The root joint is the ultimate joint in the model. Every other joint eventually finds its way back to this one joint. Any operations done on the root joint, whether they be translation or rotation, affect every vertex in the model. You can think of the root joint as the joint that controls all other joints. By simply modifying the root joint, you could make the character walk upright, or you could rotate him so he is upside down and let him walk on the ceiling—all with a single call.

There is only one root joint in each model, and it has no parent joint.

The root joint is generally located in a place where many bones come together, yet in a place where little animation is required. Examples of this include the middle and lower back. But there is nothing that dictates exactly where the root joint is to be located in a model; it could be different for each model if you desire. Figure 5.4 shows what would happen to a model if you modified the position and orientation of the root joint.