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OffscreenCanvas: transferToImageBitmap() method

The OffscreenCanvas.transferToImageBitmap() method creates an ImageBitmap object from the most recently rendered image of the OffscreenCanvas. The OffscreenCanvas allocates a new image for its subsequent rendering.

Syntax

transferToImageBitmap()

Parameters

None.

Return value

A newly-allocated ImageBitmap.

This ImageBitmap references a potentially large graphics resource, and to ensure your web application remains robust, it is important to avoid allocating too many of these resources at any point in time. For this reason it is important to ensure that the ImageBitmap is either consumed or closed.

As described in the OffscreenCanvas examples, passing this ImageBitmap to ImageBitmapRenderingContext.transferFromImageBitmap() consumes the ImageBitmap object; it no longer references the underlying graphics resource, and can not be passed to any other web APIs.

If your goal is to pass the ImageBitmap to other web APIs which do not consume it - for example, CanvasRenderingContext2D.drawImage() - then you should close it when you're done with it by calling ImageBitmap.close(). Don't simply drop the JavaScript reference to the ImageBitmap; doing so will keep its graphics resource alive until the next time the garbage collector runs.

If you call transferToImageBitmap() and don't intend to pass it to ImageBitmapRenderingContext.transferFromImageBitmap(), consider whether you need to call transferToImageBitmap() at all. Many web APIs which accept ImageBitmap also accept OffscreenCanvas as an argument.

Examples

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

// Perform some drawing using the gl context

offscreen.transferToImageBitmap();
// ImageBitmap { width: 256, height: 256 }

// Either:
// Pass this `ImageBitmap` to `ImageBitmapRenderingContext.transferFromImageBitmap`
// or:
// Use the `ImageBitmap` with other web APIs, and call `ImageBitmap.close()`!

Specifications

Specification
HTML Standard # dom-offscreencanvas-transfertoimagebitmap-dev

Browser compatibility