Now, let me state up front, that all four of these are competent engines and I could see situations where I would recommend three of the four engines for client projects and could easily imagine situations where all four of them would be good choices.These four do not make an exhaustive list; there are other engines out there, including some really good ones, but these were the four that we looked at for this specific project. As much as I like the UDK, for example, I don't like it enough to spend quality time in Windows, so that one is off the table for me until the fine folks at Epic decide to port their dev and content tools to the Mac.
Unity Overview
Unity3D actually predates the iPhone, and of the four engines, it's the one that feels the most mature and has the most robust developer tools. It also has one of the most active developer communities. Unity3D is a closed-source commercial product that you must pay for, which might be a turn-off for some, but it is worth every penny of the license fee.Unity Pros
Unity is actually fairly easy to learn, yet has a feature set that is compares favorably to most other engines. Unity's asset pipeline is really robust and their tools give you the ability to very quickly make changes and test those changes.One of the best things about Unity3D is the fact that it supports both Windows and Mac OS equally as development platforms. Regardless of which platform you develop on, you can generate games that runs on every platform Unity supports (assuming you're licensed for it). You can even have part of your team developing on Macs and the other part on Windows with no problems.
At the time I'm writing this, it is possible to generate games from Unity that run on Mac OS X and Windows (both native apps as well as games playable through a web plug-in), iOS, Android, Wii, Xbox 360, and PS3. The Unity folks are also actively working on adding Linux support. Console licenses are negotiated on an individual basis and are likely quite expensive, but it's nice to have the option to take a successful game to so many platforms without having to do a full port each time.
Having had to port OpenGL ES apps from iOS to Android more than once (which is no fun), I can honestly state that the Android support should be a huge selling point you're giving any thought to supporting both iOS and Android.
Unity Cons
The downsides to Unity3D are relatively few, actually.It is the most expensive of the engines I looked at, and the costs are completely front-loaded. You pay a flat per-developer license per platform up front, but then you can create as many games as you want for the platforms you're licensed for. For the basic iOS license, it's $400 per seat (one developer using up to two machines), for the Pro license, you're looking at $3,000 (because the $1,500 iOS Pro requires the $1,500 Unity Pro), which might sound like a lot of money, but it's really a pittance compared to the amount of development time it can save you. The "Basic" version of Unity, which allows you to generate Mac, Windows, and Web games (though excludes some of the more advanced features) is available completely free of charge.The other potential downside in some situations is that the underlying C++ source code is not available. You work primarily in either JavaScript or C# (MonoScript) and at a higher level of abstraction. Unity supports a few other languages besides Javascript and C#, but only those two languages work for mobile development.
After about two weeks of spending my evenings with Unity, I actually came to the conclusion that not having access to the source isn't really much of a drawback, and for many developers, working this way will be better than working in C++. The engine takes care of almost all the low-level stuff you'd need to do, but even if it doesn't, Unity has a shader language that lets you write code that runs on the GPU and anything that needs to run on the CPU can be done by scripting inside Unity.
Although the programming in Unity3D is done with scripting languages, the scripts you write are actually compiled, so there's not a huge performance overhead to using them. In the exceedingly rare situation where C# or Javascript isn't sufficient, it is possible to send messages into and out of Unity from your application's C or Objective-C code.
Learning Curve
I found Unity surprisingly easy to learn. There are some really good resources out there, including lots of tutorials and instructional videos. I actually had a game functioning after about two hours of playing. It was ugly and the game mechanics were simple, but it was playable. Unity also has a fairly active developer community forum where you can go and get help when you get stuck.If you don't already know something about graphics programming, you still don't need to be too scared or intimidated. Most of the gnarly stuff is squirreled away where you won't see it until you need it, and you can do a surprising amount by just configuring things in the development GUI. You can create, for example, a fairly full-featured physics simulations without ever writing a line of code. Want to stack up a bunch of crates and roll a ball into them and watch them all fall? You can do it without ever opening a text editor. Heck, you can even do that without opening a 3D program.
Complexity
I was scared of one thing going into learning Unity: I was concerned that because it was so easy (at least I had heard it was), that it was going to be a dumbed down game maker that sacrificed more advanced features for the sake of lowering the obstacles to entry. My fears were completely misplaced, though. That's not the case at all. The Unity folks have done a really great job of making their tools easy to use while still giving you the ability to do most anything you'd ever need to do. It's even possible (though the process is a bit convoluted) to integrate UIKit and Unity3D within the same application.Asset Pipeline
The thing that actually impressed me most about Unity is what they call their "asset pipeline", which is the process by which you get 3d models, textures, and other assets into your game. If you've ever developed a game or game mod for the UDK, a Valve Source game, or other commercial engine, usually there's kind of a convoluted process you have to go through to get your game assets including characters, textures, props, and environments, into the game. You usually have to use separate applications to specify shaders and physics options, sometimes write a compile script or other text document to define certain trains, and then compile the object and package it all up into some kind of bundle or package. While developing a single mod, you typically iterate through this process many, many times. It can be a bit tedious and hard to learn and usually requires the use of multiple tools and lots of trial and error.Unity bypasses almost all of this tedium. When you save your assets from your 3D program or Photoshop, you simply save them in your Unity project's Assets folder. When you launch Unity, or navigate back to it if it's already open, Unity detects the new file, or any changes you've made to an existing file. It imports it and adds it to the list of available assets.Anything you need to do that can't be done in your 3D program, such as specify a shader or identify the physics engine properties for the object, you do right in Unity, and most of that can be done without writing code (though everything you can configure without code can also be changed from code).
Which 3D program do you need to use? Pretty much any one you're comfortable with. Unity3D has direct import support for the native file formats from several programs including Blender , but any package that can export to Autodesk FBX or Collada can be used with full support for all features like bones, textures, animations, etc.It also supports native Photoshop files (.psd) for textures, flattening layers in a non-destructive manner when you build your app.Assets can even be given different characteristics for different platforms. You could have, for example, a 2048x2048 texture asset for the desktop version of your game but tell Unity to use a 1024x1024 version for game consoles and a 512x512 version on iOS and Android.
The Bottom Line
As I stated earlier, there are situations where I could see any of the four engines I looked at being a good choice, as could many of the other engines I didn't look at. But, honestly, if I had to make an engine decision without detailed information, Unity3D would be my first recommendation. The tools are solid, the company and community support is awesome, it supports many platforms, and getting assets into your game couldn't be easier. It's relatively easy to start using for both experienced developers and those who aren't.A good, experienced graphics programmer working in mobile right now can easily ask $200 per hour or more because the demand far outstrips the supply. Figure it out. Unity's full iOS license is basically the equivalent of 15 hours of a graphics programmer's time. Yet, Unity will easily cut five times that many hours off of any decent size software game project's schedule, probably a lot more.
I'm giving Unity3D two big thumbs up. I'm using it on a project now and hope to use it on many more in the future because it's fun to use and removes much of the tedium associated with 3D programming without removing power.
1 Many of the more advanced features require the more expensive "Pro" license, however, and really cutting edge features from the latest games generally take a little while to show up in the tools
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