Sappa, Aifanti, Grammalidis & Malassiotis

each arm and each leg. The illustration presented in Figure 1 (left) corresponds
to an articulated model defined by 22 DOF.
On the contrary, in computer graphics, highly accurate representations consisting
of more than 50 DOF are generally selected. Aubel, Boulic & Thalmann
(2000) propose an articulated structure composed of 68 DOF. They correspond
to the real human joints, plus a few global mobility nodes that are used to orient
and position the virtual human in the world.
The simplest 3D articulated structure is a stick representation with no associated
volume or surface (Figure 1 (left)). Planar 2D representations, such as the
cardboard model, have also been widely used (Figure 1 (right)). However,
volumetric representations are preferred in order to generate more realistic
models (Figure 2). Different volumetric approaches have been proposed,
depending upon whether the application is in the computer vision or the computer
graphics field. On one hand, in computer vision, where the model is not the
purpose, but the means to recover the 3D world, there is a trade-off between
accuracy of representation and complexity. The utilized models should be quite
realistic, but they should have a low number of parameters in order to be
processed in real-time. Volumetric representations such as parallelepipeds