Face and Body Animation in the MPEG-4 Standard

The MPEG-4 SNHC (Synthetic and Natural Hybrid Coding) group has standardized
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.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 describes
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.