I’ve been working on a game engine recently, and here are some of my experiences and lessons learned. Despite the title, there are many ways to approach this problem, and this is just the one I took.

So, what’s massively cross-platform? It’s a rejection of the ideology of picking a single toolkit or environment (Flash, Unity, XNA, iOS, Android, etc) to base code in. It’s about making the game itself the model in MVC programming with the controller and view being handled by whatever environment I’m porting it to. Many of these toolkits are cross-platform, but sometimes they have poor performance, limited functionality or don’t support many of the targets. I wanted to support everything and have it perform well across the board, which involves four major areas…

The Desktop. Linux, OSX, and Windows. The easiest to target, due to the ubiquity of free and open-source tools for these platforms.

Mobile. Android and iOS (maybe Windows Mobile 7). More limited in options, and wildly different in some ways, but the basic set of tools are readily available.

The Web. IE, Chrome, Opera, Firefox, Safari. The most unusual of the four targets, because of the limited choice of languages.

Consoles. The Xbox 360 (and/or XNA), PS3, and Wii. Excluding XNA, expensive to target. Still, there’s a lot of similarities between them and the desktop target. I haven’t gotten around to this part yet because it’s expensive, so it’s not covered here.

So, basically I want to write a game engine that can support 9+ wildly different platforms, and have it be pretty easy as well. Turns out it can be done.

Choosing a language (or the core environment)

So, at the core of this game, I want to write game code once that can be shared amongst the different ports. I also wanted the nice warm embrace of a quality scripting language, with minimal impact on speed. Here’s some of the options I went through until I found the right one.

Javascript

The web target is basically the hardest to target, since there’s really only Javascript, or Flash, which is also basically Javascript. I could go the Unity route as well, but a good web developer should avoid requiring plugins whenever possible. There’s also Java applets, but I’ve had lots of problems with applets in the past and they’re not particularly user friendly.

So, why not use Javascript itself and clear up the web target problems easily? I tried finding a portable Javascript runtime but had trouble. Rhino, the Javascript interpreter for Java, seemed plausible for Android. I could probably manage with V8 on the desktop. Initial research suggested I couldn’t use iOS’s Javascript interpreter easily, and V8 wouldn’t meet iOS’s code execution guidelines. This seemed like a minefield of potential problems, plus I had a huge bias, I don’t like Javascript over some of the other possible choices. I decided to look elsewhere first, but ended up never looking back.

PyPy / RPython

At this point I felt if I could get something to compile to C or LLVM bitcode I could make it work. I found a project called emscripten that converts LLVM bitcode to Javascript. Additionally, if this didn’t work there was always Alchemy, which does basically the same thing for Flash.

I started checking out PyPy, or more specifically, RPython. Python being my favorite language to code in, it might be perfect for the job. I could even get PyPy to generate C that seemed vaguely usable. PyPy however seemed to be made solely for creating binaries, not C code or llvm bitcode. Additionally, many cool Python features were not available in RPython, so there was just no way I was going to get the full Python experience. I moved on.

Ruby

Perhaps… Ruby? Rubinius compiles to llvm. Unfortunately, it was easy to determine that this was not an option. Oh well.

Haskell

I tried getting ghc to generate LLVM bitcode, but this was consistently troublesome. It could also generate vanilla C, but this was also difficult. I tried getting ghc to use Alchemy’s tools directly, but they just never worked.

Then… Lua

To me, Lua was a toy language, something that non-programmers used to program. This isn’t true. It ended up being my final choice and proved itself to be a top tier programming language. I was impressed by it quickly, and was confident I could get it onto my desktop, mobile, and console targets with ease. Still, there was the web target, but I found ways around this problem, which I detail below.

Choices I didn’t investigate fully

Lisp. A solid lisp implementation could be easily ported everywhere. I think this would’ve been my choice had I not found Lua.

Javascript. I abandoned this choice pretty early. While I think Lua is a better language to work with for this kind of thing, Javascript still remains a valid possibility.

haXe. Created by Flash demigod Nicolas Cannasse, it could potentially be compiled to every target mentioned. It didn’t fit in well with the manner in which I wanted to develop this game though, and the C++ targeting didn’t seem mature enough, so I looked checked out other options first.

EDIT: playn. This was suggested in the comments, I never tried it out during this project. It does not currently support iOS and Console environments, and relies on Java, but it’s open-source and so it’s possible I could do that myself. Worth investigating.

Porting Lua to everything

Each platform usually had it’s own quirks and needs, so I had to figure out the best way to make Lua work on each of them.

Lua on the Desktop

There were no real problems here. I used Lua 5.1, and it just worked. Eventually I switched to luajit 2. Not because I needed the performance boost, which luajit did give me, but to familiarize myself with luajit’s much more complicated build process so I could use it in other targets. Both are fantastic pieces of software, but I would say only use luajit if speed is very important.

Lua on the Web

I first tried compiling Lua using Alchemy. Lua compiled easily, but some hastily made speed tests placed it at a few hundred operations per second, which is extremely low. I decided to try working with emscripten instead. It was also pretty easy, but my first live test of lua code running via the lua runtime via emscripten via a Javascript interpreter was also extremely slow (EDIT: This may have changed, emscripten now has emcc, a tool which may offer significantly better speeds than what I experienced). It seems obvious in retrospect, but I was hoping for the best. In the end it could barely manage 10 fps, even with rendering turned off.

