R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

54 Medical modelling
4.20 The effect of deviation settings: (left) large deviation = small fi le
and (right) small deviation = large fi le.
When creating STL fi les from medical scan data, a similar effect can be
achieved. Typically, rather than setting a maximum deviation, the STL fi le
resolution is determined by relating the triangulation to the voxel size of
the CT data. If every voxel is triangulated the maximum resolution will be
produced; however, the fi le size will be correspondingly large. By triangulating
multiple voxels, a simpler but smaller STL fi le will be produced.
Typically, a compromise is achieved between fi le size and surface quality.
Some software will simplify this by allowing the user to specify low, medium
or high quality settings.
For example, for the data previously illustrated in Fig. 4.9, the highest
possible resolution settings result in an STL fi le of over 94 MB. Compare
this to the results of the preset software setting that range from ‘low’ at
4.3 MB, ‘medium’ at 11.7 MB and ‘high’ at 34 MB. The two extremes of the
effect of these settings can be seen in the close up views of the resulting
STL fi les shown in Fig. 4.21.
STL fi les can be post-processed in several ways, usually to enable them
to be produced by RP methods more effi ciently. Many software applications
are available that allow STL fi les to be split into separate parts, or
re-oriented in space. In addition, the quality of the STL fi le can be manipulated
somewhat. File size can be reduced by triangle reduction techniques
and smoothing algorithms can be applied.
4.6.3 VRML
Early versions of Virtual Reality Modelling Language (VRML) are also
triangular faceted surface representations. They are in essence very similar
to STL fi les but are typically created at very small fi le sizes and are coded