[Mesh] TriangleMesh's "+=" operator appends UVs regardless of the presence of existing features

[cdbharath]

Allows appending of UV coordinates, textures and material IDs with += operator overload in TriangleMesh class irrespective of the presence of these attributes in the existing mesh (mesh on LHS to += operator)

Type

• Bug fix (non-breaking change which fixes an issue): Fixes # TriangleMesh's operator "+=" gets rid of UV coordinates when no texture is attached to the mesh. #4794
• 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 #

Motivation and Context

The += operator overload of TriangleMesh class does not append UV coordinates, textures and material IDs when the existing mesh (mesh on LHS to += operator) already contains any of these attributes. There is no reason to limit += for UVs to TriangleMesh with the attributes already assigned as it currently happens here.

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

The current change doesn't check if the presence of UV coordinates, textures and material IDs in the existing mesh and appends only based on their availability in the incoming mesh.

I am attaching a screenshot that shows the addition of UV coordinates based on += operator. The test code is as follows

    auto box = geometry::TriangleMesh::CreateBox(1.0, 1.0, 1.0, true, true);    
    auto sphere = geometry::TriangleMesh::CreateSphere(1.0, 6, true);   
    rendering::MaterialRecord material;
    material.albedo_img = texture_image;
    material.name = "material";
            
    auto scene = std::make_shared<geometry::TriangleMesh>();
        
    // Append new meshes using +=
    *scene += *box;
    *scene += *sphere;
            
    gui::Application::GetInstance().Initialize(argc, argv);
    auto vis1 = std::make_shared<visualizer::O3DVisualizer>("Textures", 1024, 768);
    vis1->AddGeometry("scene", scene, &material);
    vis1->ResetCameraToDefault();
    gui::Application::GetInstance().AddWindow(vis1);
    gui::Application::GetInstance().Run();

The following is the resulting mesh with its texture UVs.

[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.

[benjaminum]

@cdbharath Thanks for your PR, please have a look at the comment

[benjaminum]

If this does not have any UV coordinates then triangle_uvs_.size() will be different from the total number of triangles in the resulting mesh. This would break with the assumption that triangle_uvs_.size() == 3*number of triangles