How does Unity export to so many platforms?

So for my first article I picked a not so simple theme, but one that has been extremely intriguing to me lately. This week a friend, during a talk about game development and Unity3D, asked me: “How does Unity export to so many and different platforms?” I started to think and formulate my answer, struggled and I couldn’t really answer why. I never really thought about it. So I started doing some research about it.

So let’s start. Unity3D is probably the most famous Game Development Engine on the market. It’s user friendly (15 yo kids can use it after a few tutorials, trust me), it’s available for both Windows and MacOS X (a Linux port is on experimental phase)  and finally and the most important in this article: it can export to basically any existing device. I’m not kidding, check the list at their website:

  • iOS
  • Android
  • Windows Phone
  • BlackBerry
  • Windows
  • Windows Store Apps
  • Mac OS X
  • Linux
  • Their native Web Player
  • PS 3
  • Xbox 360
  • Wii U
  • PS 4
  • Xbox One
  • Tizen
  • SamsungTV
  • PS Vita
  • Gear VR
  • Oculus Rift

It’s a legit myriad of devices and environments. If you don’t know Unity3D and want to give a try, the free version lets you export to Mac OX, Linux, Windows, iOS, Android, BlackBerry, Web Player, SamsungTV, WebGL, Tizen and Windows Phone. It’s pretty awesome, try it! (unfortunately I’m not making money by advertising, I just truly think it’s awesome).

Update: As pointed by Jashan in the comments section, I focused only on scripting in this article, so I decided to write a bit about how Unity3D exports other features to so many platforms. As you will notice whilst reading the next paragraphs, although Unity Technologies does a great job with Unity3D, it relies on several companies to help them to bring some features into it. When a new platform is announced, the UT developers check if every single one of these features are supported by one of their partners, and if so, the time dedicated to support it is reduced drastically. Now let’s take a look at some of the main features.

Graphics
Let’s start with graphics. A few graphics API are supported by Unity: OpenGL, OpenGL ES, WebGL, Metal and DirectX, each of these APIs targeting a different platform. OpenGL is widely used, from MacOS X and some iOS devices, to Linux and even Windows. OpenGL ES is compatible with mobile devices, mostly Android and some iOS. WebGL is the new promise on browser-based graphic application and games, eliminating the need of plugins like Flash and Unity web player. Metal is Apple’s new graphics API, compatible with the most recent iOS devices and computers from the Californian company. Finally, DirectX is Microsoft’s own graphics API solution, compatible with Windows, Windows Phone and Xbox. So even though the folks from UT dedicate a lot of time for graphics, most of the time is devoted to integrate these tools to the engine, and not to write their own API from scratch.

Physics
When it comes to physics, Unity trusts solely in one tool: Nvidia’s PhysX, which supports every single platform Unity builds to. It is, hands down, one of the best physics engines in the market, and it’s been trustful and efficient since the first version of the game engine, when PhysX was called Novodex and didn’t belong to Nvidia yet. The main reason to have a single physics solution for all platforms is consistency: all collisions and movements must behave identically in every device you support, otherwise some platforms can be favored in game.

Lighting
Once again, Unity relies on externals tools to implement lighting, both baked and realtime. Before Unity 5, Autodesk’s Beast was used as a baked lighting tool, been replaced by Geomeric’s Enlighten, which is now used for both realtime GI and baked lighting on the new versions (5.x) of the engine.

Networking
In 2014, Unity Technologies announced UNET (Unity Networking), which is their own networking and multiplayer solution, made in house. In the past, some networking solutions were common, the most famous being Photon. The new tool consists of two parts: the Networking APIs (with high and low level APIs) and the paid Multiplayer services. Since this is a internal project, UT has to port the code to all supported platforms, differently from graphics, physics and lighting described above.

Finally: Scripting

STOP: If you are a developer, is really confident about concepts like managed and native code, execution environments and different platforms, this section is for you. If you are an 3D artist, 2D artist or you are just curious about how this works, I’m really sorry but you may consider stopping here and accepting an awful answer: it’s magic. Seriously, you may jump to the last two paragraphs of this section that summarizes it. If you are still curious, give it a try!


