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STL is a file format native to the stereolithography CAD software created by 3D Systems of Valencia, CA, USA. STL files may be ASCII or binary data, although binary is far more common due to the resulting size of the CAD data when saved to the ASCII format.
An STL file describes a raw unstructured triangulated surface by the unit normal and vertices (ordered by the right-hand rule) of the triangles using a three-dimensional Cartesian coordinate system.
An ASCII STL file begins with the line:
solid *
where * is an optional string, continues with any number of triangles:
facet normal n1 n2 n3 outer loop vertex v11 v12 v13 vertex v21 v22 v23 vertex v31 v32 v33 endloop endfacet
and concludes with:
endsolid *
History of use
Stereolithography machines are basically 3D printers that can build any volume shape as a series of slices. Ultimately these machines require a series of closed 2D contours that are filled in with solidified material as the layers are fused together.
The natural file format for such a machine would be a series of closed polygons corresponding to different Z-values. However, since it's possible to vary the layer thicknesses for a faster though less precise build, it seemed easier to define the model to be built as a closed polyhedron that could be sliced at the necessary horizontal levels.
The STL file format appears capable of defining a polyhedron with any polygonal facet, but in practice it's only ever used for triangles, which means that much of the syntax of the file is superfluous. It is also the case that the value of the normal shouldn't be necessary, since that is a direct calculation from the coordinates of the triangle with the orientation being controlled by the left hand rule.
Also, STL files are supposed to be closed and connected like a combinatorial surface, where every triangular edge being part of exactly two triangles, and not self-intersecting. Since the syntax does not enforce this property, it can be ignored for applications where the closedness doesn't matter.
The closedness only matters insofar as the software which slices the triangles requires it to ensure that the resulting 2D polygons are closed. Sometimes such software can be written to cleanup small discrepancies by moving endpoints of edges that are close together so that they coincide. The results are not predictable, but it is often sufficient to get the job done.
Obviously, there is much scope for "improvement" of this file format, which in its present form is nothing more than a listing of groups of 9 (or 12 if you care about the normals) floating point numbers embedded in some unnecessary syntax. Since each vertex is on average going to be used in six different triangles, considerable savings in memory could be obtained by listing all the points in a table at the beginning of the file, and concluding with a list of triangle definitions comprised of triplets of integers that referenced this table.
However, for the purpose of a generating a single contour slice using a very lightweight piece of software on a computer with little memory, this format is perfect since it can be processed in one pass regardless of file size. Wide use of STL outside this application. Many Computer-aided design systems are able to output the STL file format among their other formats because it's quick and easy to implement, if you ignore the connection criteria of the triangles. Many Computer-aided manufacturing systems require triangulated models as the basis of their calculation.
Since an STL file output, of a sorts, is almost always available from the CAD system, it's often used as a quick method for importing the necessary triangulated geometry into the CAM system.
Once it works, there is very little motivation to change, even though it is far from the most memory and computationally efficient method for transferring this data. Many integrated CAD and CAM systems transfer their geometric data using this accidental file format, because it's impossible to go wrong.
There are many other file formats capable of encoding triangles available, such as VRML, DXF, but they have the disadvantage that it's possible to put things other than triangles into it, and thus produce something ambiguous or unusable.
See also
- RepRap is an OpenSource project that uses STL file input and generates solid objects as output.
External links
- University of California, San Diego. Tele-Manufacturing Facility Project: STL Format description
- MeshLab: an open source application for visualizing and converting three dimensional meshes to or from the STL file format.
- How to Create an STL file and Free STL Viewer download courtesy of ProtoCAM
- Freesteel with a web-interface where you can upload an STL file and render it into an image in your browser.
"STL (file format)"