R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

58 Medical modelling
scan data of a human hip with control points varying from 6 to 100. The
areas of darker grey show where the IGES surface patch closely approximates
the scan surface whilst the lighter areas show that the IGES surface
patch is below the scanned surface. The IGES surface patch is also shown
as a mesh showing the complexity of the surface as the number of control
points increases.
Depending on the software being used, the quality of the fi t between the
IGES surface and the original data can be visually inspected on screen as
shown in Fig. 4.22 or numerically quantifi ed. For the surface with only six
control points, the average gap between original surface and the IGES
patch is 0.818 mm. In comparison, the surface shown with 100 control points
is a much closer fi t with the average gap almost negligible at 0.00678 mm.
As NURBS surfaces are controlled by mathematical equations, the
patches themselves have to obey certain criteria in order for the equations
to solve. Failure to obey these constraints will result in no surface patch
being generated or a patch that fl ips, creases or twists. Typically, surfaces
that exhibit these faults cannot be physically manufactured or they give rise
to other problems.
To create solid models from surface patches, all of the surface area of
the object must be covered by NURBS patches. In addition, the patches
must meet at common edges and not overlap. These surfaces can then be
‘stitched’ together to form a true three-dimensional solid computer model
of the object.