R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

46 Medical modelling
faceted meshes can be easily stored in the STL fi le format. Applying other
shapes of polygon mesh, such as square or hexagonal, may be more appropriate,
particularly for use in fi nite element analysis techniques.
Often surface scan data will extend beyond the principal area of interest
and unnecessary points should be removed immediately. This reduces the
memory requirement for any subsequent operations. Once a surface has
been generated from the available points, any gaps will become apparent.
At this point patches can be created to fi ll these gaps to form a single coherent
surface.
Surfaces that are more sophisticated can be generated on the point clouds
by applying mathematically defi ned curves to points and creating surfaces
from them. This is typically done in reverse engineering, enabling the
surface to be taken into traditional engineering CAD systems. This requires
a great deal of skill and expensive software and is only necessary when it
is desirable to perform precise changes to the surfaces. If the ultimate aim
is to produce a physical model by RP techniques then this step is redundant,
as the data will only have to be converted into the STL format again.
Whichever route is taken, if a physical model is to be made, the surface
has to be turned into a single bound volume. This can be achieved by
offsetting the surface and fi lling the gap, resulting in a model with a de -
fi nite thickness, as can be seen in Fig. 4.12, which shows two views of a
three-dimensional model created from scan data of a face. The oblique
view clearly shows the thickness of the model. To reduce fi le size the
rear surface may be simplifi ed as it has no purpose other than to give the
model bulk.
4.12 A bound volume STL fi le created from an offset surface.