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 with image/jpeg
images to specify the quality of the output.
SyntaxEdit
void 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 isimage/png
. qualityArgument
Optional- A
Number
between0
and1
indicating image quality if the requested type isimage/jpeg
orimage/webp
. If this argument is anything else, the default value for image quality is used. Other arguments are ignored.
Return value
None.
ExamplesEdit
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');
SpecificationsEdit
Specification | Status | Comment |
---|---|---|
WHATWG HTML Living Standard The definition of 'HTMLCanvasElement.toBlob' in that specification. |
Living Standard | No change since the latest snapshot, HTML5 |
HTML5.1 The definition of 'HTMLCanvasElement.toBlob' in that specification. |
Working Draft | No change |
HTML5 The definition of 'HTMLCanvasElement.toBlob' in that specification. |
Recommendation | Snapshot of the WHATWG HTML Living Standard containing the initial definition. |
Browser compatibilityEdit
[1] WebKit does not implement this feature yet. See WebKit bug 71270.
PolyfillEdit
A low performance polyfill based on toDataURL.
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'} ) );
}
});
}
See alsoEdit
- The interface defining it,
HTMLCanvasElement
. Blob