Virtual Character Definition and Animation within the MPEG-4 Standard - 3D Modeling and Animation: Synthesis and Analysis Techniques for the Human Body - page 52

The SBSkinnedModel node is the root used to define one SMS virtual character
and it contains the definition parameters of the entire seamless model or of a
seamless part of the model. Mainly, this node contains the model geometry and
the skeleton hierarchy. The geometry is specified by skinCoord field — (a list
containing the 3D coordinates of all the vertices of the seamless model) and the
skin field — (a collection of shapes which share the same skinCoord) . This
mechanism allows us to consider the model as a continuous mesh and, at the
same time, to attach different attributes (e.g., colour, texture, etc.) to different
parts of the model. The skeleton field contains the root of the bone hierarchy. All
the bones and muscles belonging to the skinned model are contained in dedicated
lists.
Once the skinned model is defined in a static position, the animation is obtained
by updating, at time samples, the geometric transformation of the bones and
muscles. In order to ensure a compact representation of these parameters, the
MPEG-4 standard specifies a dedicated stream, the so-called BBA stream.
Skeleton, Muscle, and Skin Animation Stream
Animation principle and resource representation
To address streamed animation, MPEG-4 considers the animation data indepen-
dent of the model parameters. Thus, the model is transmitted or loaded at the
beginning of the animation session and the animation parameters are sequentially
transmitted at each frame. Two representation techniques for the animation data
format are supported: the first one corresponds to a non-compressed (human
readable) format. This is useful when editing the animation parameters. In this
case, the file format is XMT (Kim, 2000) compliant in order to allow easy editing
and data exchange. This representation is called “SMS textual.” The second
representation is a compressed data format. By using appropriate compression
schemes, low bit-rate animation transmission is performed. This representation
is called “SMS binary.”
Conceptually, both animation data formats use the same representation of the
geometrical transformation parameters. The next sections describe this repre-
sentation.
Animation parameter representation
The key point for ensuring a compact representation of the SMS animation
parameters consists of decomposing the geometric transformations into elemen-