STL file format

STL file is composed of a set of many unordered triangular facets. Its BNF format is defined as followed:

STL file has two formats, ASCII format and BINARY format.

The ASCII format is described as followed:

The first line in the file is the description line, which contains the STL filename. Facets begin from the second line: first is the normal of the facet, then the vertices. The coordinates of the three vertices are given in the order that comply with the ‘right-hand rule’. When the information of this triangle ends, then begins the next one. In this way, the whole model is recorded. Here is an example of ASCII format STL file:

solid sample.stl created by Wei Feng on 15th OCT. 1994
facet normal -1.000000 0.000000 0.000000
    outer loop
      vertex 140.502634 233.993075 -38.310362
      vertex 140.502634 229.424780 -38.359042
      vertex 140.502634 242.525774 -27.097848
    endloop
endfacet
facet normal 0.903689 0.004563 0.428166
    outerloop
      vertex 134.521310 273.427873 30.342009
      vertex 134.521310 308.505852 30.715799
      vertex 140.502634 334.576026 18.369396
    endloop
endfacet
facet normal -0.903689 0.004563 0.428166
    outer loop
      vertex 140.502634 334.576026 18.369396
      vertex 140.502634 294.929752 17.946926
      vertex 134.521310 273.427873 30.342009
    endloop
endfacet
... ...
endsolid sample.stl

BINARY STL file format is accessed by byte. The format is as follows: the first 80 bytes are used for description, and the next 4 bytes represents the total number of the facets(Long Int), followed by the facet information (normal and 3 vertices), the normal and vertices are stored in floating point format, each occupying 4 bytes. At the end of each facet information section, there are two bytes spaces, then the next facet is repeated till the end of the file. When BINARY format is used to describe STL file, the data size is much smaller than ASCII format, so most STL files available now use BINARY format.

This page was last updated on 20th Sept. 1997 by Dr. Wei Feng