Skeletal Animation - Technique

Technique

This technique is used by constructing a series of 'bones,' sometimes referred to as rigging. Each bone has a three dimensional transformation (which includes its position, scale and orientation), and an optional parent bone. The bones therefore form a hierarchy. The full transform of a child node is the product of its parent transform and its own transform. So moving a thigh-bone will move the lower leg too. As the character is animated, the bones change their transformation over time, under the influence of some animation controller. A rig is generally composed of both forward kinematics and inverse kinematics parts that may interact with each other. Skeletal animation is referring to the forward kinematics part of the rig, where a complete set of bones configurations identifies a unique pose.

Each bone in the skeleton is associated with some portion of the character's visual representation. Skinning is the process of creating this association. In the most common case of a polygonal mesh character, the bone is associated with a group of vertices; for example, in a model of a human being, the 'thigh' bone would be associated with the vertices making up the polygons in the model's thigh. Portions of the character's skin can normally be associated with multiple bones, each one having a scaling factors called vertex weights, or blend weights. The movement of skin near the joints of two bones, can therefore be influenced by both bones. In most state-of-the-art graphical engines, the skinning process is done on the GPU thanks to a shader program.

For a polygonal mesh, each vertex can have a blend weight for each bone. To calculate the final position of the vertex, each bone transformation is applied to the vertex position, scaled by its corresponding weight. This algorithm is called matrix palette skinning, because the set of bone transformations (stored as transform matrices) form a palette for the skin vertex to choose from.