Game Architecture Series

Stylish logo of two engaged gears with the text XNA Game Architecture


I’m planning to start a short article series:

There are a lot of XNA tutorials out there that explain the basics – how to display a sprite, how to do collision detection and how to render a bunch of colorful particles with additive blending. But there aren’t many articles that explain to you how you’re supposed to put it all together – how to structure a game so that it is easy to extend and remains manageable when the amount of code begins to grow.

The discipline that deals with this issue is called software architecture. Like programming, or any other creative process, it relies a lot on tacit knowledge – finding a good solution without first running down an alley of dead ends (that you can identify using the Principles of Object-Oriented Design) requires a lot of experience.

What I will do in this series is let you look over my shoulder as I design a small game and try to explain my motivations for choosing one design over another while I do so. This will give you a solid starting point and an understanding of the design process that you can apply to your own game projects.

The logo of the WiX XNA intaller, a cheap montage of the WiX and XNA logos

WiX XNA Installer 3.1

WiX XNA Installer Logo

I just uploaded a new release of my WiX XNA Installer template that has been updated to XNA 3.1!

If you happen to have a customized installer built on the XNA 3.0 template, fear not, for the required changes are very small! Use your favorite Diff/Merge tool and copy over any changes referring to Xna_3_1 into your existing installer.

I have declared this release a beta because I haven’t gotten around to testing it on all possible operating systems.

Efficiently Rendering Dynamic Vertices

Sometimes, games have to render highly dynamic geometry such as sparks, bullet trails, muzzle flashes and lightning arcs. Sometimes it’s possible to off-load the work of simulating these things to the GPU, but there are effects than can’t be done by the GPU alone.

These cases usually occur when effects require heavy interaction with level geometry or when they require lots of conditionals to mutate a persistent effect state. And sometimes, the effort of simulating an effect on the GPU is just not worth the results. If you have maybe a hundred instances of a bullet trail at once, letting the GPU orient the constrained billboards for the trails instead of generating the vertices on the CPU might just not yield any tangible benefits.

However, there are still a lot of traps you can run into. A typical mistake of the unknowing developer is to render the primitives one-by-one either using one Draw[Indexed]UserPrimitives() call per spark/trail/arc.

This is not a good idea because modern GPUs are optimized for rendering large numbers of polygons at once. When you call Draw[Indexed]UserPrimitives(), XNA will call into Direct3D, which will cause a call into driver (which means a call from code running in user mode to code running in kernel mode, which is especially slow). Then the vertices are added to the GPU’s processing queue.

By sending single primitives to the GPU, this per-call overhead is multiplied and can become a real bottleneck. To demonstrate the effects of making lots of small drawing calls, I wrote a small benchmark and measured the results on my GeForce 8800 GTS 512 and on whatever the XBox 360 uses as its GPU.

Chart showing a drastic performance bottleneck for excessive DrawPrimitive() calls

Read More
The logo of the WiX XNA intaller, a cheap montage of the WiX and XNA logos

WiX XNA Installer 3.0

WiX XNA Installer Logo

This past week I’ve been working on my WiX XNA installer template again because I really wanted to integrate installer generation into my continuous integration builds. That way, I can hand test versions to friends without explaining in detail how to get it to run and it’s one less worry I have when I release the game.

After some FAQ reading and some questions on the XNA Forums, I had the certainty that XNA 3.0 can be deployed with .NET 2.0 only (if you change your project configurations to target .NET 2.0). This is good news because the .NET 3.5 installer is huge and, on a fresh system, I’ve had about a 1 in 10 success quote of the installer finishing without an error, so my trust in the .NET 3.5 installer is completely shattered.

Screenshot of an installer showing that DirectX 9.0c and XNA 3.0 are installed

Read More

Island War Day 7

Today, I’ve got some new screen shots! Read on :-)

Over the weekend, I implemented proper camera and terrain selection controls. Getting keyboard, mouse and gamepad working at the same time proved a bit of a nightmare. PC RTS players will expect the arrow keys to move around their view and the mouse to be usable as a selection tool. XBox 360 RTS players need some way of telling the game where to place buildings, but the gamepad isn’t suited for this task as well as the mouse is.

