Bringing Cut the Rope to Life in an HTML5 Browser
Award-winning mobile game Cut the Rope is an immediate favorite for anyone who plays it. It’s as fun as it is adorable. So we had an idea: let’s make this great game available to an even bigger audience by offering it on the web using the power of HTML5.
To do this, Microsoft’s Internet Explorer team partnered with ZeptoLab (the creators of the game) and the specialists at Pixel Lab to bring Cut the Rope to life in a browser. The end result is an authentic translation of the game for the web, showcasing some of the best that HTML5 has to offer: canvas-rendered graphics, browser-based audio and video, CSS3 styling and the personality of WOFF fonts.
You can play the HTML5 version of Cut the Rope at: www.cuttherope.ie.
We think that the HTML5 version makes the web more fun and it demonstrates the advances in standards support made in the latest version of Internet Explorer. With that in mind, we want to share some of the cool “behind the scenes” technical details used on this project to help you build your own HTML5 sites and ultimately get ready for the Windows 8 Store!
Cut the Rope running in IE9 as an HTML5 application, ported from the original iOS source code.
A screenshot of one of the specially designed levels that are uniquely available in this version.
In bringing Cut the Rope to a new platform, we wanted to ensure we preserved the unique physics, motion, and personality of the experience. So early on we decided to approach this game as a “port” from the native iOS version (rather than a rewrite). We kicked off the project with an extensive investigation of the original Objective-C codebase. Turns out it’s a big and complex game. The native iOS version consists of about 15,000 lines of code (excluding libraries)! The most complex parts of the codebase are the physics, animation, and drawing engines. They’re complex on their own, but made even more so by the fact that all three are tightly connected and highly optimized.
An example of drawing the ropes with aliased lines using the canvas API.
Surprisingly, we encountered a few areas where the Canvas provides more functionality than does the version of OpenGL ES that was used by the mobile version of Cut the Rope. One example is drawing anti-alias lines. Drawing anti-aliased lines in OpenGL requires tessellating a line into a triangle strip and fading the opacity of the end caps to complete transparency. The HTML5 canvas automatically handles anti-aliasing for lines drawn with its line API. This meant we actually needed to remove code from the OpenGL version. Unwinding the array of triangle vertices in the OpenGL code also gave us much better performance over trying to natively copy the triangle strip method of drawing lines.
In the end, we have nearly 15,000 lines of code executing in the browser (it’s been minified so if you view the source code in your browser, it will be many lines fewer than that). Anticipating the complexity associated with that much code, Denis Morozov (the Director of Development at ZeptoLab) asked a fair question early on: will HTML5 give us the speed and performance that we need for this game?
To answer that, we created an early “performance” milestone, one where we focused on getting a minimal version of the most intense parts of the game running. Namely, we wanted to see what the ropes looked like and whether we could handle the complex physics engine in the browser.
Three weeks into the project, we finally had the basics of the physics and drawing engines in place with a simple timer to bootstrap the animation. We could now render a couple of ropes, a star, and an Om Nom sprite into the game scene. Progress! By week four, we had included some basic mouse interaction and with that we could actually play the game! We were testing performance along the way, but we wanted to let the team at ZeptoLab give us their feedback.
Framerate test results early in the project (note that framerates are capped at 60FPS)
Based on those numbers, we set 30 FPS as our minimum bar. We decided that when the browser goes below that threshold, we would notify the user. They could still play the game, but we’d inform them that it could feel a little bit sluggish. This ensures that we support the huge diversity of hardware and software that’s out there and provide the best experience we can to all of the game’s visitors.
Two things we want to point out. One, the current version of the game works best on desktop PCs and Macs with a mouse. We have not added the support for touch based input yet, but this is something we’re considering for future versions.
Second, the current version of Chrome (version 16) has known issues related to media playback that make sound unpredictable in Cut the Rope. We researched workarounds and have attempted to re-encode the media in multiple formats (including WebM), but haven’t found a format or MIME configuration or anything else that will reliably fix the problem. These appear to be browser bugs and known issues. More importantly, the game continues to be playable and enjoyable in spite of the intermittent audio. In light of that, while we can say Internet Explorer 9 users get a great plug-in free experience, Chrome and some Firefox users could have run into an audio problem but will notice we fall back to a flash plugin to ensure that sound effects and music will work.
Code Editor and Development Environment
Visual Web Developer 2010 Express is a free download and a great editor for even experienced web developers.
A screenshot from the profiler that shows the disproportionate amount of time being spent in Calc2PointBezier, a function that’s used to caculate the positions of the rope segments.
Check out our Resource Loader!
Cut the Rope has unique and very detailed visual styling – lots of media in the form of images, sounds and video – which comes at a small cost. The result is that the whole game is much bigger than an average website. Combined, it’s around 6 MB (compared to 200-300K for a typical site). That much media can take a little while to download and we can’t start the game until we know everything is there. A traditional web page is pretty forgiving if you’re missing an image or two but the HTML5 canvas API (drawImage) will fail if the image isn’t available.
To tackle this challenge, we wanted to create a resource loader that downloads all of the content we need for the page and gives us good feedback as things are downloaded. This bit of code does a bunch of smart things:
1. It deals with the peculiarities of how different browsers handle downloads and how they inform you of their progress.
2. It lets you make smart decisions about the order in which things are downloaded (you might want to start big files first, for example, or maybe download all of the menu images before you get the game images).
3. Finally, it gives you smart events as things arrive so that you can show the user progress or even start part of the game when the first group is completed.
Performance Tools in Internet Explorer
We did some initial code reviews, but nothing was jumping out. We loaded the game with the profiler and immediately saw that we were spending a lot of time inside the satisfyConstraints() function. That function calculates some of the math related to the physics of the ropes. The Objective-C implementation which we had ported was written recursively, passing a new object into each successively deeper call.
With some guidance from Microsoft, we decided to replace the recursive function with an “unpacked” iterative version of the same code. The results were amazing. We saw a 10x improvement in every browser! Frankly, we would have never found that without the profiling tools in Internet Explorer 9.
At BUILD in September, Microsoft showed a developer preview of Windows 8. With this announcement, the HTML5 story got more interesting because Metro style applications can be created using a variety of developer toolsets, including HTML5. This means that web developers can take code that was written for the web and easily and seamlessly port it to Windows 8. The investment in immersive experiences online now can pay off in real profits later with the Windows Store.
In fact, with very little extra work, we were able to port this HTML5 experience to a Windows 8 Metro style app. Read about Cut the Rope and its integration with the Windows Store in this blog post.
We are excited to see what developers can build today with HTML5. You can download Internet Explorer 9 and find other beautiful sites at www.beautyoftheweb.com, or download the Developer Preview of Windows 8 at dev.windows.com.