R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Case studies 185
back as shown in Fig. 6.51. In addition, the simulated physical properties
of the virtual clay can be altered to represent differing hardness and tack
strength. When using the system, the user can feel tactile resistance when
removing material, and when they reach the surface of the skull resistance
is total. Therefore, the approach is a digital equivalent of waxing up on a
medical model.
However, operating in the digital domain enables some useful techniques
to be exploited. In this case, control curves were created on the surface of
the plate. These curves can then be moved using control points on the
surface or tangents to the surface. A single control curve applied to the
sagittal plane is shown in Fig. 6.52. Many control curves can be added in
various planes to achieve a very smooth and well-defi ned natural curvature
that matches the surrounding tissue. In addition, whole areas can be selected
by painting on the model with the stylus. Then various operations can be
performed on the entire selected area in a single operation. For example,
the area could be pulled out to create a bulge or pushed in to create a
depression.
The material removal and smoothing process was then repeated for the
inner surface illustrated in Fig. 6.53. Then the buck (skull) is subtracted
6.51 Working the material back.