Game Development Reference
In-Depth Information
Skeleton, Muscle and Skin Framework
Bone and muscle controllers for animating an articulated object
An articulated virtual character and, generally, an articulated synthetic object,
also called kinematics linkage, is composed of a set of rigid links which are
connected at the joints. In the case of a seamless virtual character, a rigid link
is associated with each anatomical bone of the skeleton.
In order to define the static 3D posture of an articulated virtual character,
including geometry, colour and texture attributes, the approach here proposed
consists of considering the entire virtual character as a single 3D mesh referred
to as skin.
During the animation stage, the anatomical bones can only be affected by rigid
transformations and cannot be locally deformed. Nevertheless, realistic anima-
tions can be obtained by local skin deformations for simulating muscular activity
effects. In order to fulfil this requirement, curve-based entities are attached at
an arbitrary level of the skeleton.
Two issues are addressed by the SMS framework. The first one deals with the
definition of the skinned model as a static model described by its geometry and
its appearance. In order to perform this task, a hierarchical skeleton and a
collection of muscles are introduced. The second issue deals with the animation
of articulated models together with a compressed representation of the animation
parameters.
Defining a SMS virtual character requires us to specify a set of static attributes
(geometry, texture, etc.), as well as a deformation model. From a geometric point
of view, an SMS synthetic model is represented in such a way that the set of
vertices which belong to the skin of the virtual character is specified as a unique
list. In order to define various appearances at different levels of the SMS virtual
character, distinct shapes can be concatenated, provided that all of the shapes
composing the skin share the same list of vertices. This type of representation
avoids getting seams on the skin during the animation stage, while preserving the
possibility to define various sets of colour, texture and material attributes at
different levels of the skin.
The animation behaviour of a skinned model is defined by means of skeleton and
muscle layers together with their properties. The skeleton is a hierarchical
structure built from bones. A bone is defined as a segment of length
l
by means
of: (1) a geometric transformation of the bone, defined with respect to its parent
in the skeleton hierarchy; (2) a model of influence of the bone movement on the
surface of the articulated model; and (3) inverse kinematics constraints. A
muscle layer is a collection of individual muscles, each one being attached to a
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