Learn how to use the `requestVideoFrameCallback()` to work more efficiently with videos in the browser.
 Thomas Steiner [GitHub](https://github.com/tomayac) [LinkedIn](https://www.linkedin.com/in/thomassteinerlinkedin) [Mastodon](https://toot.cafe/@tomayac) [Bluesky](https://bsky.app/profile/tomayac.com) [Homepage](https://blog.tomayac.com/)
Published: January 8, 2023
Browser Support
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[Source](https://developer.mozilla.org/docs/Web/API/HTMLVideoElement/requestVideoFrameCallback)
The [`HTMLVideoElement.requestVideoFrameCallback()`](https://wicg.github.io/video-rvfc/)
method allows web authors to register a callback
that runs in the rendering steps when a new video frame is sent to the compositor.
This allows developers to perform efficient per-video-frame operations on video,
such as video processing and painting to a canvas, video analysis,
or synchronization with external audio sources.
## Difference with requestAnimationFrame()
Operations made with this API, such as drawing a video frame to a canvas with
[`drawImage()`](https://developer.mozilla.org/docs/Web/API/CanvasRenderingContext2D/drawImage),
are synchronized as a best effort with the frame rate of the video playing on
screen. This is different from
[`window.requestAnimationFrame()`](https://developer.mozilla.org/docs/Web/API/window/requestAnimationFrame),
which usually fires about 60 times per second.
`requestVideoFrameCallback()` is bound to the actual video frame rate---with an important
[exception](https://wicg.github.io/video-rvfc/#ref-for-update-the-rendering%E2%91%A2:%7E:text=Note%3A%20The%20effective%20rate%20at%20which,browser%20would%20fire%20callbacks%20at%2060hz.):
> The effective rate at which callbacks are run is the lesser rate between the video's rate
> and the browser's rate.
> This means a 25fps video playing in a browser that paints at 60Hz
> would fire callbacks at 25Hz.
> A 120fps video in that same 60Hz browser would fire callbacks at 60Hz.
## Feature detection
```javascript
if ('requestVideoFrameCallback' in HTMLVideoElement.prototype) {
// The API is supported!
}
```
### Polyfill
A [polyfill for the `requestVideoFrameCallback()` method](https://github.com/ThaUnknown/rvfc-polyfill)
based on
[`Window.requestAnimationFrame()`](https://developer.mozilla.org/docs/Web/API/window/requestAnimationFrame)
and
[`HTMLVideoElement.getVideoPlaybackQuality()`](https://developer.mozilla.org/docs/Web/API/HTMLVideoElement/getVideoPlaybackQuality)
is available. Before using this, be aware of the
[limitations mentioned in the `README`](https://github.com/ThaUnknown/rvfc-polyfill#requestvideoframecallback-polyfill).
## Use the method
If you use the `requestAnimationFrame()` method, you'll recognize the
`requestVideoFrameCallback()` method. Register an initial callback once, and
then re-register whenever the callback fires.
```javascript
const doSomethingWithTheFrame = (now, metadata) => {
// Do something with the frame.
console.log(now, metadata);
// Re-register the callback to be notified about the next frame.
video.requestVideoFrameCallback(doSomethingWithTheFrame);
};
// Initially register the callback to be notified about the first frame.
video.requestVideoFrameCallback(doSomethingWithTheFrame);
```
In the callback, `now` is a
[`DOMHighResTimeStamp`](https://developer.mozilla.org/docs/Web/API/DOMHighResTimeStamp)
and `metadata` is a [`VideoFrameMetadata`](https://wicg.github.io/video-rvfc/#dictdef-videoframemetadata)
dictionary with the following properties:
- `presentationTime`, of type `DOMHighResTimeStamp`: The time at which the user agent submitted the frame for composition.
- `expectedDisplayTime`, of type `DOMHighResTimeStamp`: The time at which the user agent expects the frame to be visible.
- `width`, of type `unsigned long`: The width of the video frame, in media pixels.
- `height`, of type `unsigned long`: The height of the video frame, in media pixels.
- `mediaTime`, of type `double`: The media presentation timestamp (PTS) in seconds of the frame presented (such as its timestamp on the `video.currentTime` timeline).
- `presentedFrames`, of type `unsigned long`: A count of the number of frames submitted for composition. Allows clients to determine if frames were missed between instances of `VideoFrameRequestCallback`.
