Fix for Issue #5924:

[dbs4261]

Fix for Issue #5924:

Changed TriangleMesh::materials_ to be a std::vector<std::pair<std::string, Material>> so that the order of materials when loading a mesh is respected. Using the unordered map caused issues when a Default Material and a texture were loaded and the texture would end up as the second material in the iterator. Now TriangleMesh::triangle_material_ids_ will indicate what material index should be used. A warning is given when passing a TriangleMesh to the visualizer and more than one material index is found, and the minimum value found in TriangleMesh::triangle_material_ids_ is used. If no triangle_material_ids_ exist then the first material in the order they are loaded will be used.

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves For TriangleMesh triangle_material_ids_ do not relate to the materials_ #5924

Motivation and Context

When loading a textured mesh with Assimp or TinyOBJ the materials are loaded in order, and a material index is provided for each triangle. However these materials are stored in an unordered map. A common occurrence is loaded a textured mesh along with a default material. The texture might be loaded as index 1 which can be seen in the TriangleMesh::triangle_material_ids_. By instead loading to a vector, the order is maintained.

This change breaks any access to the TriangleMesh::materials_ parameter. Though no instances of this access occur within the Open3d C++ library, it is exposed publicly in the Python API. Instead of a dictionary, the user would get a list of pairs.

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

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This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[ssheorey]

Hi @dbs4261 please check the CI failures.

[dbs4261]

I made some updates to address the merge issues. Lets try the workflows again.

[errissa]

Reviewed 5 of 7 files at r1, 2 of 2 files at r2, all commit messages.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @dbs4261)