R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

50 Medical modelling
be generated with suffi cient resolution such that curved surfaces on RP
models derived from them appear smooth.
The format was developed by 3D Systems as an alternative input fi le for
their stereolithography technology. If an SLC fi le can be generated from
the original source data, it effectively bypasses the need to create STL fi les.
For a given data set, an SLC fi le will be much smaller than an STL fi le of
comparable resolution. For example, for the data illustrated in Fig. 4.9 the
SLC fi le would be 28.0 MB, which would compare favourably with the
equivalent quality STL fi le which would be 94.3 MB. Therefore, the SLC
provides a highly effi cient data transfer format for medical modelling using
stereolithography.
4.5.3 CLI
Common Layer Interface is a contour fi le format that describes crosssectional
slices through an object in much the same way as the SLC
fi le. However, the format is commonly available to many software developers
and may be used with a variety of RP technologies. For comparison,
the CLI fi le of the data illustrated in Fig. 4.9 would be 13.8 MB.
4.5.4 SLI
Also derived from the word ‘slice’, the SLI fi le format is similar to the SLC
fi le format but, rather than being an input fi le, it is an intermediate fi le
format created during the preparation of stereolithography builds. The fi le,
developed by 3D Systems, not only describes the perimeter of the slices but
also includes raster lines that make up the cross-sectional area within the
boundary. These raster scan lines are usually referred to as hatches. The
fi le format also includes different types of hatch for up-facing and downfacing
layers. This is important in stereolithography because up-facing and
down-facing layers are built differently to optimise accuracy. As with the
SLC fi le format, the ability to generate this fi le format directly from medical
scan data provides an effi cient data transfer when generating stereolithography
fi les. The extra hatch information contained in the fi le increases its
size in comparison to the SLC. For example, the SLI fi le of the data illustrated
in Fig. 4.9 would be 46.1 MB.
4.6 True three-dimensional formats
4.6.1 Polygon faceted surfaces
Unlike the contour or slice-based formats described above, true threedimensional
data formats generate computer models that have a surface.