Virtual Character Definition and Animation within the MPEG-4 Standard - 3D Modeling and Animation: Synthesis and Analysis Techniques for the Human Body - page 30

the scene, streamed animations, integration of 2D objects and advanced time
control.
In terms of functionalities related to virtual characters, both VRML and MPEG-4
standards define a set of nodes in the scene graph to allow for a representation
of an avatar. However, only the MPEG-4 SNHC specifications deal with
streamed avatar animations. A major difference is that an MPEG-4 compliant
avatar can coexist in a hybrid environment and its animation can be natively
synchronized with other types of media objects, while the H-Anim avatar can
only exist in a VRML world and must be animated by VRML generic, usually
non-compressed, animation tools.
Now that the reasons of virtual character standardization within MPEG-4
become clearer, the question is how to find the good compromise between the
need for freedom in content creation and the need for interoperability? What
exactly should be standardized, fixed, invariant and in the meantime, ideally
impose no constraints on the designer creativity? The long-term experience that
the MPEG community has makes it possible to formulate a straight and solid
resolution: in the complex chain of content producing-transmitting-consuming,
the interoperability is ensured by only standardizing the data representation
format at the decoder side. Pushing this concept to its extreme, an MPEG ideal
tool is that one for which two requirements are satisfied: the designer can use any
production tool he/she possesses to create the content and it can be possible to
build a full conversion/mapping tool between this content and an MPEG
compliant one. The same principle has been followed when MPEG released the
specifications concerning the definition and the animation of the virtual charac-
ters, and specifically human avatars: there are no “limits” on the complexity of
the avatar with respect to its geometry or appearance and no constraints on the
motion capabilities.
The animation method of a synthetic object is strongly related to its definition
model. A simple approach, often used in cartoons, is to consider the virtual
character as a hierarchical collection of rigid geometric objects called segments,
and to obtain the animation by transforming these objects with respect to their
direct parents. The second method consists in considering the geometry of the
virtual character as a unique mesh and to animate it by continuously deforming
its shape. While the former offers low animation complexity, with the price of the
seams at the joints between the segments, the latter ensures a higher realism of
the representation, but requires more computation. Both modeling/animation
methods are supported by the MPEG-4 standard, as will be extensively shown
in this chapter. Its structure is as follows. The first section presents the tools
adopted in the MPEG-4 standard related to the specification and encoding of the
synthetic object's geometry in general. Specifically, techniques based on
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