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headingVec = Transformation.Matrix.Forward; strafeVec = Transformation.Matrix.Right; upVec = Transformation.Matrix.Up; }

In the Update method, you first update the unit s animated model, passing the elapsed time since the last update and a parent matrix used to transform the animated model. Because there is no need to transform the animated model, you can pass the identity matrix to update it. After that, you update the unit s linear and angular velocity. If the unit s linearVelocity or gravityVelocity is not zero, the unit is moving and you need to call the UpdateHeight method to assure that the unit is correctly positioned over the terrain. You also need to set the needUpdateCollision flag to true, to update the position of the unit s collision volumes. Last, if the unit s angularVelocity is not zero, you call the NormalizeBaseVectors method to update its orientation vectors (heading, strafe, and up vectors). You can extract these vectors from the transformation matrix of the unit s animated model.

You can check the collision between the scene objects using some different approaches. An accurate approach would check the intersection between two objects using its mesh, which is composed of many triangles. This method is the most accurate one, but it is also the least efficient one. For example, to test the collision between two meshes having 2,000 triangles each, you would need to make 2000 * 2000 collision tests. Instead of testing the collision using the mesh of the objects, you can use collision volumes. Collision volumes provide a faster, although more inaccurate, way of checking the intersection between objects. In your game, you ll use two different collision volumes for each unit a box and a sphere to check its collision against other objects. When the collision volume is a box, it s called a bounding box, whereas when the volume is a sphere, it s called a bounding sphere. You can build the box you ll use for the collision aligned to the world axes. In this case, the box is called an axis-aligned bounding box (AABB). One of the advantages of the AABB is that the collision test with it is simple. However, the AABB can t be rotated because it needs to keep its axes aligned with the world s axes. If the box used for collision is oriented with the unit s axes, it s then called an object oriented bounding box (OOBB). A collision test using an OOBB is slower than one using an AABB, but the OOBB provides a box that is always oriented with the unit. Figure 12-5 illustrates the creation of an AABB and an OOBB for a unit with two different orientations.

Figure 12-5. Creating an AABB and an OOBB for a model. (Left) The AABB and the OOBB are the same when the model has the same orientation as the world. (Middle) The AABB created for the new model orientation. (Right) The OOBB created for the new model orientation.

Because XNA already has a class to handle an AABB, you ll use it as the box volume for the unit. So, each unit will have an AABB and a bounding sphere volume, represented using XNA s BoundingBox and BoundingSphere classes. The default model processor of the Content Pipeline generates a bounding sphere volume for each mesh present in a model that is processed. In this way, you have a bounding sphere for each model s mesh. You can avoid testing the collision with each mesh of the model, creating a bounding sphere for the entire model. Also, because the default model processor doesn t generate a bounding box (AABB) volume, you need to generate one for the model. You can create the bounding box and bounding sphere for the unit by modifying its model processor, which is the AnimatedModelProcessor class created in 11. First, open the AnimatedModelProcessor class, which is inside the AnimatedModelProcessorWin project. Then, create a method named GetModelVertices to extract all the vertices of the model s meshes. You ll use these vertices to create the collision volumes of the model, through the CreateFromPoints method of XNA s BoundingBox and BoundingSphere classes. The CreateFromPoints method creates a volume for the model from its vertices. Following is the code for the GetModelVertices method:

private void GetModelVertices(NodeContent node, List<Vector3> vertexList) { MeshContent meshContent = node as MeshContent; if (meshContent != null) { for (int i = 0; i < meshContent.Geometry.Count; i++) { GeometryContent geometryContent = meshContent.Geometry[i]; for (int j = 0; j < geometryContent.Vertices.Positions.Count; j++)