STL File Format

STL (Stereolithography or Standard Tessellation Language) is the standard file format for 3D printing. It describes the surface of a 3D model as a mesh of triangles, with no colour, texture or units.

Each triangle is stored as three vertices (x, y, z) and a unit normal vector pointing outward. Formats: ASCII (readable, large) and binary (compact, common).

ASCII STL structure

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

Advantages

• Universally supported by CAD and slicing software.
• Simple structure, easy to generate.
• Fine mesh gives good accuracy.

Limitations

• Approximates curves, so accuracy depends on triangle count; a finer mesh gives large files.
• No colour, material, texture or unit information.
• Errors are common: gaps, flipped normals, overlapping or non-manifold edges, needing repair.
• Only surface, not internal structure.

Exam tip

Draw a triangle mesh, show the ASCII facet and list 3 advantages and 3 limitations.