Boris Smus

interaction engineering

Screen video capture for Chrome OS

This post is about video capture in Chrome that doesn't rely on any external dependencies like Flash (no fun), NPAPI (not supported on Chrome OS) and Native Client (not yet supported on Chrome OS).

I take screenshots all the time for bug reporting, image editing, etc. On OS X, this functionality is conveniently built in, and available through Command - Shift - 4. As a web denizen, I find it very useful to auto-upload these captures to a remote server, so I wrote this minimal image uploader which replaces the default behavior on OS X to capture the screenshot, and also uploads it to a picture hosting service.

Taking video capture of various UIs is also immensely useful for showing demonstrations, complex interactions, and subtle bugs. I recently re-discovered that QuickTime on OS X comes with this functionality built in. Prior to that I used (paid) ScreenFlow, which also has very nice dimension cropping and time dilation features.

What if we're on a web-only device, such as a Chromebook running Chrome OS? There is a still screenshotting API, but capturing video is less trivial. I've released an extension that captures and play backs video captures inside Chrome, and also lets you share stills to Picasa (using the OAuth 2 extension library). It's available on the webstore, and the source is on github. Read on to learn how it works, and see how you can help.

Screenshots in Chrome

Chrome provides the captureVisibleTab extension API for taking a screenshot of a tab. It requires host permissions on the page, but as usual the permission will enable the API across all pages (with some exceptions). A few successful extensions, such as Awesome Screenshot, use this API and allow cropping, annotation and sharing of screen grabs.

What if you want to capture video of a tab? Chrome provides no pre-existing API for this purpose, however, we can piggyback on the still screenshot API, executing it repeatedly from the background page for every frame we want to capture:

var images = [];
var FPS = 30;
var QUALITY = 50;
timer = setInterval(function() {
  chrome.tabs.captureVisibleTab(null, {quality: QUALITY},
    function(img) {
}, 1000 / FPS);

As we capture, we store the base64-encoded strings representing video frames in an array. Once we're done capturing, we can simulate video playback by rapidly swapping the images in and out:

var background = chrome.extension.getBackgroundPage();
timer = setInterval(function() {
  if (currentIndex >= images.length - 1) {
  setIndex(currentIndex + 1);
}, 1000 / background.FPS);

This approach turns out to be surprisingly efficient, with the extension being able to capture at 30 FPS on a MacBook Air, and 10 FPS on a Chromebook without too much noticeable slowdown.

Note that we rely on a fixed FPS for ease of implementation, however one could imagine using requestAnimationFrame and tracking the variable frame rate so that the playback speed is reasonable. However, there are definitely precision issues with JavaScript's timers, so this is a much more challenging approach.

So we can capture and playback videos inside the browser, but getting it out of the browser is another matter entirely. As a temporary measure, my colleague Ido Green built a screen stitching service which encodes multiple images into a movie using ffmpeg. Ideally, of course, we would encode in the browser. Perhaps a JavaScript video encoder could be implemented, though the performance may be too poor for practical use. Alternatively, a ffmpeg Native Client-based approach might be suitable, especially given that ffmpeg has already been ported.

Free ideas

There are a few logical next steps for this sample. As already mentioned, encoding video in the browser is a top priority, but there are a slew of other interesting directions, some of which can be seen as features, and others as separate products.

  • The captureVisibleTab API doesn't track the mouse cursor. This could be done by injecting an overlay onto the current page and tracking mousemove and click events. This data could then either be drawn onto a canvas context, or encoded separately as mouseData, and then drawn with JavaScript at playback time.

  • Cropping the video dimensions, modifying the video time schedule (speedup, slowdown, truncation) and annotation are all desired video-editing class features that could be implemented by treating images as canvases.

  • A compelling use case for this technology is creating screen sharing sessions for demos and presentations. Thus, it would be very useful to stream the video to a server, and broadcast it to multiple clients in real time. Binary websockets are now available in Chrome, and this could be a great application.

  • Audio annotations on screen captures make perfect sense, and are widely supported by desktop applications. APIs for sound capture have been a long time coming, but finally we may have an answer via the WebRTC ecosystem, and the getUserMedia call.

By the way, I've switched to exclusively using Markdown for all of my published writing, and wrote an markdown preview for Chrome to make my life a bit easier.