meta:
  id: stl
  application: 3D Systems Stereolithography
  file-extension: stl
  xref:
    justsolve: STL
    loc: fdd000505
    pronom: fmt/865
    wikidata: Q1238229
  license: CC0-1.0
  endian: le
doc: |
  STL files are used to represent simple 3D models, defined using
  triangular 3D faces.

  Initially it was introduced as native format for 3D Systems
  Stereolithography CAD system, but due to its extreme simplicity, it
  was adopted by a wide range of 3D modelling, CAD, rapid prototyping
  and 3D printing applications as the simplest 3D model exchange
  format.

  STL is extremely bare-bones format: there are no complex headers, no
  texture / color support, no units specifications, no distinct vertex
  arrays. Whole model is specified as a collection of triangular
  faces.

  There are two versions of the format (text and binary), this spec
  describes binary version.
seq:
  - id: header
    size: 80
  - id: num_triangles
    type: u4
  - id: triangles
    type: triangle
    repeat: expr
    repeat-expr: num_triangles
types:
  triangle:
    doc: |
      Each STL triangle is defined by its 3 points in 3D space and a
      normal vector, which is generally used to determine where is
      "inside" and "outside" of the model.
    seq:
      - id: normal
        type: vec3d
      - id: vertices
        type: vec3d
        repeat: expr
        repeat-expr: 3
      - id: abr
        type: u2
        doc: |
          In theory (per standard), it's "attribute byte count" with
          no other details given on what "attribute" is and what
          should be stored in this field.

          In practice, software dealing with STL either expected to
          see 0 here, or uses this 16-bit field per se to store
          additional attributes (such as RGB color of a vertex or
          color index).
  vec3d:
    seq:
      - id: x
        type: f4
      - id: y
        type: f4
      - id: z
        type: f4