R_Bibb_Medical_Modelling_The_Application_of_Adv.pdf

Working with medical scan data 51
If a computer model has a surface it may be rendered and visualised on
screen and be manipulated with a higher degree of sophistication than 2 1 / 2D
data. However, fi le sizes are typically higher.
One of the simplest methods of generating a three-dimensional computer
model is to create a polygon faceted surface. This is achieved by approximating
the original data as a large number of tessellating polygon facets.
As the triangle is the simplest polygon, it is frequently exploited in polygon
faceted representations. However, other polygons are also used, particularly
in fi nite element analysis (FEA).
4.6.2 STL
Derived from the word ‘stereolithography’, the STL fi le is a simple fi le
format that describes objects as a series of triangular facets that form its
surface. For example, if we view a simple object as an STL fi le we can see
the triangles. It can be seen that large fl at areas require few facets, whereas
curved surfaces require more facets to approximate the original surface
closely, see Fig. 4.16. The format was originally developed by 3D Systems
to provide a transfer data format from CAD systems to their stereolithography
technology, but it has subsequently been adopted as the de facto
standard in the RP industry.
The STL fi le simply lists a description of each of the triangular facets,
which make up the surface of a three-dimensional model. Figure 4.17 shows
the beginning and end of an STL fi le in text format. The fi rst line describes
the direction of the facet normal. This indicates which surface is the outside
of the facet. The next three lines give the co-ordinates of the three corners,
or vertices, of the facet.
The simplicity of the triangle makes mathematical operations such as
scaling, rotation, translation and surface area and volume calculations
straightforward. The format also allows the angle of facets to be identifi ed,
which is necessary for stereolithography.
STL fi les can be in binary or text (ASCII) format. Binary format fi les are
much smaller and should be used unless there is a specifi c reason why the
text format is required. STL fi les can vary in size from around 50 K to hundreds
of megabytes. Highly complex parts may result in excessively large
STL fi les, which may make data operations and fi le transfer diffi cult. Figure
4.18 shows how a large number of facets describe the surface of a complex
anatomical shape. However, highly effective compression software is freely
available that can reduce an STL fi le to a small fraction of its normal size
to enable easy transfer of fi les over the Internet or by email.
To be used successfully in RP, the STL fi le must form a single enclosed
volume; meaning that it should have no gaps between facets and all the
facets should have their normals facing away from the part (i.e. identifying