ASCII STL grammar: solid, facet, loop and vertex

STL comes from 3D Systems’ StereoLithography Interface Specification (July 1988, revised October 1989), which defines an ASCII form and a binary form. The specification itself is not published online; this page follows the reprint of its rules in Marshall Burns’ Automated Fabrication (1993, section 6.5), and says where today’s files and readers depart from them.

Syntax

solid name
  facet normal ni nj nk
    outer loop
      vertex v1x v1y v1z
      vertex v2x v2y v2z
      vertex v3x v3y v3z
    endloop
  endfacet
  …the facet block repeated, one or more times…
endsolid name

The grammar of an ASCII STL file.

Keyword	Followed by	Meaning
`solid`	An optional name, to the end of the line	Starts the file
`facet normal`	Three numbers	Starts a triangle and gives its unit normal
`outer loop`	Nothing	Starts the triangle’s list of corners
`vertex`	Three numbers	One corner: x, y and z. Exactly three per loop
`endloop`	Nothing	Ends the list of corners
`endfacet`	Nothing	Ends the triangle
`endsolid`	The same optional name	Ends the file

The words outer loop suggest that a facet could have other loops, or loops of more than three corners, but the specification describes only triangles, and readers expect exactly one loop of three vertices per facet.

Rules of the specification

Rule	In practice
Keywords are lower case.	Followed by almost all writers.
`facet normal` and `outer loop` each contain one space; the keywords beginning `end` contain none.	Readers rely on it: `endloop`, not `end loop`.
Indentation is with spaces; tabs are not allowed.	Most readers accept any white space, including tabs and blank lines, between words and numbers, but never inside one.
Numbers are single-precision floats in exponent form, such as `2.648000e-002`.	Plain decimals such as `0.5` or `1` are written and read just as widely.
A normal coordinate may be negative; a vertex coordinate may not. The object lies in the all-positive octant, every vertex coordinate greater than zero.	No longer enforced: zero and negative coordinates are common today, as in the example below.
The file has no units; coordinates are in arbitrary units.	Unchanged. Programs ask, or assume millimetres.
Sorting facets by ascending z is recommended, not required, to help the slicer.	Rarely done.

The name after solid is optional. When it is left out, some software expects a space after solid anyway. The rest of that line is free text and is sometimes used for the file name, the author or a date.

Facet orientation

Each facet is part of the boundary between the inside and the outside of the object, and the specification states which side is outside twice, in two ways that must agree: the normal points outwards, and the three vertices run anticlockwise when the object is seen from outside (the right-hand rule).

The normal should be a unit vector. Many programs write 0 0 0 and leave the reader to compute the normal from the vertex order, and many readers ignore the stored normal altogether. A file that gives both a correct normal and the right vertex order works everywhere.

The vertex-to-vertex rule

Each triangle must share two vertices with each triangle next to it. A vertex of one triangle must never lie partway along the side of another. With the orientation rules this makes the surface closed: every edge belongs to exactly two facets, with no gaps, flipped facets or overlaps. Nothing in the syntax enforces it, which is why repair tools exist.

Each facet repeats its three vertices in full. The file has no shared vertex list, so a reader that needs the connections between facets has to find matching coordinates itself.

Example

solid tetrahedron
  facet normal 0.000000e+000 0.000000e+000 -1.000000e+000
    outer loop
      vertex 0.000000e+000 0.000000e+000 0.000000e+000
      vertex 0.000000e+000 1.000000e+000 0.000000e+000
      vertex 1.000000e+000 0.000000e+000 0.000000e+000
    endloop
  endfacet
  facet normal 0.000000e+000 -1.000000e+000 0.000000e+000
    outer loop
      vertex 0.000000e+000 0.000000e+000 0.000000e+000
      vertex 1.000000e+000 0.000000e+000 0.000000e+000
      vertex 0.000000e+000 0.000000e+000 1.000000e+000
    endloop
  endfacet
  facet normal -1.000000e+000 0.000000e+000 0.000000e+000
    outer loop
      vertex 0.000000e+000 0.000000e+000 0.000000e+000
      vertex 0.000000e+000 0.000000e+000 1.000000e+000
      vertex 0.000000e+000 1.000000e+000 0.000000e+000
    endloop
  endfacet
  facet normal 5.773503e-001 5.773503e-001 5.773503e-001
    outer loop
      vertex 1.000000e+000 0.000000e+000 0.000000e+000
      vertex 0.000000e+000 1.000000e+000 0.000000e+000
      vertex 0.000000e+000 0.000000e+000 1.000000e+000
    endloop
  endfacet
endsolid tetrahedron

A closed tetrahedron: four facets, each with an outward unit normal and its corners anticlockwise seen from outside.

Telling ASCII from binary

An ASCII file starts with solid, but so do some binary files, whose free 80-byte header happens to begin with that word. A reliable test reads the 32-bit count at offset 80 as if the file were binary: if the file size is exactly 84 + 50 × count, it is binary. Otherwise, finding facet and endsolid confirms ASCII.