Lecture 13 --- Building a 3D-game using XNA
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First part
 General thoughts on game engine architecture,
and some XNA considerations
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Second part
 Adding a skybox and mouse-look to our previous
code
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Engine (library)
 Shared package (library)
 The actual game
 Editor
 Playpen (sandbox)
 Content pipelineextensions
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Break down code into smaller components
 RenderingComponent
 PhysicsComponent
▪ Collision
▪ Response
 SoundComponent
 Components should not be aware of eachother
 You might use the already existing
GameComponent-class
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When working in teams, naturally breaks
down responsibilities
Easier to replace code
Alot easier to nail down bugs and
performance problems
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Since components are not aware of
eachother, they have to use an indirect way
to communicate
 This can be accomplished using events
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EventManager
 Basically contains a dictionary from event type to
a list of listeners for each event
 When an event is sent, all the listeners are notified
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Examples
 PlayerDiedEvent
 GameAboutToPauseEvent
 LevelCompletedEvent
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Break it down into separable parts:
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Find a good way to quickly add new effects
For example, decide upon common variable /
semantic names for different common
 The application can then set the parameters
automatically for you
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SceneManager
CameraManager
LightManager
EffectManager
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A tree containing the entire scene
Objects can be placed and rotated relatively
to eachother
Easy to group objects
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Example1: EnemyObject
 Contains common stuff like position, model etc
 Enemies might have different behaviors:
ChaseBehavior, JumpAroundBehavior...
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Example2: Camera
 Camera does not control itself, instead a
CameraController-class is moving the camera
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Use a fixed time-step or multiply all your
movement calculations with the time passed
since the last frame
 Game.IsFixedTimeStep = true/false
No ”best” approach: Shawn Hargreaves has a
good (as always) article about this:
http://blogs.msdn.com/b/shawnhar/archive/200
7/07/25/understanding-gametime.aspx
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Flips the back-buffer when the electron beam
travels to the top-left of the screen (a state
called vertical retrace)
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SynchronizeWithVerticalRetrace property of
GraphicsDeviceManager
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The most important rule:
 Never allocate reference type objects (classes)
while the game is being played
 Value types are ok (structs, primitive types...)
 The garbage collector is a much bigger problem
on the Xbox360
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Garbage collection
 Memory allocated on the heap exceeds a given limit:
▪ GC kicks in to release unused memory
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Unacceptable 1: GC triggers too often
Unacceptable 2: GC triggers infrequently but it
takes a looong time for it to complete
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Read more here:
http://blogs.msdn.com/b/shawnhar/archive/2007
/07/02/twin-paths-to-garbage-collectornirvana.aspx
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Use pooling
 For particle systems, enemies etc
 Should use this in non-garbage collected
environments anyway
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So, allocate everything while the game is
loading
 You might also force garbage collection through
System.GC.Collect() which will reset memory
count to zero
▪ Do not do this while playing the game though!
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CPU or GPU bound?
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How can you tell if you are GPU/CPU bound?
 Some detective work is needed
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Example: run the game at a tiny resolution.
Does the framerate improve?
 In that case, you are bound by the pixel shader
stage
▪ Try doing cheaper pixel calculations
▪ Or if you are happy with the framerate, you can do more
heavy vertex processing for free
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Two blog-posts I recommend:
 http://blogs.msdn.com/b/shawnhar/archive/2008/
03/31/an-elf-in-a-box.aspx
 http://blogs.msdn.com/b/shawnhar/archive/2008/
04/11/santa-s-production-line.aspx
 (Read them in that order)
 [email protected]
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Subscribe to Shawn Hargreaves blog:
 http://blogs.msdn.com/b/shawnhar/
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Hang around at the Creators Club Forum:
 http://forums.xna.com/forums/
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People there are really friendly, and if you’re
lucky, Shawn himself will answer your
questions 
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A cheap way to simulate scenery far far away
Imagine walking around in the center of a
textured sphere, where the sphere is always
moving along with you
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We can use a cubemap for the sky-texture
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Use the local position of the vertex as a direction
vector for lookup into the cube texture
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Turn off backside culling for sphere or change
cullmode (we’ll be inside the sphere)
Always pass depth test and write maximum
depth (1.0) for each pixel to depth-buffer
 Nothing in the scene should be farther away than
the sky
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Cubemaps constructed with special tools
CubeMapGen is a free tool from AMD:
http://developer.amd.com/gpu/cubemapgen/
pages/default.aspx
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