Graphics Reference
In-Depth Information
FIGURE 4.27
Deformation by translating the deformation tool relative to an object.
Chadwick et al. [ 8 ] described a layered approach to animation in which FFDs are used to animate a
human form. The FFDs are animated in two ways. In the first technique, the positions of the FFD grid
vertices are located relative to a wire skeleton the animator uses to move a figure. As the skeleton is
manipulated, the grid vertices are repositioned relative to the skeleton automatically. The skin of the
figure is then located relative to this local FFD coordinate grid. The FFDs thus play the role of muscular
deformation of the skin. Joint articulation modifies the FFD grid, which in turn modifies the surface of
the figure. The FFD grid is a mechanism external to the skin that effects the muscle deformation. The
“muscles” in this case are meant not to be anatomical representations of real muscles but to provide for
a more artistic style.
As a simple example, a hinge joint with adjacent links is shown in Figure 4.30 ; this is the object to be
manipulated by the animator by specifying a joint angle. There is a surface associated with this struc-
ture that is intended to represent the skin. There are three FFDs: one for each of the two links and one for
the joint. The FFDs associated with the links will deform the skin according to a stylized muscle, and
the purpose of the FFD associated with the joint is to prevent interpenetration of the skin surface in
highly bent configurations. As the joint bends, the central points of the link FFDs will displace upward
and the interior panels of the joint FFD will rotate toward each other at the concave end in order to
squeeze the skin together without letting it penetrate itself. Each of the grids is 5
4, and the joint grid
is shown using dotted lines so that the three grids can be distinguished. Notice that the grids share a
common set of control points where they meet.
Moving the FFD lattice points based on joint angle is strictly a kinematic method. The second tech-
nique employed by Chadwick [ 8 ] uses physically based animation of the FFD lattice points to animate
 
Search WWH ::




Custom Search