XNA 3D Primer 978-0-470-59693-7

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Michael C. Neel

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XNA 3D Primer Wiley Publishing, Inc.

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Contents Who Is This Book For? 3D Overview Basic 3D Math Right-Hand Rule Working with Matrices Identity, Scale, Rotate, Orbit, Translate (ISROT) Working with Vectors Unit Vectors Working with Quaternions

Controlling the Camera Basic Camera Follow Camera ViewPorts BoundingFrustum

3D Models Modeling Programs and Formats Loading a Model

Collision Detection Skeletal Animations Extending the Content Pipeline Manipulating Bones at Run Time Using Model Animations

About Michael C. Neel

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XNA 3D Primer In early computer graphics, graphic adapters were 2D (two-dimensional) only. Games created a frame by setting pixel colors in the graphics adapter’s memory, then telling the graphics adapter to make the buffer active in a simple process called frame buffering. If a game was in 3D (three dimensions), it meant that the programmers wrote 3D-to-2D conversion methods, sometimes in C, often in Assembly, to generate 2D frames from 3D sources. Fortunately, it’s no longer 1980, and graphics adapter advancements include processing 3D images. With XNA, it’s possible to write high-performance 3D games in managed code that will run on a Windows PC and Xbox 360. 3D game development has never been easier, but that doesn’t mean it is easy. Learning the terms and concepts in 3D graphics programming should qualify as a foreign language credit.

Who Is This Book For? This Wrox Blox is an introduction to 3D game programming with XNA, but it is not an introduction to XNA. You should be familiar with the basics of XNA (working through a few tutorials at http://creators.xna.com should be sufficient) and also be comfortable with C# and .NET 3.5. Although this Wrox Blox covers the concepts of 3D at a high level, it does not dive into details such as calculating vector cross-products. The focus is on the methods in the XNA Framework and when you should use them. This is a primer on the world of 3D programming, but you may need to explore some areas more deeply if you intend to complete a fully working 3D game. Hopefully, this Wrox Blox will have provided you with enough tools to learn what you don’t know.

XNA 3D Primer

3D Over view In XNA the basic steps to using 3D content in your game are:

1. . 3. 4. 5.

Loading or creating the model Setting up the camera Configuring the graphics device Configuring the basic effect (or shader) Rendering the result

A simple example of a game rendering a 3D object is as follows: public class GameMain : Microsoft.Xna.Framework.Game { GraphicsDeviceManager graphics; VertexPositionColor[] diamond; Matrix cameraProjection, cameraView; public GameMain() { graphics = new GraphicsDeviceManager(this); } protected override void Initialize() { graphics.PreferredBackBufferHeight = 450; graphics.PreferredBackBufferWidth = 450; graphics.ApplyChanges(); cameraProjection = Matrix.CreatePerspectiveFieldOfView( MathHelper.PiOver2, 1f, 1f, 10000f); cameraView = Matrix.CreateLookAt( new Vector3(0, 0, 4), Vector3.Zero, Vector3.Up); base.Initialize(); } protected override void LoadContent() { diamond = new VertexPositionColor[] { new VertexPositionColor(new Vector3(0, 0, 1), Color.Red), new VertexPositionColor(new Vector3(-1, 0, 0), Color.Green), new VertexPositionColor(new Vector3(0, 1, 0), Color.Blue), new VertexPositionColor(new Vector3(0, 0, -1), Color.Red), new VertexPositionColor(new Vector3(0, 1, 0), Color.Green), new VertexPositionColor(new Vector3(1, 0, 0), Color.Blue), new VertexPositionColor(new Vector3(0, 0, 1), Color.Red), new VertexPositionColor(new Vector3(0, -1, 0), Color.Green), new VertexPositionColor(new Vector3(-1, 0, 0), Color.Blue), new VertexPositionColor(new Vector3(0, -1, 0), Color.Red), new VertexPositionColor(new Vector3(0, 0, -1), Color.Green), new VertexPositionColor(new Vector3(1, 0, 0), Color.Blue), }; }

XNA 3D Primer protected override void Update(GameTime gameTime) { KeyboardState keys = Keyboard.GetState(); if (keys.IsKeyDown(Keys.Escape)) this.Exit(); base.Update(gameTime); } protected override void Draw(GameTime gameTime) { graphics.GraphicsDevice.Clear(Color.CornflowerBlue); graphics.GraphicsDevice.VertexDeclaration = new VertexDeclaration( graphics.GraphicsDevice, VertexPositionColor.VertexElements); BasicEffect effect = new BasicEffect(graphics.GraphicsDevice, null); effect.VertexColorEnabled = true; effect.Projection = cameraProjection; effect.View = cameraView; effect.World = Matrix.Identity; effect.Begin(); foreach (EffectPass pass in effect.CurrentTechnique.Passes) { pass.Begin(); effect.GraphicsDevice.DrawUserPrimitives( PrimitiveType.TriangleStrip, diamond, 0, diamond.Length - 2); pass.End(); } effect.End(); base.Draw(gameTime); } }

This code is available for download in the example solution as part of the “1_3D_Overview” project. The class starts by creating a GraphicsDeviceManager, which provides access to the video card. In Initialize, the PreferredBackBufferHeight and PreferredBackBufferWidth properties are set to the desired window height and width, respectively. The next lines describe the camera, which is covered in detail later, but for now, know that the projection is the lens of the camera and the view is the position and orientation. In LoadContent you create a simple list of vertices with color information. A vertex is a point on a polygon, and in this case, it is a polygon of a three-dimensional diamond. It is common in 3D programming to refer to a vertex collection as a mesh. A model refers to a mesh or collection of meshes with color or texture data, and possibly animation data as well. In the Draw method, setting the VertexDeclaration tells the GraphicsDevice what type of data is being sent. Before the data can be displayed, a BasicEffect shader is configured. A shader is code that will run on the GPU of the graphics adapter, saving the CPU for other tasks such as physics calculations, input handling, artificial intelligence, and so on. Shaders for XNA are written in High-Level Shader Language (HLSL), which is similar to C. Shaders commonly have the extension .fx.

