STL (file format)

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In the STL interface (Surface Tesselation Language, German as language for describing the surface by triangles or Standard Triangulation Language and of unknown origin and Standard Tessellation Language) is a ( quasi- ) standard interface to many CAD systems.

This data interface is primarily used to provide geometric information from three-dimensional data models out for production using generative manufacturing processes or rapid prototyping systems.

The term has stereolithography interface has its foundation in the fact that stereolithography systems ( SLA), the first commercially available systems were that were just run with this geometry description.

  • 2.1 Implementation problems
  • 2.2 Syntactic representation error

Definition

The STL format contains the description of the surface of 3D objects with the help of triangular facets (English tesselation = " tiling "). Each triangular facet is characterized by the three vertices and the associated surface normal of the triangle. This entire geometrical values ​​are needed in a defined form for further data processing ( eg positioning, slicing ) for the construction process.

By the fact that at least three triangles most likely four, five or more triangles having a common vertex, therefore each point appears at least three times. In addition, the surface normal is stored, which could be defined as well by the order in which the description of a single triangle. This and the storage in ASCII code cause the amount of data can take enormous sizes. The format also exists in a binary stored variant, which already results in a considerable reduction, in addition to compression can be a reduction to manageable file sizes reach.

A format that has the same structure ( triangulated surfaces ), but eliminates redundancy ( multiple answers), VRML (Virtual Reality Modeling Language, also *. Vrml ). In this format, there are two numbered lists: the first contains the coordinates of the triangle vertices in the second, the triangles are defined by the position number of the points list, as well as the surface normal of the order of the triangle points. This geometry element is required to distinguish inside and outside can. To build up the surface of the body, ie, a single passage of the second list of the VRML file is required. When STL format, the triangles are directly read out the points as mentioned above, but read several times. The time required for the development of a virtual model is in both formats is not very different, but the memory requirements and the data transfer operation VRML is a clear advantage. For this reason, the format finds its preferred use in computer games, in the "virtual reality" and in 3D applications on the Internet.

Curved surfaces are approximated by triangles only. The lower the number of triangles, the greater are the differences; the more precise must be the approach, the more individual triangles are needed. Therefore, the amount of data increases rapidly with higher accuracy.

The description of the model surface in the form of the STL format is actually an industry standard dar. VRML is not correctly generated or read what has prevented a substitution of all systems. VRML offers the possibility to provide the surfaces with colors or textures, or even a fourth dimension, which is displayed on a temporal change of geometry elements ( point coordinates). At least here, however, the standardization ends (similar to the dialect versions of NC - code).

In STL format This data can without re-importing turned into a CAD program and increased or decreased to compensate for the shrinkage of the resin in stereolithography, for example.

ASCII format

Each STL file in ASCII code is as follows:

Solid name   facet normal n1 n2 n3    outer loop     vertex p1x P1Y p1z     vertex p2x p2y P2Z     vertex p3x P3Y P3Z    endloop   endfacet endsolid name name is the file name, the block of facet to endfacet represents a triangle and is repeated according to the number of triangles. ni is the normal vector of the triangle, P1J p3j to the x -, y -and z- coordinates of the vertices of the triangle.

Binary format

Since models in ASCII format easily lead to very large files is often stored in binary format. A binary STL file begins with a file header ( header) of 80 bytes. The content of the header is ignored when processing, but it can not start with solid, as this is the key for STL files in ASCII format. The header is followed by four bytes representing an unsigned integer, which indicates the number of triangles (respectively facet records) in the file. Thereafter, the data for the individual triangles follow. The file ends after the last triangle.

Each triangle is represented by floating-point numbers each of twelve 32-bit: three for the normal and for the three respective X, Y and Z coordinates of the vertices of the triangle. After two bytes representing an unsigned integer followed (attribute byte count ). Usually the value is zero, the majority of the existing software understands any other values.

Floating point numbers are represented according to IEEE 754, the byte order little endian is being used.

UINT8 - header ( header) UINT32 - Number of triangles foreach triangle    REAL32 - normal vector    REAL32 - Vertex 1    REAL32 - Vertex 2    REAL32 - Vertex 3    UINT16 - Attribute byte count end Bugs and Problems

The implementation of the CAD internal geometry data errors can occur. They are divided as follows:

Implementation problems

  • No direct implementation of curved form elements possible
  • Distortion of form elements through the selected degree of approximation ( minimum size and maximum density of the triangular facets used )

The implementation errors lead to a distortion of the manufactured geometry in relation to the constructive basis. However, they have no influence on the processability of the data on the stereo system. The magnitude of the error depends on the CAD system used with its predefined properties as well as the handling of the system ( and in particular the interface STL ) by the operator. A special factor is the degree of approximation used for curved shape elements. This defines the number of triangular facets to be used per curved surface and thus the fineness of the implementation. The more facets are used, the more accurate is the description and the greater the created file. Here is a compromise between data quantity and form deviation found for the descriptive to the workpiece.

For the production of the parts therefore, these values ​​are of fundamental importance.

Syntactic representation error

When errors occur in these data files a production is not subject to restrictions or. This error could be:

  • Gaps between triangular facets
  • Double triangular facets
  • Incorrect orientation of individual facets
  • Fold
Polygon mesh
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