Biomedical Engineering Reference
In-Depth Information
a post-processing shape deformation component. Zordan et al. [
3
], in their breath-
ing simulation model, recorded the trajectories of pre-selected points attached to the
model used as control vertices of a NURBS surface. The surface shape is then up-
dated to show skin deformation. However, the surface shape is implicitly defined by
how the control vertices are selected and limited to the captured data. High-quality
rendering of the skin can be considered to be done offline using advanced skin ren-
dering engines like Pixar's RenderMan [
34
] or NVIDIA's Mental Ray [
35
], to enrich
the visualization referenced to real human skin.
6.2.3 Acquisition and Processing of the Human Anatomy
In this section we present the typical pipelines for automatic generation of volumetric
meshes of anatomical structures. We introduce techniques to acquire anatomical
structures from the real-world and to process these structures into a musculoskeletal
model that can be simulated.
6.2.3.1 Artistic Anatomy
In computer animation and graphics applications, most human figure models use a
simplified articulated skeleton consisting of relatively few connected segments. The
bones are constrained by joints which allow them to move relative to one another
within limits. Since the focus is on the capability of articulation, it is usually not
necessary for the skeleton geometry to conform to the real world, even when mov-
able joints are modeled with sets of curves [
36
,
37
]. Nonetheless, subject-specific
and accurate geometries are generally required in biomedical research like surgery
simulation.
In an anatomical human (see Fig.
6.4
), the musculature system is typically more
complex than the skeletal system. Indeed in the human anatomy, muscles are arranged
side-by-side and in layers on top of bones and other muscles. They often spanmultiple
joints and typically consist of different types of tissue, allowing some portions to be
contractile and others not. Depending on their state of contraction, muscles have
different shapes and influence their surface form in different ways.
In an effort to achieve real-time performances, muscles are usually constructed
fromNURBS or spline patch (see Fig.
6.5
). Scheepers et al. [
11
] developed anatomy-
based models of skeletal muscles used to flesh-out a skeleton. They use ellipsoids
to represent muscle bellies and deformation is achieved by updating the volume
when the lengths of the principal axes are adjusted. As a general muscle model, they
construct a bicubic patch mesh by sweeping a varying ellipse along a cubic Bezier
curve, and reach the deformation by manipulating the control points sampled along
the curves. Pratscher et al. [
39
] use elliptical muscle models and a procedural defor-
mation for the muscle simulation in their anatomy-based character rigging system.
