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A human body is represented by an articulated object model. The model consists of 15 nodes. Each node corresponds to a part of human body. The nodes are connected to each other in a tree graph so that one node includes one joint that has at most 3 rotational axes. Joint rotational angles of the node i at time t are expressed by a vector . The subscript k means the rotational axis of the joint rotational angle. The minimum and maximum value of are defined so that a human model prohibit unrealistic motion.
The motion is said to be estimated if the angles of all the joints are calculated for every frame in an image sequence.