OffscreenCanvas: convertToBlob() method

Baseline 2023 *

Newly available

Since March 2023, this feature works across the latest devices and browser versions. This feature might not work in older devices or browsers.

* Some parts of this feature may have varying levels of support.

Note: This feature is available in Web Workers.

The OffscreenCanvas.convertToBlob() method creates a Blob object representing the image contained in the canvas.

The desired file format and image quality may be specified. If the file format is not specified, or if the given format is not supported, then the data will be exported as image/png. Browsers are required to support image/png; many will support additional formats including image/jpeg and image/webp.

The created image will have a resolution of 96dpi for file formats that support encoding resolution metadata.

Syntax

convertToBlob()
convertToBlob(options)

Parameters

options Optional

An object with the following properties:

type: A string indicating the image format. The default type is image/png; this image format will be also used if the specified type is not supported.
quality: A Number between 0 and 1 indicating the image quality to be used when creating images using file formats that support lossy compression (such as image/jpeg or image/webp). A user agent will use its default quality value if this option is not specified, or if the number is outside the allowed range.

Return value

A Promise returning a Blob object representing the image contained in the canvas.

Exceptions

The promise may be rejected with the following exceptions:

InvalidStateError DOMException: The OffscreenCanvas is not detached; in other words it still associated with the DOM and not the current worker.
SecurityError DOMException: The canvas context mode is 2d and the bitmap is not origin-clean; at least some of its contents have or may have been loaded from a site other than the one from which the document itself was loaded.
IndexSizeError DOMException: The canvas bitmap has no pixels (either the horizontal or vertical dimension is zero).
EncodingError DOMException: The blob could not be created due to an encoding error.

Examples

const offscreen = new OffscreenCanvas(256, 256);
const gl = offscreen.getContext("webgl");

// Perform some drawing using the gl context

offscreen.convertToBlob().then((blob) => console.log(blob));
// Blob { size: 334, type: "image/png" }

Specifications

Specification
HTML # dom-offscreencanvas-converttoblob-dev