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stl to scad conversion program design

Initial reference information

My current repository

Goals

Create as many separate objects / (closed) surfaces as possible from each input stl file, without changing the set of vertex points, or adding or removing any facets. That is, create a separate scad object from each disjoint surface in each stl file.

Initially, the scad options are all polyhedrons. Later enhancements are intended to build objects from primitive objects and operations.

Future

  • Turn the detected polyhedron objects into union, intersection, differences of primitive scad objects, operations.
  • detect duplicate objects
    • in a single file
    • across multiple stl file processed together
    • as sub components of a single object

data validation

How much is practical to do as part of the data format conversion, and how much should use an external, possibly 3rd party tool?

  • manifold
    • a manifold is not necessarily closed ??
  • interpenetrating objects
  • interpenetrating surfaces of a single object

geometry discussion

disjoint surfaces can touch at one or more vertex points, edges, or faces. They can also interpenetrate.

test cases

2 completely disjoint tetrahedrons 2 tetrahedrons that meet at one vertex 2 tetrahedrons that meed at one edges a single surface that meets itself at a single point a single surface that meets itself at a single edge a single surface that meets itself at a single face a single surface that meets itself at multiple single points, edges, faces disjoint surfaces that meet each other at multiple single points, edges, faces Given the surface/face walk processing, interpenetrating surfaces are not a problem when the surface faces do not share any vertices, edges, faces.

intersecting (with or without interpenetrating) surface segments, with either a single or disjoint surfaces. negative surface / volume (hollow interior)

python

sample calls

python stl2scad -i$'\t' file.stl

definitions

¦ terms ¦ meanings ¦ acronyms ¦ backronyms ¦ alias ¦

  • stl
    • STereoLithography
    • backronyms
      • Standard Triangle Language
      • Standard Tessellation Language
    • surface geometry of a 3D object

data file format

stl ASCII

solid <<name>>
«indent» «facet»+
endsolid <<name>>
Loading
  • «name» is optional, but the preceding space is not
  • name is a simple text string
    • no spaces?
  • indent is one or more spaces (no tabs)
facet normal «n~i~» «n~j~» «n~k~»
«indent» «oloop»
endfacet
  • the normal is a vector or length 1.0. Each «n» is (the decimal number text representation of) a single precision float. «n» may be negative.
outer loop
«indent» «vert»
«indent» «vert»
«indent» «vert»
endloop

vertex «v~x~» «v~y~» «v~z~»
  • a vertex is a 3 dimensional coordinate in the all-positive octant. Each «v» must be positive-definite (non-negative, and nonzero).
  • standard format specification says there are exactly 3 vertex entries in an outer loop (triangle definition). Other information says that technically there could be more data points, as long as they are all co-planar.

parsing

stl binary

80 bytes ASCII characters ¦ header data
4 bytes unsigned long integer ¦ number of facets in file
«facet»…«4:»

«normal»
«vert»
«vert»
«vert»
2 bytes unsigned integer ¦ attribute byte count (0)

4 bytes float ¦ i
4 bytes float ¦ j
4 bytes float ¦ k

4 bytes float ¦ x
4 bytes float ¦ y
4 bytes float ¦ z

concepts

  • hollow, void, manifold, polyhedron
  • stl ASCII vertex format text is close enough to a point in an scad polyhedron to not need to convert to floating point and back. Could just trim extra spaces, insert commas, and wrap with "[]".

splitDisjointObjects( mdl )

processing environment

Inputs

The model structure contains an objects array (normally with a single entry) that holds dictionary entries with faces and points data as numpy arrays. The points data is shape (-1, 3). Each point entry holds the x, y, and z coordinates of a single vertex on the surface defined by the object. The faces data is shape (-1, 3). Each face is a triangle, and has 3 index values, one for each vertex. There are no duplicate points. Faces that share vertices with connected faces include 1 or 2 of the same point indexes.

Outputs

The objects array updated to hold the disjoint surfaces from the input object(s). Each input object entry will generate 1 or more dictionary elements with the same structure as the input. Each dictionary will contain only the points from the input that define a single complete surface. Starting from any face it will be possible to recursively traverse to 3 adjacent faces (based on the command edges), and visit every face in the set. Common edges are used, not common vertices, because 2 (or more) surface could meet at a single point, without sharing any faces. Those cases will create separate disjoint objects.

processing ideas

  • directed edge as hash of 2 endpoint indexes
  • generate face, edge, reverse edge, face lookup before start disjoint detection

current

  • create empty list of disjoint objects
  • for each input object, create directed edges for every face, empty list of disjoint surfaces, and a set of faces (all of them) that are not on any disjoint surface
    • while there are any faces not on a disjoint surface
      • create a new empty disjoint surface
      • add any one face that is not part of a disjoint surface to the surface
        • add edges of the face to the surface too
      • process edges while there are unprocessed edges in the disjoint surface
        • if the reverse of the edge is not in the surface, locate the face for the reverse edge, and add it and its edges to the surface
      • add the surface to the list of disjoint surfaces for the object
      • update the set of faces that are not yet on a disjoint surface
    • for each disjoint surface, create a new scad polyhedron object, and add it to the list of disjoint objects
  • replace the objects list in the model with the list of disjoint objects

blue sky features

  • calculation and reporting
    • surface area
    • volume
  • check for and repair manifold
  • split to multiple objects
    • disjoint face sets
    • handle void as a negated (from stl) object, differenced out of outer object
  • convert polyhedron to operations on more primitive objects
    • during conversion? separate program that used scad file as input?
  • merge co-planar adjacent faces

scad simplify, optimize, compress

Generate series of primitive scad operations + actions that will generate an equivalent 3D model.

  • /home/phil/Documents/evernote/openscad.md # language, primitives

  • curve fitting

    • 'knife' edges
    • merge co-planar adjacent faces
      • intermediate step

environment

¦ virtualenv ¦ venv ¦ python2 ¦ python3 ¦ https://virtualenv.pypa.io/en/

notes

  • /home/phil/Documents/evernote/stl file manipulation.md

Need a sample stl file with separate objects connected at a single edge

IDEA remove test stl file(s) from repository that are generated programmatically (ie export from OpenScad) -- that would rely on other versions of OpenScad creating the same content

NOTE: Remember to post a note to Rick van Hattem ¦ numpy-stl when have a usable / public version of stl2scad

Existing code is actually fairly simple. It seems to extract all data points from the stl facets, generating matching points in a polyhedron, then creating faces from the points. Could be a lot smarter. As a start, duplicate points could be merged across facets, reducing the number of data points to about 1 third (with triangular mesh faces, each point will typically be used in 3 different faces).

Do heavy math lifting to find edges to generate simpler 3d solids. Or 2d extrusions.

  • curve fitting : points, edges
    • limited set of equations to attempt to match : limited number of graphic object primitives
      • other than polygon / polyhedron, which the intent is to avoid / minimize usage of.
    • watch for 2D extrude: straight lines in one dimension, arbitrary polygon shape 90° from that.
      • twist, scale can turn 'simple' extrude into complex forms. Going to be interesting to detect
      • rotate_extrude: watch for circles with common axis
    • locate disjoint subsets, and process separately (in initial passes)

Use ?oct?-i-tree? data structure for storing and finding 'near' points. And higher level entities / objects. overlapping cubes at multiple scales.

virtualenv --python=python3 -v --prompt=py3 basepy3
virtualenv --python=python2 -v --prompt=py2 basepy2

Is it practical to user virtualenv for python < 3.3, and venv for 3.3+?

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