66.1 STL FILE FORMAT

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Originally developed for use with stereolithography, but now used by many other processes.

The standard for Rapid prototyping systems

Basically connected 3D triangles

3D smooth surfaces are tessellated into triangles. the higher the degree of tessellation the closer the surface approximates the smooth surface.

The triangles are defined

1. with a direction of nodes defined clockwise for the out direction

2. with similar nodes at the corners of triangles. If the triangles don't overlap, the model will have gaps and be invalid.

A general approach to determining a rapid prototype slice is to use a ray projection through the collection of polygons. When the ray strikes a triangle it is in/out of the solid. (This is a simple geometrical problem.) A set of lines constitute a slice.

3. Fire a "ray" through the triangles, and find intersections

For the stereolithography machine we need to develop a "scan line" for the laser. When the laser scan-line (the ray) is "in" the part, the laser is on, and thus developing the light hardening polymer.

If polygon vertices don't match up, then there will be gaps between the polygons. This can result in non contact, that leads to material added/subtracted unexpectedly.

Another problem can arise from mobius strip representations. Because the outside is defined by the order of node definitions, a mobius strip will lead to a back touching a front.