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video.requestAnimationFrame() Explainer

Introduction

Today <video> elements have no means by which to signal when a video frame has been presented for composition nor any means to provide metadata about that frame.

We propose a new HTMLVideoElement.requestAnimationFrame() method with an associated VideoFrameRequestCallback to allow web authors to identify when and which frame has been presented for composition.

Use cases

Today sites using WebGL or <canvas> with a <video> element rely on a set of heuristics to determine if the first frame has been presented (video.currentTime > 0, video canplaythrough event, etc). These heuristics vary across browsers. For subsequent frames sites must blindly call Canvas.drawImage() or GLContext.texImage2D() for access to the pixel data. Our proposed API would allow reliable access to the presented video frame.

In an era of Media Source Extensions based adaptive video playback (e.g., YouTube, Netflix, etc) and boutique WebRTC streaming solutions (e.g., Rainway, Stadia), the inherent raciness of access operations and the lack of metadata exposed about the frames limits quality and automated analysis. Our proposed API would allow reliable access to metadata about which frame has been presented when for correlation with other page level events (user input, etc).

Additionally, our proposal will enable will enable a host of frame-accurate web-platform-tests which can be shared across browsers that were heretofore impossible or otherwise flaky. E.g., while the HTMLMediaElement spec defines readyStates in terms of buffering, it does not define when a frame will be present on the screen.

Specific use case examples:

  • WebGL applications would like to composite at the video rate and not the display rate to save on processing complexity.
  • WebRTC applications would like to synchronize user events such as a key press with the frame that was displayed to the user when the event was triggered.

Proposed API

dictionary VideoFrameMetadata {
    // The time at which the user agent submitted the frame for composition.
    required DOMHighResTimeStamp presentationTime;

    // The time at which the user agent expects the frame to be visible.
    required DOMHighResTimeStamp expectedPresentationTime;

    // The width and height of the presented video frame.
    required unsigned long width;
    required unsigned long height;

    // The presentation timestamp in seconds of the frame presented. May not be
    // known to the compositor or exist in all cases.
    double presentationTimestamp;  // optional

    // The elapsed time in seconds from submission of the encoded packet with
    // the same presentationTimestamp as this frame to the decoder until the
    // decoded frame was ready for presentation.
    //
    // In addition to decoding time, may include processing time. E.g., YUV
    // conversion and/or staging into GPU backed memory.
    double elapsedProcessingTime;  // optional

    // A count of the number of frames submitted for composition. Allows clients
    // to determine if frames were missed between VideoFrameRequestCallbacks.
    //
    // https://wiki.whatwg.org/wiki/Video_Metrics#presentedFrames
    unsigned long presentedFrames;  // optional

    // For video frames coming from either a local or remote source, this is the
    // time the encoded packet with the same presentationTimestamp as this frame
    // was received by the platform. E.g., for a local camera, this is the time
    // at which the frame was captured by the camera. For a remote source, this
    // would be the time at which the packet was received over the network.
    //
    // TODO: For remote sources should this instead be an estimate of the
    // capture time on the remote endpoint?
    DOMHighResTimeStamp captureTime;  // optional

    // The RTP timestamp associated with this video frame.
    //
    // https://w3c.github.io/webrtc-pc/#dom-rtcrtpcontributingsource
    unsigned long rtpTimestamp;  // optional

    // Potentially other useful properties?
};

callback VideoFrameRequestCallback = void(DOMHighResTimeStamp time, VideoFrameMetadata);

partial interface HTMLVideoElement {
    // Extends the AnimationFrameProvider mixin with the addition of an
    // VideoFrameMetadata parameter on the FrameRequestCallback.
    //
    // https://html.spec.whatwg.org/multipage/imagebitmap-and-animations.html#animation-frames
    unsigned long requestAnimationFrame(VideoFrameRequestCallback callback);
    void cancelAnimationFrame(unsigned long handle);
};

partial interface WebGLVideoTexture {
    // Allows DOM-less WebGL usage where texImage2D() is the only compositor to
    // avoid the need for requestAnimationFrame() to get video frame metadata.
    VideoFrameMetadata VideoElementTargetVideoTexture(GLenum target, HTMLVideoElement video)
}

Example

  let video = document.createElement('video');
  let canvas = document.createElement('canvas');
  let canvasContext = canvas.getContext('2d');

  let frameInfoCallback = (_, metadata) => {
    console.log(
      `Presented frame ${metadata.presentationTimestamp}s ` +
      `(${metadata.width}x${metadata.height}) at ` +
      `${metadata.presentationTime}ms for display at ` +
      `${expectedPresentationTime}ms`);

    canvasContext.drawImage(video, 0, 0, metadata.width, metadata.height);
    video.requestAnimationFrame(frameInfoCallback);
  };

  video.requestAnimationFrame(frameInfoCallback);
  video.src = 'foo.mp4';

Output:

Presented frame 0s (1280x720) at 1000ms for display at 1016ms.

Implementation Details

  • When texImage2D() or drawImage() is called during an active VideoFrameRequestCallback, implementations shall ensure that the video frame used is the one matching the active callback.
  • Just like window.requestAnimationFrame(), callbacks are one-shot. video.requestAnimationFrame() must be called again to get the next frame.
  • Since VideoFrameRequestCallback will only occur on new frames, error states
  • In cases where VideoFrameMetadata can't be surfaced (e.g., encrypted media) implementations may never satisfy the requestAnimationFrame.

Open Questions / Notes / Links

  • The API as proposed will miss some frames when compositing happens off the main thread if a subsequent video.requestAnimationFrame() call does not happen in time. To rectify this we would need to make callbacks repeating.
  • Link to GitHub repository.
  • Link to Chromium Prototype.
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