- `processingDuration`, of type `double`: The elapsed duration in seconds from submission of the encoded packet with the same presentation timestamp (PTS) as this frame (e.g., same as the `mediaTime`) to the decoder until the decoded frame was ready for presentation.
For WebRTC applications, additional properties may appear:
- `captureTime`, of type `DOMHighResTimeStamp`: For video frames coming from either a local or remote source, this is the time at which the frame was captured by the camera. For a remote source, the capture time is estimated using clock synchronization and RTCP sender reports to convert RTP timestamps to capture time.
- `receiveTime`, of type `DOMHighResTimeStamp`: For video frames coming from a remote source, this is the time the encoded frame was received by the platform, that is, the time at which the last packet belonging to this frame was received over the network.
- `rtpTimestamp`, of type `unsigned long`: The RTP timestamp associated with this video frame.
> [!NOTE]
> **Note:** Note that `width` and `height` might differ from `videoWidth` and `videoHeight` in certain cases (e.g., an anamorphic video might have rectangular pixels).
Of special interest in this list is `mediaTime`.
Chromium's implementation uses the audio clock as the time source that backs `video.currentTime`,
whereas the `mediaTime` is directly populated by the `presentationTimestamp` of the frame.
The `mediaTime` is what you should use if you want to exactly identify frames in a reproducible way,
including to identify exactly which frames you missed.
> [!NOTE]
> **Note:** Unfortunately, the video element does not guarantee frame-accurate *seeking* . This has been an ongoing [subject of discussion](https://github.com/w3c/media-and-entertainment/issues/4). The [WebCodecs API](https://developer.mozilla.org/docs/Web/API/WebCodecs_API) allows for frame accurate applications.
### If things seem one frame off
Vertical synchronization (or just vsync), is a graphics technology that
synchronizes the frame rate of a video and the refresh rate of a monitor.
Since `requestVideoFrameCallback()` runs on the main thread, but, under the
hood, video compositing happens on the compositor thread,
everything from this API is a best effort, and the browser doesn't offer any
strict guarantees.
It's possible the API is one vsync late relative to when a video frame is
rendered. It takes one vsync for changes made to the web page through the API to
appear on screen (same as `window.requestAnimationFrame()`).
So if you keep updating the `mediaTime` or frame number on your web page and compare that
against the numbered video frames, eventually the video will look like it's one frame ahead.
What is really happening is that the frame is ready at vsync x, the
callback is fired and the frame is rendered at vsync x+1,
and changes made in the callback are rendered at vsync x+2.
You can check whether the callback is a vsync late (and the frame is already rendered on screen)
by checking whether the `metadata.expectedDisplayTime` is roughly `now` or one vsync in the future.
If it is within about five to ten microseconds of `now`, the frame is already rendered;
if the `expectedDisplayTime` is approximately sixteen milliseconds in the future
(assuming your browser is refreshing at 60Hz), then you are in sync with the frame.
## Demo
I have created a small
[demo](https://googlechrome.github.io/samepls/requestvideoframecallback/)
that shows how frames are drawn on a canvas at exactly
the frame rate of the video and
where the frame metadata is logged for debugging purposes.
```javascript
let paintCount = 0;
let startTime = 0.0;
const updateCanvas = (now, metadata) => {
if (startTime === 0.0) {
startTime = now;
}
ctx.drawImage(video, 0, 0, canvas.width, canvas.height);
const elapsed = (now - startTime) / 1000.0;
const fps = (++paintCount / elapsed).toFixed(3);
fpsInfo.innerText = `video fps: ${fps}`;
metadataInfo.innerText = JSON.stringify(metadata, null, 2);
video.requestVideoFrameCallback(updateCanvas);
};
video.requestVideoFrameCallback(updateCanvas);
```
## Conclusions
People have done frame-level processing for a long time---without having access to
the actual frames, only based on `video.currentTime`.
The `requestVideoFrameCallback()` method greatly improves upon this workaround.
## Acknowledgements
The `requestVideoFrameCallback` API was specified and implemented by
[Thomas Guilbert](https://github.com/tguilbert-google).
This post was reviewed by [Joe Medley](https://github.com/jpmedley)
and [Kayce Basques](https://github.com/kaycebasques).