R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

56 Medical modelling
Some of these routines also operate in three dimensions in order to create
complex curved surfaces called ‘patches’. An object may require a number
of patches to cover the whole object surface. The patches differ according
to the complexity of the mathematical curve routine used.
This kind of surface modelling produces highly sophisticated surface
models that are typically used in the automotive, motor sports and aerospace
industries. Usually, objects are designed using surface modelling
packages. However, surface patches are also often used in reverse engineering
to create useful CAD geometry from digitised physical objects.
4.6.6 IGES surfaces
As stated before, IGES is an international standard that describes computer-aided
design data as mathematically defi ned geometries positioned
in the three-dimensional space (see http://www.nist.gov/iges/). By converting
data from the original source through an intermediary three-dimensional
format, such as the STL fi le, some CAD packages may be able to
generate IGES curves and surfaces based on the original data. The nature
of the surface and the degree to which it accurately reproduces the original
anatomy will depend greatly on the data formats and CAD packages used.
Some CAD packages also enable the user to create IGES curves and surfaces
from point cloud data obtained using touch probe or non-contact
surface scanners.
IGES curves and surfaces are mathematically described and are normally
relatively smooth and simple surfaces. As such, they are typically used to
defi ne the smooth but accurate surfaces of objects in product, aerospace
and automotive design. This makes them less well suited to the highly
complex surfaces of human anatomy. However, there are many cases where
it may prove to be a useful approach, particularly when attempting to integrate
human anatomy with the design of products that must accommodate
or fi t around people.
The curves and surfaces are created by positioning the control points and
boundary lines that defi ne the surface patch onto the surface of the source
data. The surface patch itself is then mathematically created according to
the type of surface the software uses. The most complex type of surface
patch is defi ned by NURBS surfaces (non-uniform rational B-spline). The
degree to which this kind of surface patch matches the source data can be
controlled by altering the number of control points in the surface. More
control points enable the surface patch to be more complex and therefore
follow the original data more closely. The number of control points is normally
set by the user as a variable when creating the patch.
The effect of altering the number of control points can be seen in
Fig. 4.22, which shows an IGES surface patch created from non-contact