.STL File Format

Introduction

ASCII STL is the text form of the stereolithography format. It holds the same data as binary STL, a list of triangles with a normal each, but spelt out in keywords and decimal numbers. It is useful for small test files and for reading a model by eye, and several times larger than the binary form.

Structure

The file starts with solid and an optional name, and ends with endsolid and the same name. Between them, each triangle is a facet with its normal, then an outer loop of three vertex lines giving the corners, then endloop and endfacet. The corners are listed anticlockwise when seen from outside, and many programs recompute the normal from them rather than trust the stored one.

solid part
  facet normal 0 0 1
    outer loop
      vertex 0 0 0
      vertex 1 0 0
      vertex 0 1 0
    endloop
  endfacet
endsolid part

A file that starts with solid is not always text: some programs write binary files whose 80-byte header begins with the same word. Comparing the file size with 84 + 50 × count tells the two apart.

Variants

• ASCII STL – the text form of STL: each triangle a facet of three vertex lines
• Binary STL – the usual form of STL: a header, a count, then 50 bytes per triangle

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