Advances in Vision-Based Human Body Modeling - 3D Modeling and Animation: Synthesis and Analysis Techniques for the Human Body - page 9

In addition, complex 3D deformations that can result from the movement of
specific body parts (e.g., muscle contraction, clothing folds, etc.) can be modeled
by using Face/Body Animation Tables (FAT/BATs). These tables specify a set
of vertices that undergo non-rigid motion and a function to describe this motion
with respect to the values of specific FAPs/BAPs. However, a significant
problem with using FAT/BAT Tables is that they are body model-dependent and
require a complex modeling stage. On the other hand, BATs can prevent
undesired body animation effects, such as broken meshes between two linked
segments. In order to solve such problems, MPEG-4 addresses new animation
functionalities in the framework of AFX group (a preliminary specification has
been released in January 2002) by including also a generic seamless virtual model
definition and bone-based animation. Particularly, the AFX specification de-
scribes state of the art components for rendering geometry, textures, volumes
and animation. A hierarchy of geometry, modeling, physics and biomechanical
models are described along with advanced tools for animating these models.
AFX Extensions for Humanoid Animation
The new Humanoid Animation Framework, defined by MPEG-4 SNHC (Preda,
2002; Preda & Prêteux, 2001) is defined as a biomechanical model in AFX and
is based on a rigid skeleton. The skeleton consists of bones, which are rigid
objects that can be transformed (rotated around specific joints), but not de-
formed. Attached to the skeleton, a skin model is defined, which smoothly
follows any skeleton movement.
More specifically, defining a skinned model involves specifying its static and
dynamic (animation) properties. From a geometric point of view, a skinned model
consists of a single list of vertices, connected as an indexed face set. All the
shapes, which form the skin, share the same list of vertices, thus avoiding seams
at the skin level during animation. However, each skin facet can contain its own
set of color, texture and material attributes.
The dynamic properties of a skinned model are defined by means of a skeleton
and its properties. The skeleton is a hierarchical structure constructed from
bones, each having an influence on the skin surface. When bone position or
orientation changes, e.g., by applying a set of Body Animation Parameters,
specific skin vertices are affected. For each bone, the list of vertices affected
by the bone motion and corresponding weight values are provided. The weighting
factors can be specified either explicitly for each vertex or more compactly by
defining two influence regions (inner and outer) around the bone. The new
position of each vertex is calculated by taking into account the influence of each
bone, with the corresponding weight factor. BAPs are now applied to bone nodes