Can't download canvas content

Question

So Im trying to build an app using electron, the program is pretty basic, the user should upload a couple of pictures, they press a button, I draw them into a <canvas> element and they should be able to download it, the problem that Im having is that when I try to make a downloadable button, a security error is thrown, I did some research and find out that it's a feature of HTML5, the think is that i can't add the crossOrigin = 'Anonymous' because the user is going to load the picture from its own computer, so my question is...

a)- is there a way to "cheat the system" and be able to download the content of the canvas?,

b)- if (a) is not posible...is there an alternative that could work similary to a canvas element?

I'm already using FileReader() to get the picture the user selected, and i'm already drawing that into the canvas, and it looks great, the problem that I have is that i can't download the content that I created for that security error. the .toDataURL can't be called because that's the one that throws the error.

The code to read the pictures from the inputs:

 function handleBox1(evt) {
    var files = evt.target.files; // FileList object

    for (var i = 0, f; f = files[i]; i++) {
      // Only process image files.
      if (!f.type.match('image.*')) {
        continue;
      }
      var reader = new FileReader();

      reader.onload = (function(theFile) {
        return function(e) {

            $('#imgBox1').attr("src",e.target.result);              

        };
      })(f);
      // Read in the image file as a data URL.
      reader.readAsDataURL(f);
    }
}

this is a simple function to see if I could get the Url from the canvas:

descargar = function(){
  var w=window.open('about:blank','image from canvas');
  w.document.write("<img src='"+ totem.toDataURL("image/png")+ "' alt='from canvas'/>");
}

Answer 1

the user should upload a couple of pictures, I draw them into a element and they should be able to download it

You can utilize for loop containing an IIFE to process files object from input type="file" element, new Image, onload event of <img> element, URL.createObjectURL(), Array.prototype.sort() to determine greatest width of the two uploaded images; Promise.all(), Array.prototype.forEach() to process img elements; canvas.toBlob() polyfill to create an objectURL of of image; <a> element having download attribute set to objectURL representing single image created from two images; revoke objectURL at click of a element after download of image completes.

// if (!HTMLCanvasElement.prototype.toBlob) {
Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
  value: function(callback, type, quality) {

    var binStr = atob(this.toDataURL(type, quality).split(',')[1]),
      len = binStr.length,
      arr = new Uint8Array(len);

    for (var i = 0; i < len; i++) {
      arr[i] = binStr.charCodeAt(i);
    }

    callback(new Blob([arr], {
      type: type || 'image/png'
    }));
  }
});
//}

var input = document.querySelector("input");
var canvas = document.querySelector("canvas");
var a = document.querySelector("a");
var ctx = canvas.getContext("2d");
var urls = [];

input.onchange = function(e) {
  var images = [];

  for (var i = 0; i < e.target.files.length; i++) {
    (function(j) {
      // var reader = new FileReader();
      // reader.onload = function(event) {
        var img = new Image;
        img.onload = function() {
          images.push(Promise.resolve(this));
          if (images.length === e.target.files.length) {
            Promise.all(images)
              .then(function(data) {
                data.sort(function(a, b) {
                  return a.naturalHeight < b.naturalHeight;
                })
                data.forEach(function(el, index) {
                  if (index === 0) {
                    canvas.width = el.naturalWidth;
                    canvas.height = el.naturalHeight 
                                    + data[index + 1].naturalHeight;
                    ctx.drawImage(el, 0, 0);
                  } else {
                    ctx.drawImage(el, 0, canvas.height - el.naturalHeight);
                  }

                });
                canvas.toBlob(function(blob) {
                  var url = URL.createObjectURL(blob);
                  console.log(blob);
                  a.href = url;
                  a.style.display = "block";
                  urls.push(url);
                });
              })
          }
        }
        img.src = URL.createObjectURL(e.target.files[j]);
      // }
      // reader.readAsDataURL(e.target.files[j]);
    }(i))
  }
}

a.onclick = function() {
  setTimeout(function() {
    urls.forEach(function(obj) {
      URL.revokeObjectURL(obj)
    })
  })
}

<a download="" href="" style="display:none">download</a>
<input type="file" accept="image/*" multiple />
<br>
<canvas></canvas>