Because of demands for rapid feedback and the limitations ofpresent-day technology, human figures are often represented withstick figures, curves, or simple geometric primitives. This approachsacrifices realism of representation for display efficiency. Recently,a layered approach to the representation of human figures has beenadopted in which skeletons support one or morelayers, typically muscle, fatty tissue, skin, and clothing layers. Theadditional layers serve to flesh-out the skeleton and to enhance therealism of the representation.

Most human figure models use a simplified articulated skeletonconsisting of relatively few jointed segments. Magnenat-Thalmannand Thalmann challenged researchers to develop more accuratearticulated models for the skeletal support of human figures.They observe that complex motion control algorithms which havebeen developed for primitive articulated models better suit robotlikecharacters than they do human figures. To address this issue,researchers have revisited the skeletal layer of human figure modelsto solve some specific problems. In Jack, the shoulder ismodeled accurately as a clavicle and shoulder pair. The spatial relationshipbetween the clavicle and shoulder is adjusted based onthe position and orientation of the upper arm. In another treatmentof the shoulder-arm complex, the Thalmanns use a movingjoint based on lengthening the clavicle which produces good results.Monheit and Badler developed a kinematic model of thehuman spine that improves on the realism with which the torso canbe bent or twisted. Scheepers et al. developed a skeleton modelwhich supports anatomically accurate pronation and supination ofthe two forearm bones. Gourret et al. use realistic bones in theirhand skeleton to assist in producing appropriate deformations of thefingers in a grasping task.
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