So I decided to split my terrain into a regular grid in which a terrain cursor can be moved. The grid is in units of 3×3 terrain quads, which allows me to add some detail to the terrain on the sub-grid-cell level. Otherwise, it would look fairly obvious to the player that the terrain is based on a heightmap with regular X/Y samples.

Screenshot of an island with flak buildings intercepting an incoming missile

Read More

Island War Day 6

During the past week, I got missiles working to a point where they will launch, gain altitude, head towards their target and dive for the attack. They will damage the island (which means scraping off texture layers and possibly alter the terrain, not sure whether I want this, however). Buildings in the vicinity are then destroyed.

For the missile trajectory, I went for the simplest thing that could possibly work:

  • Until the missile has reached its cruise altitude, it will ascend at a 45 degree angle. If during this phase, missile closes in to the target so much that it needs to start diving, the diving phase will be entered.

  • Once at its cruise altitude, the missile will simply fly in a horizontal line towards its target. If the missile gets closes enough to its target, it will enter the diving phase.

  • In the diving phase, the missile accelerates to maximum speed and descends towards its target at a 45 degree angle. Once it hits the ground, it explodes.

Read More
The logo of Microsoft's XNA platform, a framework for writing games in .NET

Why Beginners Should Start with XNA

Today, I watched an interesting discussion on where someone asked why everyone is recommending beginners to start off with XNA and C# whereas the entire gaming industry is based on C/C++.

I would have expected to read something along the lines of

“We recommend XNA because it’s very easy to learn and you will hit the ground running fast. It teaches beginners the kind of thinking required to lay out the logic of a program and doesn’t discourage them by forcing them to write boring console applications for months until they know the semantics of the language well enough to use a graphics library such as SDL, DirectX or OpenGL. Some people may not even want to enter the industry, so C# and XNA is a fine choice for them. Those that do can switch over to C/C++ after they’re fluent in C# and it will be a mostly easy going, incremental learning process.”

Instead, an ugly mess resulted with people firmly stating that .NET/Mono is the way to go for games, that the gaming industry is using it for prototyping, scripting and development tools and that the only reasons for not adopting C# yet are legacy code bases and unwillingness by developers to learn new stuff or to change proven ways. People began dissecting each other’s posts one by one and, well, if you’ve been on the internet some time, you can probably imagine how it turns out.

Read More

Island War Day 5

Last time, I wrote about how I set sail for the Island War project and how my motivation faltered slowly over time. And I promised to write a follow-up post that explains how I believe to get back on track and finish development of my Island War project. Well, here it is, folks!

I see the primary reason for my loss of motivation in my tendency to write and fully implement systems instead of just the piece of code I need. While I knew that I had to “write a game, not an engine”, I still fell for the same old trap: I didn’t technically write an engine, but I started to design entire systems and fully implement them when all I needed was a small routine to get the job done.

Read More

Island War Day 4

It’s baack!

Slowly, the Island War project begins to grind its wheels again. Time for a review of what’s happened so far. When I started this project, I was still intoxicated from the successful completion of my just-for-fun project “Ball Race”. I was eager to build a real, full-blown game taking no shortcuts and doing everything in the best way imaginable.

This turned out to be a real time waster and motivation sapper.

Read More
The logo of the WiX XNA intaller, a cheap montage of the WiX and XNA logos

WiX Installer for XNA Games

My GameSetupHelper library is making good progress. I was finally able to get some insight into the workings of WiX and Windows Installer.

I’m just about to create a small installer template for XNA games that will automatically check whether the required system components are available (for XNA games, these are DirectX 9.0c with the D3DX library from October 2006, the .NET Framework 2.0 and of course the XNA Framework).

Screenshot of a windows installer GUI showing the .NET/XNA/DirectX detection results

What I’m looking for now is feedback on the installer (you can find a download link for an example .msi at the end of this article).

  • Is the detection working for you?
  • Do you think the presentation is okay?
  • Do the logo bitmaps look abhorrent? (I’d happily accept any contributions *wink*)
  • Would people be scared by the additional slide during setup? (I tried to be honest about what the installer does to your system and at the same time follow MS recommendation to just silently install anything that’s required)
  • Do I violate any licenses with this? (I am including the official redistributables inside the MSI file, which to my understanding is legal)
  • Anything else that you think might be wrong or missing?

I plan to release the sources of this template for free once I’m done. If you absolutely need to get that installer sources NOW, just send me a mail :)