Vladimir Vukićević

I Make Fun Stuff

Unreal JavaScript

At the 2013 Game Developers’ Conference, Alon and I from Mozilla and Josh Adams from Epic Games presented a talk called “Fast and Awesome HTML5 Games”. We surprised people by showing off Unreal Engine 3 running in Firefox — compiled from C++ source with Emscripten, running smoothly and efficiently. Today, Epic is making the Epic Citadel demo available, so that you can try it out for yourself.

For best results, it needs a recent Firefox Nightly (Firefox 23 or better). However, because the core technologies are just standard web technologies, it will run in Firefox 20 (the current released version) — but with some performance degradation and a lack of Web Audio-dependant audio effects. We’ve had success in running it in other browsers, but it’s somewhat hit and miss – it heavily depends on the quality of the WebGL implementation, memory management, and JavaScript engine. Now that the demo is available, we expect that they will fix any remaining issues quickly.

Here’s a video (1080p!) of both the Epic Citadel demo, as well as some gameplay footage from the unreleased “Sanctuary” demo:


Working with Epic helped us prove that the Emscripten approach was viable even for large, performance-intensive codebases. We also wanted to demonstrate that with Emscripten, the web becomes just another porting and build target that integrates nicely in existing frameworks. During the week working with Epic to build the demo, we went from things failing to compile to shooting bots in a UT3 level. Having an early build of an asm.js-enabled Firefox (with Odinmonkey) got us the performance boost we needed to hit great frame rates and a very playable experience.

Technical and Porting Details

The engine is compiled down to 60MB of minified asm.js-flavoured JavaScript. This is a large chunk of JS for browsers to digest. Initially, our parse and compile time was close to 40-45s, which is not a great experience. Thanks to some efforts by Nick Nethercote, Sean Stangl, and others, that’s down to around 10s currently. (less on some hardware, highly dependant on CPU speed.) The work to improve parsing and compilation of this large codebase also translated into gains on more normal JS workloads as well. We have plans in progress to reduce this further by doing a custom parser for asm.js, as well as enable compilation caching in IndexedDB.

On the platform side, this was the first big test of our new Web Audio implementation. Ehsan wrote an OpenAL-compatible wrapper that mapped to Web Audio, and this worked very well with the existing OpenAL code that was in UE. The Sanctuary demo was enhanced by adding non-full-screen pointer lock support — a web page can request pointer lock independent from fullscreen, making games that want to lock the mousepossible as part of web pages instead of needing to enter a full screen mode.

The experience really reinforced the main porting strategy that we suggest to others for getting existing content on the Web using Emscripten: make it work natively first using the same libraries that Emscripten is mapping. If you can launch the application direct from the command line, using SDL and OpenGL ES 2.0 for video, OpenAL for audio, and otherwise using pure C libraries, you should see quick results on the Web.

The Web Is The Platform

With Emscripten, the web is just another platform to target. It can be targetted alongside other platforms, with the additional development cost similar to porting to another OS. No longer does choosing to deploy on the web mean rewriting in JavaScript, and worse, continuing to maintain that JavaScript version in parallel with the native version.

Personally, it’s really exciting to get to this point a short two years since the the first WebGL specification was introduced at GDC 2011. With WebGL, Emscripten, and asm.js, the pieces really fell into place to make this possible. It’s even more exciting to be able to do this with high profile engine, known for its quality and performance. Seeing the web get to this point is pretty amazing, and I look forward to seeing what the industry does with it.

More Information

For more information, please check out the slides from our talk:

Vladimir Vukićević – The Web Is Your Next Platform
Josh Adams – Unreal Engine 3 in JavaScript
Alon Zakai – Compiling C/C++ to JavaScript

Also, please visit the new Mozilla Developer’s Network Games landing page, which we’ll be expanding in the coming weeks. The Emscripten project and information about asm.js are also useful if you’d like to take a look at what it would take to port your own games or other apps.