I still stuck with Lua however, and wrote a Lua->Javascript source code translator called lua.js. This would avoid any speed problems due to Alchemy and emscripten. Javascript turned out to be a good host for translated Lua applications, approaching near-Javascript speeds.

I’ve open-sourced this translator, which you can find here.

Lua on Android

Originally I used standard Lua which compiled easily for Android. When performance was a problem, and improvements to the rendering had already been made I switched to luajit. Luajit 2 is in beta right now, and for unknown reasons crashed on Android with JIT turned on, but it can be turned off. There was a slight speed boost, but overall the rendering was still the problem so it may not have been necessary. I talk more about that below.

Lua on iOS

I didn’t waste any time here and went straight to luajit. Not much needs to be said about it, although the JIT compiler cannot be used on iOS because of Apple’s code execution guidlines. I have seen some suggestions that this is not true in certain cases, but if it didn’t seem necessary anyway.

Graphics

The easiest path here is to keep the art simple, at least at first, so I decided to make a 2D game. Generally speaking 3D games are more time-consuming and expensive as well. Knowing what I know now, it’s very possible that each target could handle a simple 3D game. For my own sanity though, I kept it 2D. Take a source image, draw it to the screen at a location. That’s it.

Drawing on the Desktop

I first went with SDL 1.2. It’s stable, wildly popular and portable, and also surprisingly slow. It turns out 1.2 is pretty much exclusively a software-rendering system with no vsync. The result was choppy animation that tears, and has a lower framerate than I’d like. I tried SFML, but found the API lacking, and for a while settled on Allegro 5.0.4. Allegro 5.0.4 has a lot of potential, but is rough around the edges, little niceties like the transition to fullscreen on OS X were missing.

I then decided on SDL 1.3, which is still being developed, but I haven’t had any problems. The core set of features I wanted all have worked flawlessly. It basically combined all the nice things about SDL and Allegro, with none of the bad things. Performance improved and the game looked smooth on all platforms.

Drawing on the Web

Originally, I figured Flash was the best option for this, since traditionally it’s been much faster to render in Flash. As I discovered, this changed with the advent of Canvas and HTML5, but I still wanted to support Flash for any users that might not have Canvas available. I tried several different drawing methods (copyPixels, using Bitmaps) but performance was worse than Canvas in every browser I tested, regardless of the method used. Compared to Canvas on Chrome, it was around 4x slower. With some extreme effort, I’m sure Flash could improve, but even still it didn’t think I could ever reach the dizzying highs of 60fps in Chrome. I eventually dropped the Flash target entirely, since it couldn’t meet my standards. I figured letting users play a poorly performing game would give them a bad impression, and soliciting them to upgrade their browsers was actually a better choice.

Drawing on Android

I first used Android’s Canvas, but it was way too slow. Apparently there’s hardware acceleration for Canvas in Android 3+ but I couldn’t see a performance difference when I tried to enable it, and I still wanted to support 2.x if possible. I then wrote my own OpenGL renderer, that mostly relied on glDrawTexfOES to draw images. It was much faster but still too slow.

I managed to find libgdx, and was immediately impressed. The fps doubled immediately compared to my more naive solution. libgdx is so good, I’d use it on the desktop targets if it didn’t require the user to have a Java VM installed.

Drawing on iOS

I was expecting this to be easy since iOS is popular and libgdx left me feeling positive about rendering libraries for mobile platforms, but all the choices on iOS either didn’t fit into my display model or weren’t free. Mostly both. I reluctantly wrote my own OpenGL renderer for iOS, but this time I learned a little bit more about what keeps performance high on mobile devices and relied on a method that used glBufferData and glDrawElements instead. The performance ended up being what I wanted, even on an iPhone 3G.

Audio

Like the art, I needed to keep audio simple. There are event sounds, which play once, and background sounds, which loop forever but can be stopped at any time.

Audio on the Desktop

Originally I planned to use whatever audio system was available with my display library, but after switching around I disabled sound in whatever library I was currently using and looked elsewhere. The first was libao, but it was prohibitively licensed. I investigated a couple alternatives, including PortAudio, until I eventually I found OpenAL. Despite a high learning curve it met all my needs, including some I didn’t think I had. It also favored pushing data over polling data (callback-based audio playback being pretty common), which was great since I wanted event sounds to be as responsive as possible.

OpenAL just plays sounds, it doesn’t decode them, so I embedded libogg and libvorbis, so I could play Ogg Vorbis files. Unlike other formats, using Vorbis doesn’t require me to pay a license. I eventually switched to stb_vorbis though, which is an entire Ogg Vorbis decoder in a single file, because it simplified my build process and appeared to be faster as well.

Audio on the Web

There’s only one real choice here, the HTML5 audio tag. This was also the most worrying, since delays in sound playback can’t really be controlled and I don’t have the option to seek an alternative. Overall though, it seemed to work great across all browsers.

Audio on Android

MediaPlayer seemed to work just fine.

Audio on iOS

I had some performance issues here when I used AVAudioPlayer, so I wrote an OpenAL version instead. It was better overall, but the game still runs significantly slower during sound playback. This is actually an ongoing problem, so I’d say my next option would be to try a good sound playback library for iOS, since the selection seems a lot better than the rendering libraries for iOS.

EDIT: Audio issues were CPU-bound on my iPhone 3G, so I found a compressed audio codec that iOS supports called IMA4. The files it generates are much bigger, but CPU usage is much much better. I found details on how to encode IMA4/CAFF files here. I use Extended Audio File Services to decode the files, and the data is then passed to OpenAL.