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XNA 3D Primer At a glance, shaders fall into two types — vertex shaders and pixel shaders. A vertex shader applies transformations to the vertices being rendered such as changing their position, and a pixel shader processes color information to determine the color of the pixels on screen. Shaders are covered in greater detail in the animation section; for now, just know that BasicEffect is a vertex and pixel shader included with the XNA Framework. The location of our diamond on screen is determined by the Projection, View, and World matrices set on effect. The projection and view matrices come from our camera, and the World matrix is how the model space maps to world space. Use this matrix to adjust a mesh or model to world space, such as scaling up a model that was too small compared to the other models in the game. All of this comes together to produce the rather boring screen shown in Figure 1.

Figure 1

Basic 3D Math Before I can improve upon the first example, I must cover some 3D math. Please stay with me — I promise this won’t be like that college calculus class with the evil professor who lived to fail students!

Right-Hand Rule The coordinate system used in XNA 3D follows the Right-Hand Rule. This is a simple mnemonic to remember which direction is positive in 3D space. To use this mnemonic, hold your right hand so that you are looking at your palm. Make a fist and extend your thumb to the right, index finger up, and middle finger straight so that it is pointing directly at you. Your fingers are now pointing in the positive directions of each axis.

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XNA 3D Primer Another way to remember this (one that I prefer) is to look at the screen and raise your right hand. Your right hand is the positive direction for the X-, Y-, and Z-axes. X values increase as you move to the right, Y values increase as you move up, and Z values increase as you move toward the front of the screen. For the positive rotation direction around an axis, picture yourself standing behind the screen. With your right hand, reach out and grab an axis. The direction your fingers wrap around the axis is the positive direction when rotating around that axis.

Working with Matrices At the heart of 3D math is the matrix. A matrix can be thought of as a machine on a conveyer belt, processing vertices. Each matrix is programmed to perform a transformation to a vertex, and can be combined with other matrices to perform multiple transformations at once. Strictly speaking, you are using 4×4 matrices; each matrix is a table for values with four columns and four rows. XNA provides several static methods on the Matrix structure for creating matrices, so I won’t get into the details of how to perform matrix math manually. Some of the common methods used to create matrices are in the following table:

Matrix Static Method

Notes

Matrix.CreateScale

Changes the scale (size)

Matrix.CreateRotationX

Rotation around an axis

Matrix.CreateRotationY Matrix.CreateRotationZ Matrix.CreateFromAxisAngle Matrix.CreateFromQuaternion Matrix.CreateTranslation

Translates (moves) the given amount

Matrix.CreateLookAt

Used to create a View matrix

Matrix.CreatePerspective

Used to create a Perspective matrix

Matrix.CreatePerspectiveFieldOfView Matrix.CreatePerspectiveOffCenter Matrix.CreateOrthographic Matrix.CreateOrthographicOffCenter Matrix.CreateWorld

Used to create a World matrix

In addition to these, there are methods for matrix math and specialized methods such as Matrix.CreateShadow, which will flatten your mesh to a Plane given a light direction. All of these

methods (and all other similar methods in the XNA Framework) assume that (0, 0, 0) is the X, Y, Z point from which we are referencing. When I say to translate (move) to (1, 2, 4), I am really saying “from (0, 0, 0)

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XNA 3D Primer move to (1, 2, 4).” Because of this, there is a general rule you should follow when combining matrices called ISROT (Identity, Scale, Rotate, Orbit, Translate), which is discussed later.

Decimals, Doubles, and Floats, Oh My! You may have noticed I’ve been using floats instead of the more common .NET decimal type. Decimals are added, subtracted, multiplied, divided, and so on in software. The reason for this is what is known as the inaccuracies of floats. At the CPU level, everything is binary, which is fine for integers, but there are problems converting floating point numbers to binary format — 1.0 might become 1.00000001 or 0.9999999. In the world of 3D graphics programming, this is OK. In the world of business applications, this is not acceptable. Imagine telling someone their account is off by a few pennies because of a computer conversion! So why is 3D programming OK with these missing pennies? The short answer is “performance.” Floats and doubles perform calculations on the CPU itself in a single step, whereas decimals are done in multiple steps: one step for the whole number and one step for the fractional number, then combining the result in a final step. Given the number of calculations in a 3D game running at 60 frames per second, the performance difference is significant. Avoid decimals unless absolutely needed. When writing code, a float can be used where a double is required. A double-to-float conversion requires a case and includes a loss of precision. In general, this loss isn’t significant, but avoid conversions if possible. One other area to be aware of is testing the values of floats and doubles. The following code may never be called since the value in myFloat will have lost some accuracy! if(myFloat == 1.5f) // do something

Consider using >= and