For those who don’t know, Unity lets us script using C# and Unityscript (eww), and without any conversion or specialized tool, export our game to any of the previously listed platforms. Simple like that. So here comes the question: HOW? I thought that just stopping and thinking about it I could come up with an answer, but even tough I had some knowledge about how Unity works under the hood, I couldn’t figure out why. So I asked my best friend in these times (Google) the question that titles this post. Let’s just say I didn’t have the best feedback ever, and later I found out why: I was asking the wrong questions.

The answer lays on the real star of this post: Mono. I knew Mono was really important to Unity, but it goes further than I thought. Let’s start with the beginning: what the hell is Mono? According to the official website, Mono is a “cross platform, open source .NET development framework”. That’s somehow confusing, so let’s go deep in the Mono history.

Right after Microsoft released the .NET framework in 2000 as a “new platform based on internet standards”, this guy called Miguel de Icaza of Xamarin really liked .NET but wanted to develop for Linux. Since Microsoft .NET didn’t support (and still doesn’t) Linux or other non-Windows platforms, he simply decided to create his own environment, the Mono open source project in 2001. So Mono is basically an open source project to “bring” the .NET Framework to other platforms including its own C# compiler and CLR (Common Language Runtime). Historically, Mono was way behind .NET when it comes to features in the beginning. Today, not only it implements most of the .NET tools, but some extra ones. So summing up, Mono is an open source, C/C++ written implementation of the .NET development framework that today is available to a whole bunch of platforms (do you see a pattern here?).

So now let’s stop talking about Mono and give our friend Unity some attention again. So how we program our scripts in Unity? Using C# or Unityscript. So there comes the questions: if we program using C#, how can an Unity game run on so many platforms? Doesn’t Android use Java and iOS use objetive-C instead of C#? Does Unity compile every single game to each of the platforms native code? So many questions.

Let’s start with the last question. No, Unity doesn’t compile every single game to each of the platforms native code. That’s just insanity. Doesn’t Android use Java? Not only. You can still develop using native code (C/C++). But no one wants do do that, right? Wrong, that’s exactly what Mono does.

Now let’s go to the main question: how does my Android device runs a game that was written in C# but there’s no runtime environment that runs C# in it? Well, Mono runs it. But hey, I didn’t install Mono on my device. So basically the question is: how does Mono “get into” my device? With each game made in Unity (and every application developed using Mono) goes a Mono runtime environment. Are you crazy? No, and I can’t prove it right now, but here’s what Xamarin (remember them?) claim on their “How it works” page, and makes perfect sense:

Write your app in C# and call any native platform APIs directly from C#. The Xamarin compiler bundles the .NET runtime and outputs a native ARM executable, packaged as an iOS or Android app.

This brings other question: a Mono developed app attaches the whole framework to itself? The answer can be found in the same website: unused classes in the framework are stripped during linking. So the only portions of the framework that are bundled and put into your app are the ones you use. If you want to read more about how it works on the Xamarin products, visit the Developer Center.

So this is it boys and girls. Mono is the savior here. He provides the .NET Framework to Unity Games. This is why you can code using C# and run the game at so many platforms. I think this is it for my first post here. I hope I can keep the blog updated with more thoughts that have been concerning me. If you like, if you don’t, if there’s an error or even if you think this is total BS, please leave a comment.

Important update (September, 2015): Unity’s IL2CPP technology is changing the way the engine’s scripting backend works, eliminating Mono’s VM and AOT compiler. Take a look at it!

What to read next? I wrote a new post about how Unity3D works under the hood and how our code interacts with the engine code here.

Published by matheusamazonas

Game developer with 4 years of experience on 2D, 3D and VR game development. Worked on Dutch, American and Brazilian markets. Shipped a total of 4 mobile games for Android and iOS and 3 VR experiences so far.

16 thoughts on “How does Unity export to so many platforms?

