Advances in Vision-Based Human Body Modeling - 3D Modeling and Animation: Synthesis and Analysis Techniques for the Human Body - page 8

a maximum 3-D displacement for each vertex. An application may uniformly
scale these displacements before applying them to the corresponding vertices.
For example, this field is used to implement Facial Definition and Animation
Parameters of the MPEG-4 standard (FDP/FAP).
Finally, the H-anim 2001 standard does not introduce any major changes, e.g.,
new nodes, but provides better support of deformation engines and animation
tools. Additional fields are provided in the Humanoid and the Joint nodes to
support continuous mesh avatars and a more general context-free grammar is
used to describe the standard (instead of pure VRML97, which is used in the two
older H-anim standards). More specifically, a skeletal hierarchy can be defined
for each H-anim humanoid figure within a Skeleton field of the Humanoid node.
Then, an H-anim humanoid figure can be defined as a continuous piece of
geometry, within a Skin field of the Humanoid node, instead of a set of discrete
segments (corresponding to each body part), as in the previous versions. This
Skin field contains an indexed face set (coordinates, topology and normals of skin
nodes). Each Joint node also contains a SkinCoordWeight field, i.e., a list of
floating point values, which describes the amount of “weighting” that should be
used to affect a particular vertex from a SkinCoord field of the Humanoid node.
Each item in this list has a corresponding index value in the SkinCoordIndex
field of the Joint node, which indicates exactly which coordinate is to be
influenced.

Face and Body Animation in the MPEG-4 Standard

The MPEG-4 SNHC (Synthetic and Natural Hybrid Coding) group has standard-
ized two types of streams in order to animate avatars:

•

The Face/Body Definition Parameters (FDP/BDP) are avatar-specific
and based on the H-anim specifications. More precisely the MPEG-4 BDP
Node contains the H-anim Humanoid Node.

• The Face/Body Animation Parameters (FAP/BAP) are used to animate

face/body models. More specifically, 168 Body Animation Parameters
(BAPs) are defined by MPEG-4 SNHC to describe almost any possible
body posture. A single set of FAPs/BAPs can be used to describe the face/
body posture of different avatars. MPEG-4 has also standardized the
compressed form of the resulting animation stream using two techniques:
DCT-based or prediction-based. Typical bit-rates for these compressed
bit-streams are two kbps for the case of facial animation or 10 to 30 kbps
for the case of body animation.