Modeling and Animating Human Figures - Computer Animation: Algorithms and Techniques - page 287

skeleton:
shoulder, elbow, forearm, wrist

Arm Example
Kinematic Muscle
Deformation

ball - hinge - hinge - universal

muscle: bicep, elbow, forearm geometric skin

flexor - tendon - flexor

smooth bend at elbow

smooth continuity across
sdjoining muscles

crease forms at elbow

FIGURE 9.2
Deformation induced by FFDs as a result of joint articulation [ 13 ] .

sheet muscles, and bendable sheet muscles. Tendons are modeled as part of the muscles and attach to
the skeleton. Muscles deform according to the articulation of the skeleton. See Figure 9.3 for an exam-
ple. These muscles populate the skeletal figure in the same manner that actual muscles are arranged in
the human body ( Figure 9.4 ). To deform the skin according to the underlying structure (muscles, ten-
dons, and fatty tissue), the user defines implicit functions so that the densities occupy the volume of the
corresponding anatomy. Ellipsoids are used for muscles, cylinders for tendons, and flattened ellipsoids
for fat pads. The implicit primitives are summed to smooth the surface and further model underlying
tissue. The skin, modeled as a B-spline surface, is defined by floating the control points of the B-spline
patches to the isosurface of the summed implicit primitives. This allows the skin to distort as the under-
lying skeletal structure articulates and muscles deform ( Figure 9.5 ) .