The HTMLCanvasElement.toBlob() method creates a Blob object representing the image contained in the canvas; this file may be cached on the disk or stored in memory at the discretion of the user agent. If type is not specified, the image type is image/png. The created image is in a resolution of 96dpi.

The third argument is used when creating images using lossy compression (namely, image/jpeg) to specify the quality of the output.

Syntax

canvas.toBlob(callback, mimeType, qualityArgument);

Parameters

callback
A callback function with the resulting Blob object as a single argument.
mimeType Optional
A DOMString indicating the image format. The default type is image/png.
qualityArgument Optional
A Number between 0 and 1 indicating image quality if the requested type is image/jpeg or image/webp. If this argument is anything else, the default value for image quality is used. Other arguments are ignored.

Return value

None.

Exceptions

SecurityError
The canvas's bitmap is not origin clean; at least some of its contents come from secure

Examples

Getting a file representing the canvas

Once you have drawn content into a canvas, you can convert it into a file of any supported image format. The code snippet below, for example, takes the image in the <canvas> element whose ID is "canvas", obtains a copy of it as a PNG image, then appends a new <img> element to the document, whose source image is the one created using the canvas.

var canvas = document.getElementById('canvas');

canvas.toBlob(function(blob) {
  var newImg = document.createElement('img'),
      url = URL.createObjectURL(blob);

  newImg.onload = function() {
    // no longer need to read the blob so it's revoked
    URL.revokeObjectURL(url);
  };

  newImg.src = url;
  document.body.appendChild(newImg);
});

Note that here we're creating a PNG image; if you add a second parameter to the toBlob() call, you can specify the image type. For example, to get the image in JPEG format:

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

A way to convert a canvas to an ico (Mozilla only)

This uses -moz-parse to convert the canvas to ico. Windows XP doesn't support converting from PNG to ico, so it uses bmp instead. A download link is created by setting the download attribute. The value of the download attribute is the name it will use as the file name.

var canvas = document.getElementById('canvas');
var d = canvas.width;
ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(d / 2, 0);
ctx.lineTo(d, d);
ctx.lineTo(0, d);
ctx.closePath();
ctx.fillStyle = 'yellow';
ctx.fill();

function blobCallback(iconName) {
  return function(b) {
    var a = document.createElement('a');
    a.textContent = 'Download';
    document.body.appendChild(a);
    a.style.display = 'block';
    a.download = iconName + '.ico';
    a.href = window.URL.createObjectURL(b);
  }
}
canvas.toBlob(blobCallback('passThisString'), 'image/vnd.microsoft.icon', 
              '-moz-parse-options:format=bmp;bpp=32');

Save toBlob to disk with OS.File (chrome/add-on context only)

This technique saves it to the desktop and is only useful in Firefox chrome context or add-on code as OS APIs are not present on web sites.

var canvas = document.getElementById('canvas');
var d = canvas.width;
ctx = canvas.getContext('2d');
ctx.beginPath();
ctx.moveTo(d / 2, 0);
ctx.lineTo(d, d);
ctx.lineTo(0, d);
ctx.closePath();
ctx.fillStyle = 'yellow';
ctx.fill();

function blobCallback(iconName) {
  return function(b) {
    var r = new FileReader();
    r.onloadend = function () {
    // r.result contains the ArrayBuffer.
    Cu.import('resource://gre/modules/osfile.jsm');
    var writePath = OS.Path.join(OS.Constants.Path.desktopDir, 
                                 iconName + '.ico');
    var promise = OS.File.writeAtomic(writePath, new Uint8Array(r.result), 
                                      {tmpPath:writePath + '.tmp'});
    promise.then(
      function() {
        console.log('successfully wrote file');
      },
      function() {
        console.log('failure writing file')
      }
    );
  };
  r.readAsArrayBuffer(b);
  }
}

canvas.toBlob(blobCallback('passThisString'), 'image/vnd.microsoft.icon',
              '-moz-parse-options:format=bmp;bpp=32');

Specifications

Specification Status Comment
HTML Living Standard
The definition of 'HTMLCanvasElement.toBlob' in that specification.
Living Standard No change since the latest snapshot, HTML5
HTML 5.1
The definition of 'HTMLCanvasElement.toBlob' in that specification.
Recommendation No change
HTML5
The definition of 'HTMLCanvasElement.toBlob' in that specification.
Recommendation Snapshot of the HTML Living Standard containing the initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidEdge MobileFirefox for AndroidOpera for AndroidiOS SafariSamsung Internet
Basic supportChrome Full support 50Edge No support NoFirefox Full support 19IE Full support 10
Prefixed
Full support 10
Prefixed
Prefixed Requires the vendor prefix: ms
Opera Full support 37Safari Full support Yes
Notes
Full support Yes
Notes
Notes See WebKit bug 71270.
WebView Android Full support 50Chrome Android Full support 50Edge Mobile No support NoFirefox Android Full support 4Opera Android Full support 37Safari iOS No support NoSamsung Internet Android Full support 5.0
Image quality parameterChrome Full support 50Edge No support NoFirefox Full support 25IE No support NoOpera Full support YesSafari No support NoWebView Android Full support 50Chrome Android No support NoEdge Mobile No support NoFirefox Android Full support 25Opera Android No support NoSafari iOS ? Samsung Internet Android No support No

Legend

Full support  
Full support
No support  
No support
Compatibility unknown  
Compatibility unknown
See implementation notes.
See implementation notes.
Requires a vendor prefix or different name for use.
Requires a vendor prefix or different name for use.

Polyfill

A low performance polyfill based on toDataURL.

if (!HTMLCanvasElement.prototype.toBlob) {
  Object.defineProperty(HTMLCanvasElement.prototype, 'toBlob', {
    value: function (callback, type, quality) {
      var dataURL = this.toDataURL(type, quality).split(',')[1];
      setTimeout(function() {

        var binStr = atob( dataURL ),
            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'} ) );

      });
    }
  });
}

See also

Document Tags and Contributors

Last updated by: DomenO,