DXF: Support negative index in VERTEX

[kenichiice]

In DXF files, negative value is allowed for the "Polyface mesh vertex index" of the VERTEX entity.

https://help.autodesk.com/view/ACD/2024/ENU/?guid=GUID-0741E831-599E-4CBF-91E1-8ADBCFD6556D

If the index is negative, the edge that begins with that vertex is invisible.

However, the current implementation of DXFImporter::ParsePolyLineVertex() converts negative index to 0 and cannot handle them correctly.

This pull request ensures that negative index is handled correctly by reading their absolute value.

[turol]

If the edge is supposed to be invisible shouldn't it be marked somehow or skipped?

[kenichiice]

The current patch only prevents the loss of faces. Still, I believe it improves the function.

As you mentioned, the invisible information should be retained if possible. Unfortunately, I haven't been able to implement this as I'm unsure how to represent invisible edges in assimp.

[kenichiice]

I think this pull request will fix the next issue:

• ERROR, DXF IMPORT #2036

[kenichiice]

I have created an example of a DXF file that cannot be read correctly without applying this PR.

• https://gist.github.com/kenichiice/d1980d6169966ca21334d532671eb46f

This file defines a square mesh consisting of two triangles. Negative indices are specified on lines 476 and 490.

• https://gist.github.com/kenichiice/d1980d6169966ca21334d532671eb46f#file-negatively-indexed-vertex-dxf-L476
• https://gist.github.com/kenichiice/d1980d6169966ca21334d532671eb46f#file-negatively-indexed-vertex-dxf-L490

When testing the code provided below, the behavior is as follows.

Using assimp before applying the PR:

num meshes = 1
mesh 0
  vertices:
    1, 1, -1
    1, 1, 1
    1, -1, 1
    1, -1, -1
  faces:
    0, 1
    2, 3

It only has 2 face vertices and does not read the triangle.

Using assimp after applying the PR:

num meshes = 1
mesh 0
  vertices:
    1, 1, -1
    1, 1, 1
    1, -1, 1
    1, 1, -1
    1, -1, 1
    1, -1, -1
  faces:
    0, 1, 2
    3, 4, 5

This one reads triangles.

The code used is as follows:

#include <iostream>

#include <assimp/Importer.hpp>
#include <assimp/scene.h>

int main(int argc, char* argv[])
{
  if (argc != 2) {
    std::cerr << "usage: " << argv[0] << " FILE" << std::endl;
    return 2;
  }

  Assimp::Importer importer;
  const auto* scene = importer.ReadFile(argv[1], 0);

  if (!scene) {
    std::cerr << "failed to read file" << std::endl;
    return 1;
  }

  std::cout << "num meshes = " << scene->mNumMeshes << std::endl;
  for (unsigned i = 0; i < scene->mNumMeshes; ++i) {
    const auto* m = scene->mMeshes[i];

    std::cout << "mesh " << i << std::endl;

    std::cout << "  vertices:" << std::endl;
    for (unsigned j = 0; j < m->mNumVertices; ++j) {
      const auto& v = m->mVertices[j];
      std::cout << "    " << v.x << ", " << v.y << ", " << v.z << std::endl;
    }

    std::cout << "  faces:" << std::endl;
    for (unsigned j = 0; j < m->mNumFaces; ++j) {
      const auto& f = m->mFaces[j];
      std::cout << "    ";
      for (unsigned k = 0; k < f.mNumIndices; ++k) {
        std::cout << f.mIndices[k];
        if (k + 1 < f.mNumIndices) {
          std::cout << ", ";
        } else {
          std::cout << std::endl;
        }
      }
    }
  }
}

[kenichiice]

The original patch conflicts with the changes made in PR #5146.
So I reworked the patch to match the latest code.

[turol]

Prefer rebase over merge, makes for a cleaner history.

[kenichiice]

I cleaned up the history by rebasing.

[kimkulling]

Merged, thanks a lot for your contribution.