Convert HTML5 Canvas into File to be uploaded?

Question

The standard HTML file upload works as follows:

Answer 1

This is what worked for me in the end.

canvas.toBlob((blob) => {
  let file = new File([blob], "fileName.jpg", { type: "image/jpeg" })
}, 'image/jpeg');

Answer 2

Currently in spec (very little support as of april '17)

Canvas.toBlob();

https://developer.mozilla.org/en-US/docs/Web/API/HTMLCanvasElement/toBlob

EDIT :

The link provides a polyfill (which seems to be slower from the wording), which code is roughtly equivalent to the @pixelomo answer, but with the same api as the native toBlob method :

A low performance polyfill based on toDataURL :

if (!HTMLCanvasElement.prototype.toBlob) {
  Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
    value: function (callback, type, quality) {
      var canvas = this;
      setTimeout(function() {

    var binStr = atob( canvas.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'} ) );

      });
    }
  });
}

To be used this way :

canvas.toBlob(function(blob){...}, 'image/jpeg', 0.95); // JPEG at 95% quality

or

canvas.toBlob(function(blob){...}); // PNG

Answer 3

Another solution: send the data in var url in a hidden field, decode and save it on the server.

Example in Python Django:

if form.is_valid():
    url = form.cleaned_data['url']
    url_decoded = b64decode(url.encode())        
    content = ContentFile(url_decoded) 
    your_model.model_field.save('image.png', content)

Answer 4

For security reasons, you can't set the value of a file-input element directly.

If you want to use a file-input element:

1. Create an image from the canvas (as you've done).
2. Display that image on a new page.
3. Have the user right-click-save-as to their local drive.
4. Then they can use your file-input element to upload that newly created file.

Alternatively, you can use Ajax to POST the canvas data:

You asked about blob:

var blobBin = atob(dataURL.split(',')[1]);
var array = [];
for(var i = 0; i < blobBin.length; i++) {
    array.push(blobBin.charCodeAt(i));
}
var file=new Blob([new Uint8Array(array)], {type: 'image/png'});


var formdata = new FormData();
formdata.append("myNewFileName", file);
$.ajax({
   url: "uploadFile.php",
   type: "POST",
   data: formdata,
   processData: false,
   contentType: false,
}).done(function(respond){
  alert(respond);
});

Note: blob is generally supported in the latest browsers.

Answer 5

The canvas image needs to be converted to base64 and then from base64 in to binary. This is done using .toDataURL() and dataURItoBlob()

It was a pretty fiddly process which required piecing together several SO answers, various blog posts and tutorials.

I've created a tutorial you can follow which walks you through the process.

In response to Ateik's comment here's a fiddle which replicates the original post in case you're having trouble viewing the original link. You can also fork my project here.

There's a lot of code but the core of what I'm doing is take a canvas element:

<canvas id="flatten" width="800" height="600"></canvas>

Set it's context to 2D

var snap = document.getElementById('flatten');
var flatten = snap.getContext('2d');

Canvas => Base64 => Binary

function postCanvasToURL() {
  // Convert canvas image to Base64
  var img = snap.toDataURL();
  // Convert Base64 image to binary
  var file = dataURItoBlob(img);
}

function dataURItoBlob(dataURI) {
// convert base64/URLEncoded data component to raw binary data held in a string
var byteString;
if (dataURI.split(',')[0].indexOf('base64') >= 0)
    byteString = atob(dataURI.split(',')[1]);
else
    byteString = unescape(dataURI.split(',')[1]);
// separate out the mime component
var mimeString = dataURI.split(',')[0].split(':')[1].split(';')[0];
// write the bytes of the string to a typed array
var ia = new Uint8Array(byteString.length);
for (var i = 0; i < byteString.length; i++) {
    ia[i] = byteString.charCodeAt(i);
}
return new Blob([ia], {type:mimeString});
}

You could stop at base64 if that's all you need, in my case I needed to convert again to binary so that I could pass the data over to twitter (using OAuth) without use of a db. It turns out you can tweet binary which is pretty cool, twitter will convert it back in to an image.