A tank model is loaded through the Content Pipeline, and then rendered using a program-controlled animation. The wheels turn, the turret swivels back and forth, and the hatch opens and closes.

This sample begins by loading a tank model. The tank is built of a hierarchy of separate meshes. The wheels, the turret, the hatch, and so on, are all separate meshes. Inside the 3D modeling package, each mesh has a descriptive name. Each is attached to the appropriate parent mesh with a suitable pivot point. For example, the cannon mesh can tilt up and down. It is attached to the turret mesh, which can also swivel from left to right, and is itself attached to the body of the tank. Because of this hierarchy, when you rotate the turret, the cannon rotates along with it.

In the XNA Framework, the Model class represents the whole model. The Model contains a ModelMesh for each separate mesh in the model. Each ModelMesh contains a ParentBone (the pivot point mentioned earlier), which controls the mesh's position and orientation relative to the model. The Model has a Root bone, which determines the model's position and orientation. Every ModelBone can have one parent and many children. The Root bone on the Model object is the ultimate parent; its children are bones on ModelMesh objects, which might have other ModelMesh bones as their children, and so on. In the example above, the body of the tank would have a ParentBone, with a child bone for the turret, which itself has a child bone for the cannon. In any given family of bones, rotating the parent bone also rotates the children, and their children, and so on.

Every bone has a transformation matrix (called Transform) that defines its position and rotation relative to the position of the parent bone. This rotation and translation applies to all the vertices in the ModelMesh (for example, all the vertices that connect to that bone). To animate a bone, you multiply the default bone transform by a new matrix. When you draw the ModelMesh, you then base your world matrix on the bone's transform.

The easiest way to incorporate transformed bones into drawing is to use the CopyAbsoluteBoneTransformsTo method. This method takes the bone transforms, which are relative to each other, and iterates over them to make them relative to the Root bone of the Model. It then returns a copy of these transforms. When you draw each ModelMesh, you can use the absolute bone transform as the first part of your world matrix. This way you won't have to worry about parent bones and their relationships.

The Tank class is a wrapper around the XNA Framework Model type. It exposes properties for each value that can be used to animate the tank. In the Load method, it uses the ContentManager to load the underlying Model data. Then it saves shortcut references to the bones that are going to be animated, and stores their original transform matrices. In the Draw method, it computes new rotation matrices based on new rotation values from the game. These new matrices are assigned to the Transform properties of the animated bones, after which the model is rendered in the usual way by using CopyAbsoluteBoneTransformsTo.

The SimpleAnimation game uses the current time value to update the animation properties on the Tank object. The wheels are rotated, while the other values are moved back and forth in sine curves of varying speeds. In a real game, you would take this data from user inputs or the physics system rather than just making everything oscillates.