Modeling and Animating Human Figures - Computer Animation: Algorithms and Techniques - page 283

Table 9.1 Selected Terms from Anatomy
Sagittal plane

Perpendicular to the ground and divides the body into right and left halves

Coronal plane Perpendicular to the ground and divides the body into front and back halves

Transverse plane Parallel to the ground and divides the body into top and bottom halves

Distal Away from the attachment of the limb

Proximal Toward the attachment of the limb

Flexion Movement of the joint that decreases the angle between two bones

Extension Movement of the joint that increases the angle between two bones

discussion in this section focuses solely on the representation of virtual humans. It addresses animation
issues where necessary to discuss how the animation techniques affect the figure's creation.

9.1.1 Representing body geometry
Many methods have been developed for creating and representing the geometry of a virtual human's
body. They vary primarily in visual quality and computational complexity. Usually these two measures
are inversely proportional.
The vast majority of human figures are modeled using a boundary representation constructed from
either polygons (often triangles) or patches (usually nonuniform rational B-splines; NURBS). These
boundary shell models are usually modeled manually in one of the common off-the-shelf modeling
packages (e.g., [ 1 ] [ 4 ] [ 35 ]). The purpose of the model being produced dictates the technique used
to create it. If the figure is constructed for real-time display in a game on a low-end PC or gaming
console, usually it will be assembled from a relatively low number of triangular polygons, which, while
giving a chunky appearance to the model, can be rendered quickly. If the figure will be used in an ani-
mation that will be rendered off-line by a high-end rendering package, it might be modeled with
NURBS patch data, to obtain smooth curved contours. Factors such as viewing distance and the impor-
tance of the figure to the scene can be used to select from various levels of detail at which to model the
figure for a particular sequence of frames.

Polygonal representations
Polygonal models typically consist of a set of vertices and a set of faces. Polygonal human figures can
be constructed out of multiple objects (frequently referred to as segments), or they can consist of a
single polygonal mesh. When multiple objects are used, they are generally arranged in a hierarchy
of joints and rigid segments. Rotating a joint rotates all of that joint's children (e.g., rotating a hip joint
rotates all of the child's leg segments and joints around the hip). If a single mesh is used, then rotating a
joint must deform the vertices surrounding that joint, as well as rotate the vertices in the affected limb.
Various constraints may be placed on the polygonal mesh's topology depending on the software
that will be displaying the human. Many real-time rendering engines require polygonal figures to
be constructed from triangles. Some modeling programs require that the object remain closed.
Polygonal representations are primarily used either when rendering speed is of the essence, as is the
case in real-time systems such as games, or when topological flexibility is required. The primary prob-
lem with using polygons as a modeling primitive is that it takes far too many of them to represent a