  1. While I certainly agree that the Mono scripting backend does make it much easier to have Unity games run multiplatform and thus plays an important role, there’s a “little” more to it than just that. They need to support various graphics APIs (Open GL, OpenGL ES, Direct X, Metal, probably some proprietary voodoo stuff on consoles) and they need to make sure that their whole engine and added middleware (stuff like PhysX or FMOD) also works smoothly on all the platforms they support. That’s a pretty big thing that this works so well even though people (including myself) do get fairly annoyed when features don’t work or don’t work properly on specific platforms (which does sometimes happen).

    So what the article gives you really is a rather limited picture: The scripting backend is important, yes. Btw, on Windows Phone / Windows Store Apps, Unity does not use Mono, it does use .NET there (which is pretty much the same from the API perspective … but there are subtle differences and it’s a completely different implementation). But there’s much more than just the scripting backend that needs to properly run multiplatform.

    1. Hi Jashan, thank you for the comment. The article lacks this information, indeed. I tried to focus only on the scripting portion, thus the limited content. Your comment showed me what some people could’ve wondered when using Unity that I didn’t at the time I wrote this article. I’ll look some content up about and update it later so it’s more complete. Thank you again.

  2. Hi Matheus

    Great post that one you made here !! Now that I know that the trick is made by Mono finally all the pieces start to be in place in my mind. I was really curious about how Unity was managing the C# code in the background

    Thanks for your research !!

  3. Thank you for your post! I want to write a little game, just to play a bit in app development (I´m still stuck on desktop in my job) and so I try to choose a technology, that exports to ios, android and my dear legacy, dying out windows phone. I thought about doing a little web app until I fount out that unity isn´t only a game engine, but a real development platform. Was really curious about how it works… I think I´ll give it a try!

    1. Hi Jessica

      Indeed, Unity has become more than a game engine, but most of its architecture still focus on interactive experiences. You can surely do a lot with it, but make sure you’re using the right tools. Maybe there’s another tool out there that is way more suitable for your purpose. I’m not an app developer, so I can’t help you much with it… but it’s worth googling a bit about it.

      Anyway, I’m glad you’re looking into app development and Unity by yourself. Maybe you get interested about game development too 🙂

      Thanks for the comment.

  4. Pingback: Site Title
  5. Hi Mathues

    I have to learn much more about Unity. I need to understand
    a. Processors
    b. Graphics
    c. Audio
    d. Controllers
    e. Storage
    f. Platform Interfaces
    Can you advise any other reading place that covers all these areas

    Thanks

  6. Sorry again. I may as well just show you my assignment as you may be able point be to great referees for me to read and research to help me complete it well.

    I do not mind buying a book or something if its a perfect reference for information on the subjects

    Really hope you can help and have some good tips and pointers

    Regards

    ——Assignment Start
    You are required to investigate four different game development/engine frameworks.

    • Unity – 3D
    • Unreal – 3D
    • Cocos2d-x – 2D
    • Stencil – 2D

    Requirements for the report are as follows

    1- Word count 4000 words Max
    2- When analysing each framework/engine you must consider the following Architecture features
    a. Processors
    b. Graphics
    c. Audio
    d. Controllers
    e. Storage
    f. Platform Interfaces

    3- As part of the report consider the Cross Platform implications for each one. These can include:
    a. Issues in achieving cross platform game operation
    b. Developing for different platforms
    c. Language and scripting
    4- Identify four mobile game genres and which framework/engine you would each and why.
    5- Finally conclude with lessons that you have learned from using engines/frameworks during your time on this degree programme
    —-Assignment End

    1. Hi,

      Most of this info about each engine is publicly available out there, usually on the own engines’ website, and I’d risk and say that even on Wikipedia pages. If not, you can just Google each aspect for each engine individually. It will really depend on how deep you want to go on each section (graphics by itself can take your entire word count).

      Furthermore, I’m not quite sure what your instruction meant with “processors”, “storage” and “platform interfaces”, but you probably know what they mean.

      I hope I helped,
      Matheus

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