c14n_stl.py

#!/usr/bin/env python
#
# OpenSCAD produces randomly ordered STL files so source control like GIT can't tell if they have changed or not.
# This scrip orders each triangle to start with the lowest vertex first (comparing x, then y, then z)
# It then sorts the triangles to start with the one with the lowest vertices first (comparing first vertex, second, then third)
# This has no effect on the model but makes the STL consistent. I.e. it makes a canonical form.
#

from __future__ import print_function

import sys


def _cmz(x):
    ''' Convert "-0" to "0". '''
    return '0' if x == '-0' else x


class Vertex:
    def __init__(self, x, y, z):
        self.x, self.y, self.z = x, y, z
        self.key = (float(x), float(y), float(z))

class Normal:
    def __init__(self, dx, dy, dz):
        self.dx, self.dy, self.dz = dx, dy, dz

class Facet:
    def __init__(self, normal, v1, v2, v3):
        self.normal = normal
        if v1.key < v2.key:
            if v1.key < v3.key:
                self.vertices = (v1, v2, v3)    #v1 is the smallest
            else:
                self.vertices = (v3, v1, v2)    #v3 is the smallest
        else:
            if v2.key < v3.key:
                self.vertices = (v2, v3, v1)    #v2 is the smallest
            else:
                self.vertices = (v3, v1, v2)    #v3 is the smallest

    def key(self):
        return (self.vertices[0].x, self.vertices[0].y, self.vertices[0].z,
                self.vertices[1].x, self.vertices[1].y, self.vertices[1].z,
                self.vertices[2].x, self.vertices[2].y, self.vertices[2].z)

class STL:
    def __init__(self, fname):
        self.facets = []

        f = open(fname)
        words = [_cmz(s.strip()) for s in f.read().split()]
        f.close()

        if words[0] == 'solid' and words[1] == 'OpenSCAD_Model':
            i = 2
            while words[i] == 'facet':
                norm = Normal(words[i + 2],  words[i + 3],  words[i + 4])
                v1   = Vertex(words[i + 8],  words[i + 9],  words[i + 10])
                v2   = Vertex(words[i + 12], words[i + 13], words[i + 14])
                v3   = Vertex(words[i + 16], words[i + 17], words[i + 18])
                i += 21
                self.facets.append(Facet(norm, v1, v2, v3))

            self.facets.sort(key = Facet.key)
        else:
            print("Not an OpenSCAD ascii STL file")
            sys.exit(1)

    def write(self, fname):
        f = open(fname,"wt")
        print('solid OpenSCAD_Model', file=f)
        for facet in self.facets:
            print('  facet normal %s %s %s' % (facet.normal.dx, facet.normal.dy, facet.normal.dz), file=f)
            print('    outer loop', file=f)
            for vertex in facet.vertices:
                print('      vertex %s %s %s' % (vertex.x, vertex.y, vertex.z), file=f)
            print('    endloop', file=f)
            print('  endfacet', file=f)
        print('endsolid OpenSCAD_Model', file=f)
        f.close()

def canonicalise(fname):
    stl = STL(fname)
    stl.write(fname)

if __name__ == '__main__':
    if len(sys.argv) == 2:
        canonicalise(sys.argv[1])
    else:
        print("usage: c14n_stl file")
        sys.exit(1)