get pixel value from canvas image, CORS restriction

Question

so obviously this is quite a problem with some browsers, because I googled a number of hits regarding the "canvas gets tainted" mumbo jumbo when you try and get pixel data from canvas images, like :

var pix = context.getImageData(3200,3200,1,1).data;

So far I already tried the crossorigin = '' and XMLHttpRequest, but I still get the same problem. Then again, I don't know if I'm implementing it wrong and just being a noob. I will greatly appreciate it if someone could please help me with this problem. Struggling with it 1.5 days, before turning to a forum question as last resort. Note that my code works for Firefox browsers, but not for Chrome... It his crucial that my implementation has cross-browser functionality. This is a small javascript code snipped I extracted from my main project to isolate, and try and fix, the problem:

<html>
<head>
  </head>
    <body>
      <canvas id="myCanvas" width="6029" height="6968"></canvas>
      <script>  <!-- Javascript section-->

      var canvas = document.getElementById('myCanvas'); //get canvas from html code via identity

      var context = canvas.getContext('2d');    //get context of canvas
      var imageObj = new Image();       //create image object 
      imageObj.src = 'gauteng.png'; //define the image of the image object


      imageObj.onload = function()  //trigger function after the image has loaded in image object
      {
        try
        {
          context.drawImage(imageObj,0,0);  //draw the image object onto context of canvas

          var imgd = context.getImageData(3200,3200,1,1);   //get pixel at x coord 3200, and y coord 3200
          var pix = imgd.data;  //get the RGB values from pixel

          // print out RGB values with alert            
          alert('R val= '+pix[0].toString()+ "\r\nG val= "+pix[1].toString()+"\r\nB val= "+pix[2].toString());

        }
        catch(err)  //display error message if exception is thrown
        {
          alert(err.toString());
        }
     };

     </script>  
  </body>
</html>

I can also include my approach for the XMLHttpRequest:

imageObj.onload = function() {

    try
    {
        var inputFile = new XMLHttpRequest(); 
        inputFile.open('get', 'gauteng.png', true); //open example file

        inputFile.onreadystatechange = function()
        {
            if(inputFile.readyState==4) //document is ready to parse
            {
                context.drawImage(imageObj, 0, 0);

                //pixel location from image
                var imgd = context.getImageData(3200,3200,6029,6968);
                var pix = imgd.data;

                alert(pix[0].toString()+ " "+pix[1].toString()+" "+pix[2].toString());
            }
        }

        inputFile.send(null); //close file after finished

    }
    catch(err){ alert(err.toString());}
  };

Thank you in advance.

Answer 1

when it is cross origin and you are working with canvas many issues are resolved by this:

imageObj.crossOrigin = 'anonymous';   
// crossOrigin attribute has to be set before setting src.

Hope it helps

get pixel value from canvas image, CORS restriction

Question

1 answers

solution1
3 ACCPTED 2014-11-07 10:15:10

get pixel value from canvas image, CORS restriction

Question

1 answers

solution1 3 ACCPTED 2014-11-07 10:15:10

solution1
3 ACCPTED 2014-11-07 10:15:10