Skeletons

• Given a model with large number of vertices, vertices can be grouped and manipulated as a unit.

  Example: in an arm, all points describing the forearm move more-or-less as a rigid body.

• Connectivity between groups can be represented by tying each group to a ``bone'' and arranging bones in an articulated ``skeleton''.
• The bone and all its associated vertices are treated as a single rigid object for the purposes of transformation.
• Movement of the bones is constrained by the joints in the skeleton.
• Different kinds of joints (revolute, hinge, ball) can support specific types of motion.
• An animation can be previewed using only the bones, speeding rendering time.
• In the final rendering, the model is shown without bones.
• Enhancement: vertices may be influenced by more than one bone.

  Results in more flexible and realistic surface near joints.

  Example: a kneecap and its skin surface affected by both the femur and shinbone, and assume a position halfway between either.