag
	  var willTaint = false;
	  // first draw on the tester
	  taintTester.drawImage(this, 0, 0);
	  // since it only one pixel wide, toDataURL is way faster
	  try {
	    taintTester.canvas.toDataURL();
	  } catch (e) {
	    // update our flag
	    willTaint = true;
	  }
	  // it will taint the canvas
	  if (willTaint) {
	    // reset our tester
	    taintTester = taintTester.canvas.cloneNode(1).getContext('2d');


	    // do something
	    ctx.fillStyle = 'rgba(0,0,0,.7)';
	    ctx.fillRect(0, 75, ctx.measureText('we won\'t diplay ' + this.src).width + 40, 60);
	    ctx.fillStyle = 'white';
	    ctx.font = '15px sans-serif';
	    ctx.fillText('we won\'t diplay ' + this.src, 20, 100);
	    ctx.fillText('canvas would have been tainted', 20, 120);
        
	  } else {
        
	    // all clear
	    canvas.width = this.width;
	    canvas.height = this.height;

	    ctx.fillStyle = 'white';
	    ctx.font = '15px sans-serif';

	    ctx.drawImage(this, 0, 0);
	  }
	};

	var error_handler = function() {
	  // remove this onerror listener to avoid an infinite loop
	  this.onerror = function() {
	    return false
	  };

	  // we need to removeAttribute() since chrome doesn't like the property=undefined way...
	  this.removeAttribute('crossorigin');
	  this.src = this.src;
	};

	img.onload = load_handler;
	img.onerror = error_handler;

	img.src = urls[0];

	btn.onclick = function() {
	  // reset the attributes and error handler
	  img.crossOrigin = 'anonymous';
	  img.onerror = error_handler;
	  img.src = urls[+!urls.indexOf(img.src)];
	};

<button id="btn">change image src</button>

- - :
IE < , svg, . Safari , <foreignObject> svg, , , UA .

, , , , try-catch, 1px 1px.

Answer 2

, Pointy Kaiido , , OpenLayers ( Pointy - ).

:

            source = new ol.source.TileWMS({
              crossOrigin: 'anonymous'
            });

, crossOrigin: anonymous. . !