What Is STL File?

How Does STL Work?

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How does STL Work?

An STL file consists of a list of facet data.  Each facet is uniquely identified by a unit normal and  three vertices.  The normal and each vertex are specified by three coordinates each, so there is a total of 12 numbers stored for each facet.

Specifications

Facet orientation.  The facets define the surface of a 3-dimensional object.  As such, each facet is part of the boundary between the interior and the exterior of the object.  The orientation of the facets  is specified redundantly in two ways which must be consistent.  First, the direction of the normal is outward.  Second, the vertices are listed in counterclockwise order when looking at the object from the outside (right-hand rule). 

Vertex-to-vertex rule.  Each triangle must share two vertices with each of its adjacent triangles.  In other words, a vertex of one triangle cannot lie on the side of another.    

The object represented must be located in the all-positive octant.  In other words, all vertex coordinates must be positive-define (nonnegative and nonzero) numbers.  The STL file does not contain any scale information, the coordinates are in the arbitrary units.

Typically, an STL file is saved with the extension "stl", case-insensitive.  The slice program may require this extension or it may allow a different extension to be specified.

The STL standard includes two data formats, ASCII and binary.  These are described separately below.

STL ASCII format.

The ASCII format is primary intended for testing new CAD interfaces.  The larger size of its files makes it impractical for general use.

The syntax for an ASCII STL file is as follows:

Bold face indicates a keyword; these must appear in lower case.  Note that there is a space in "facet normal" and in "outer loop", while there is no space in any of the  keywords beginning with "end".  Indentation must be with spaces; tabs are not allowed.  The notation {...} means that the contents of the brace brackets can be repeated one or more times.  Words in italics are variables which are to be replaced with user-specified values.  The numerical data in the facet normal and vertex lines are single precision floats, for example, 1.23456E+789.  A facet normal coordinate may have a leading minus sign; a vertex coordinate may not.

STL binary format.

The binary format uses the IEEE integer and floating point numerical representation.  The syntax for a binary file is as follow:

The header record consists of 84 bytes, the first eighty are used for information about the file, author's name and other miscellaneous comments, the last 4 bytes represent the number of triangular facets.  Next, for each facet, 50 bytes are used to represent the x, y and z components of the normal to the facets, then the x, y and z coordinates of each vertex of the triangle.  4 bytes are used for each coordinate, resulting is 48 bytes per facet.  The last two bytes are not used.

The notation {...} means that the contents of the brace brackets can be repeated one or more times.  The attribute syntax is not documented.  It is specified that the attribute byte count should be set to zero.

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Need STL Tools?

Some STL Tools are listed in here.

SolidView/Lite Solid Concepts introduces SolidView/Lite, a FREE viewer for STL, VRML, SolidWorks, and other 3D CAD formats.  The SolidView/Lite user can see all the objects and views in the STL file and in addition any other 3D view is available, along with complete dimensional information and the ability to inspect internal features.

	StlView v7.0 is a shareware written by Igor G. Tebelev Igor G. Tebelev, PO BOX 4491, Palos Verdes, CA, 90274, USA.  StlView is a Windows (Windows95, WindowsNT, Win32s) Program and Full-Server that allow you to analyze, fix and embed or link 3D solid model inside or to the document created inside your favorite word processor or other software that supports OLE2.0 (MS Word, MS Excel, Write, Cardfile and etc.).
	Magics RP is a Materialise software.  It ensures that the CAD-data is easily translated into machine files and gives STL manipulation options that are far more automated and advanced than any other system.
	VICES is developed at Clemson University for repair and verification of STL files.  IVECS stands for "Interactive Virtual Environment for Correction of .STL files".  The main goal of IVECS is to help in the correction process of faceted models issued from CAD systems.
	Imageware is SDRC's advanced 3D surface modeling and verification technology for the automotive, aerospace, and consumer products industries.  It allows the import and export of: IGES, STL, DXF, ASCII, VRML, SLC and 3D measurement data.
	DeskArtes is a Rapid Tools software of NEST Technologies.  It can be used for repair, conversion, and manipulation of STL, IGES, and VDA-FS files.

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Replace STL?

Some other formats are suggested for replacing STL file.  Here is the list of some common formats.

STEP, the Standard for The Exchange of Product model data is a proposed International Standards Organization (ISO) standard for electronic data exchange.  It is also known as the product data exchange using STEP (PDES) in the United States.

IGES, International Graphics Exchange Standard  is an international standard used by most CAD vendors.  The format is supported to allow file interchange between different vendors.  Unfortunately, many studies have shown how each CAD vendor have their own interpretation of the IGES standard, which renders some the information useless to others.  Additionally, the size of an IGES file is typically ery large and difficult to handle.  A description of IGES can be found in http://www.steptools.com/, it is also described in National Bureau of Standards document NBSIR 86-3359.

SLI, Slice format is the format used by the stereolithogrphy machines to control the laser beam.  It is a series of vector commands generated by the slicing software.  The format is proprietary, as are many machine specific formats.

SLC is 3D systems' proprietary control data, layered based format.  It is in fact a 2 1/2 dimensional contour representation of a CAD data model.  It can be generated from Solid or surface modeling software or CT scan data.  Its description is available from 3D systems.