`triangle.triangulate` output does not have segments specified

[JulianKnodt]

I'm computing a constrained delaunay triangulation of an input UV parameterization, and I want to exactly maintain all input edges, so I passed them all through the segments parameter. Currently though, I found that the output will have edges flipped as compared to the input mesh, and if I do not pass segments at all the output is identical as when I specify segments to preserve.

Here is code to load and output meshes, and the data I'm loading with it:

I'm not sure if my use is wrong, but any help would be appreciated.

import argparse
import math
import trimesh

import torch

import numpy as np

import triangle

def arguments():
  a = argparse.ArgumentParser(
    formatter_class=argparse.ArgumentDefaultsHelpFormatter,
  )
  a.add_argument("-i", "--input", required=True, help="Input mesh")
  a.add_argument("--iters", type=int, default=10000, help="Optimization iterations")
  a.add_argument("--device", default="cpu", help="device to use")
  return a.parse_args()

def main():
  args = arguments()
  device = args.device
  print("Loading mesh")
  mesh = trimesh.load_mesh(args.input, process=False)
  uv = mesh.visual.uv

  faces = torch.tensor(mesh.faces, device=device)
  uniq_edges = []

  tri_m = trimesh.Trimesh(
    vertices=np.concatenate([uv, np.zeros_like(uv[:, :1])], axis=-1),
    faces=mesh.faces,
  )
  tri_m.export("tmp_in.obj")

  og_vi = {}
  for vi, p in enumerate(uv.tolist()):
    assert(tuple(p) not in og_vi)
    og_vi[tuple(p)] = vi

  og_edges = set()
  for vis in mesh.faces:
    assert(len(vis) == 3)
    for i, v in enumerate(vis):
      assert(v >= 0)
      n = vis[(i+1)%3]
      assert(n >= 0)
      og_edges.add(tuple(sorted([int(v),int(n)])))

  corners = [[0,1],[0,0],[1,1],[1,0]]
  t = triangle.triangulate({
    "vertices": uv.tolist() + corners,
    "segments": list(og_edges),
  })

  new_uv = t["vertices"]
  new_t = t["triangles"]
  is_valid_face = [0] * len(new_t)
  se = lambda i,j: (min(i, j), max(i,j))
  for ti, vis in enumerate(new_t):
    is_valid = True
    for i, v in enumerate(vis):
      vi = og_vi.get(tuple(new_uv[v]))
      ni = og_vi.get(tuple(new_uv[vis[(i+1) % 3]]))
      is_valid = is_valid and (vi is not None) and (ni is not None) and (se(vi, ni) in og_edges)
    is_valid_face[ti] = is_valid

  #new_t = [t for ti, t in enumerate(new_t) if is_valid_face[ti]]
  tri_m = trimesh.Trimesh(
    vertices=np.concatenate([new_uv, np.zeros_like(new_uv[:, :1])], axis=-1),
    faces=new_t,
  )
  tri_m.export("tmp.obj")
  exit()

if __name__ == "__main__": main()

dense_cube copy.obj.txt