Star Trek -Part II: Adding moving 3D Objects to your XNA Game

Introduction

This is an update of the Star Trek XNA demonstration in which I ported from Visual Studio Express to Visual Studio 2005 and XNA 2.0. It adds a more realistic looking Klingon ship and a rotating 3D Planet. In this article we will discuss how to add 3D Objects and Text to your games.

Meshes

The planet consists of a sphere mesh coated with a 2D Texture. Meshes are a set of vertices in 3D space that describe a 3D Object. You can create meshes using professional tools such as Maya and 3D Studio. You can also create meshes with an open source tool such as blender. Blender takes a bit of skill to learn, but if you follow the tutorials, you'll soon be creating some interesting 3D Meshes. XNA supports the direct-x format (*.x) for meshes so you'll need to export your mesh in this format. Once you have a mesh, you can easily read and manipulate it using the objects in XNA.



Figure 1 Star Trek

Below is the code to read a Mesh into your game:

Listing 1 - Loading a 3D Mesh

ModelMesh _Sphere; // this is the mesh object
protected override void LoadGraphicsContent(bool loadAllContent)
{
if (loadAllContent)
{
// this line loads the Mesh from a direct x file (with the .x extension)
_Sphere = content.Load<Model>((@"Content\Textures\Sphere1")).
Meshes[0];
}
}

Once we have a mesh we need to position the mesh, scale the mesh, and add a texture to the mesh. All of these actions are accomplished by changing the Effects of the mesh. Each basic effect of the Effects collection has properties such as world coordinates, lighting, and textures to allow us to manipulate the mesh structure. Listing 2 shows the method for positioning our sphere on the screen and adding a bitmap texture to its surface. All transforms are performed using a 3D Matrix. XNA gives us a Matrix class that makes it simple to create our transforms. The scale matrix is created using the CreateScale method with a single scalar value relative to the game surface. A value of 1.0 indicates the entire game surface. The scale matrix is then multiplied by the World coordinates of the Effect to get the appropriate scaling of our 3D object. The sphere is moved around the screen using a translation matrix created from the CreateTranslation method using a 3D offset vector. The translation is performed in 3D space relative to View coordinates. A coordinate of (1, 0, 0) positions the planet where we want it, to the far right and centered vertically.

Textures are also assigned to the Effect through the Texture property. The 2D texture that was loaded in the LoadGraphicsContent method is assigned to an effect to give us a texture on the surface of our mesh.

Listing 2 - Changing the Effects of the Mesh

private void SetDefaultMeshEffects(ModelMesh mesh, float scale, Vector3 position, Texture2D texture) { foreach (BasicEffect effect in mesh.Effects) { effect.EnableDefaultLighting(); // set default lighting effect.World = effect.World * Matrix.CreateScale(scale); // scale the world coordinates effect.Texture = texture; effect.View = Matrix.CreateTranslation(position); effect.TextureEnabled = true; } }

Rotating the Planet

The planet is rotated by changing transforming the rotation of the sphere every time it enters the Update method of the Game object. Again, the effect of the planet mesh is multiplied by a transformation matrix to rotate the sphere. In this example we rotate the sphere .02 radians around the X and the Z axises.

Listing 3 - Rotating the Planet Every Time an Update is Called

/// <summary> /// This is called from the Update Method /// </summary> private void RotatePlanet() { // go through each effect in the sphere and rotate it's // world coordinates .02 radians around the X and Z // Axises foreach (BasicEffect effect in _Sphere.Effects) { effect.EnableDefaultLighting(); effect.World = effect.World * Matrix.CreateRotationX(-.02f) * Matrix.CreateRotationZ(-.02f); } }

Joystick Movement

XNA makes controlling an XBox joystick a snap with its built in GamePad class. By just reading the state of the pad, you can get button information and thumbstick positions with a few easy to use static methods. The method UpdateFromGamePad shown in listing 4 is called from Update in the game class. In keeping with the keypad control, we decided to use the left thumbstick to move forward and backward and the right thumbstick to rotate clockwise and counterclockwise. What is nice about the joystick classes in XNA is the code is almost self-explanatory. You can get the state of a particular button and see if it was pressed or released, or you can get the state of a thumbstick (Left or Right) and see its X and Y position vector. In listing 4 we round the left thumbstick's X vector positions in order to determine if we are moving forward or backward. A value of 1 indicates forward and a value of -1 indicates backward. We use the same technique for rotation. We round the right thumbstick's X vector positions in order to determine if we are rotating clockwise or counterclockwise. A value of 1 indicates clockwise and a value of -1 indicates counter clockwise.

Listing 4 - Reading the Joystick State

Drawing Fonts