HTML: The Living Standard

Edition for Web Developers — Last Updated 24 September 2023

1. 1. 8.10 Images
   2. 8.11 Animation frames

8.10 Images

An ImageBitmap object represents a bitmap image that can be painted to a canvas without undue latency.

The exact judgement of what is undue latency of this is left up to the implementer, but in general if making use of the bitmap requires network I/O, or even local disk I/O, then the latency is probably undue; whereas if it only requires a blocking read from a GPU or system RAM, the latency is probably acceptable.

promise = self.createImageBitmap(image [, options ])

promise = self.createImageBitmap(image, sx, sy, sw, sh [, options ])

Takes image, which can be an img element, an SVG image element, a video element, a canvas element, a Blob object, an ImageData object, or another ImageBitmap object, and returns a promise that is resolved when a new ImageBitmap is created.

If no ImageBitmap object can be constructed, for example because the provided image data is not actually an image, then the promise is rejected instead.

If sx, sy, sw, and sh arguments are provided, the source image is cropped to the given pixels, with any pixels missing in the original replaced by transparent black. These coordinates are in the source image's pixel coordinate space, not in CSS pixels.

If options is provided, the ImageBitmap object's bitmap data is modified according to options. For example, if the premultiplyAlpha option is set to "premultiply", the bitmap data's color channels are premultiplied by its alpha channel.

Rejects the promise with an "InvalidStateError" DOMException if the source image is not in a valid state (e.g., an img element that hasn't loaded successfully, an ImageBitmap object whose [[Detached]] internal slot value is true, an ImageData object whose data attribute value's [[ViewedArrayBuffer]] internal slot is detached, or a Blob whose data cannot be interpreted as a bitmap image).

Rejects the promise with a "SecurityError" DOMException if the script is not allowed to access the image data of the source image (e.g. a video that is CORS-cross-origin, or a canvas being drawn on by a script in a worker from another origin).

imageBitmap.close()

Releases imageBitmap's underlying bitmap data.

imageBitmap.width

Returns the natural width of the image, in CSS pixels.

imageBitmap.height

Returns the natural height of the image, in CSS pixels.

var sprites = {};
function loadMySprites() {
  var image = new Image();
  image.src = 'mysprites.png';
  var resolver;
  var promise = new Promise(function (arg) { resolver = arg });
  image.onload = function () {
    resolver(Promise.all([
      createImageBitmap(image,  0,  0, 40, 40).then(function (image) { sprites.person = image }),
      createImageBitmap(image, 40,  0, 40, 40).then(function (image) { sprites.grass  = image }),
      createImageBitmap(image, 80,  0, 40, 40).then(function (image) { sprites.tree   = image }),
      createImageBitmap(image,  0, 40, 40, 40).then(function (image) { sprites.hut    = image }),
      createImageBitmap(image, 40, 40, 40, 40).then(function (image) { sprites.apple  = image }),
      createImageBitmap(image, 80, 40, 40, 40).then(function (image) { sprites.snake  = image })
    ]));
  };
  return promise;
}

function runDemo() {
  var canvas = document.querySelector('canvas#demo');
  var context = canvas.getContext('2d');
  context.drawImage(sprites.tree, 30, 10);
  context.drawImage(sprites.snake, 70, 10);
}

loadMySprites().then(runDemo);

8.11 Animation frames

Some objects include the AnimationFrameProvider interface mixin.

Each AnimationFrameProvider object also has a target object that stores the provider's internal state. It is defined as follows:

If the AnimationFrameProvider is a Window

The Window's associated Document

If the AnimationFrameProvider is a DedicatedWorkerGlobalScope

The DedicatedWorkerGlobalScope

Each target object has a map of animation frame callbacks, which is an ordered map that must be initially empty, and an animation frame callback identifier, which is a number that must initially be zero.

An AnimationFrameProvider provider is considered supported if any of the following hold:

• provider is a Window.
• provider's owner set contains a Document object.
• Any of the DedicatedWorkerGlobalScope objects in provider's owner set are supported.

The requestAnimationFrame(callback) method steps are:

1. If this is not supported, then throw a "NotSupportedError" DOMException.

2. Let target be this's target object.

3. Increment target's animation frame callback identifier by one, and let handle be the result.

4. Let callbacks be target's map of animation frame callbacks.

5. Set callbacks[handle] to callback.

6. Return handle.

The cancelAnimationFrame(handle) method steps are:

1. If this is not supported, then throw a "NotSupportedError" DOMException.

2. Let callbacks be this's target object's map of animation frame callbacks.

3. Remove callbacks[handle].

To run the animation frame callbacks for a target object target with a timestamp now:

1. Let callbacks be target's map of animation frame callbacks.

2. Let callbackHandles be the result of getting the keys of callbacks.

3. For each handle in callbackHandles, if handle exists in callbacks:

   1. Let callback be callbacks[handle].

   2. Remove callbacks[handle].

   3. Invoke callback, passing now as the only argument, and if an exception is thrown, report the exception.

const offscreenCanvas = document.getElementById("c").transferControlToOffscreen();
worker.postMessage(offscreenCanvas, [offscreenCanvas]);

let ctx, pos = 0;
function draw(dt) {
  ctx.clearRect(0, 0, 100, 100);
  ctx.fillRect(pos, 0, 10, 10);
  pos += 10 * dt;
  requestAnimationFrame(draw);
}

self.onmessage = function(ev) {
  const transferredCanvas = ev.data;
  ctx = transferredCanvas.getContext("2d");
  draw();
};