In the process of building on online video editor I wanted to scratch an itch that had been bugging me for a long time. Boomerang effect. It's cheesy, fun and pretty useless. It's also been said to be impossible to do on the web in real-time.... until now.

You can try the Instant Boomerang Creator if you don't want to read the article. :) (Note: I've only properly tested in Chrome as of today.)

For the uninitiated, the boomerang effect is a video effect that is popular on Instagram - a video will play forwards and then it will play backwards in an infinte loop.

It turns out that it is not that easy to recreate on the web, especially if you want it to happen instantly, but I would like to show you how I did it.

This article assumes that you know how to record the frames from a video stream using the MediaRecorder API.

A simplistic approach would be to play a video forwards in a video element and then when the playback position reaches the end of the video, set the playbackRate to -1 and then repeat the process again when the playback position reaches the beginning of the video. This, however has a number of problems, the first that it's nearly impossible to get an accurate timestamp of where the playback is in the video, and secondly the events that fire on the <video> element aren't instant - both combined mean that the video playback won't be smooth to the user, it also means that you will need to do the video processing on the server because this would just be a simple video that is recorded in normal forwards time.

An better solution would be to create a video file that had the forwards and backwards frames in one video file and then just infinite loop that video. If we can create the video we can remove all jank from the playback.

The solution I created uses a couple of little known API's captureStream(). These API's are supported by Chromium based browsers and Firefox (there's a hint it might be in Safari too).

The captureStream() API is available on <canvas>, <video> and <audio> elements, and when called return a MediaStream object that can be used either as a source for a <video> element, or as the input to the MediaRecorder API.

It is the captureStream method on the <canvas> element that is particularly interesting. The method enables you to draw to the canvas (as you normally would) and then at your own rate call requestFrame() on the video track of the stream, which will push the contents of the <canvas> on to the MediaStream for recording or for playback.

Armed with this method, my solution to get instant looping boomerang and a recording of the single loop was as follows.

1. Get the Web Cam stream via getUserMedia
2. Create a 'frame buffer', to hold frames from the video camera stream when we want to record
3. Create a canvas element that will act as our live video element
4. Set up a requestAnimationFrame and grab the frame from the camera stream and pipe it to a hidden <video> element so that they can be drawn to a visible <canvas>.
5. Set up a MediaRecorder whose input is the result of canvas.captureStream()
6. When the user wants to record
   1. Set the MediaRecorder to record, then for each frame
   2. Continue to draw each frame of video to the canvas (this is now the forward part of the boomerang)
   3. Add the canvas pixels to 'frame buffer'
   4. When the user stops recording, clone and reverse the 'frame buffer'
   5. Join the the two frame buffers together.
   6. Stop rendering from the camera stream to the canvas, but use the frames from the 'frame buffer', starting in the middle (the point the user stopped recording)
   7. As we progress through the frame buffer, stop the MediaRecorder when at the last frame (the output will be exactly one forward loop and one reverse loop).
   8. Treat the frame buffer as a circular array, making an infinte loop of boomerangtastic video.

The code is on my Glitch but the core logic is below.

// There is some other set-up, but this will get the streams.
let canvasStream = canvasOutput.captureStream(0);
let canvasStreamTrack = canvasStream.getTracks()[0];

// The canvas stream will be played back on a video...
playbackVideo.srcObject = canvasStream;
// The canvas stream will be recorded
let rec = new MediaRecorder(canvasStream, {mimeType: mimeType});

let renderFrame = () => {
  // Every draw to the canvas will be rendered to the MediaStream
  // once requestFrame is called
  let imageData;
  if (canvasStreamTrack === undefined) return;

  if(recording) {
    // The forwards part of the loop, draw to canvas anc
    // record the frame in to the 'frame buffer'
    context.drawImage(bufferVideo, 0, 0);
    imageData = context.getImageData(0, 0, width, height);
    frames.push(imageData);
  } 
  else {
    if (playback === false) {
      // Use the camera stream straight to canvas
      context.drawImage(bufferVideo, 0, 0);
    } 
    else {
      canvasOutput.width = canvasOutput.width;
      // Loop through the frame buffer and draw to canvas
      frameIdx = (++frameIdx) % frames.length;
      context.putImageData(frames[frameIdx], 0, 0);
      if (frameIdx === 0 && rec.state != 'inactive') { 
        // We are at the last frame of the reversal, stop recording
        rec.stop();
      }
    }
  }

  // Make sure the updates to canvas are rendered on to the stream.
  canvasStreamTrack.requestFrame();
  requestAnimationFrame(renderFrame);
};

let startRecording = async () => {
  recording = true;
  frames = [];
    
  rec.start(10);
}

let stopRecording = () => {
  recording = false;
  playback = true;
    
  // Take the frames, and reverse
  frames = [...frames, ...frames.slice(0).reverse()];
  // Set the start of the playback to be the first reverse frame
  frameIdx = Math.round(frames.length / 2);
}

And that's it. Instant Live-Boomerang, with a saved recording.

I lead the Chrome Developer Relations team at Google.

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