BunnyLOD VS assimp

If you reuse the same index buffer as proposed then you won't be able to share vertices, which can be an important optimization. In my case I use a 323 cache of vert indices and just insert an index rather than a vertex if the vertex already exists and has the same material. I generate normals in the pixel shader to increase what can be shared, and then use a variant of Stan Melax's edge collapse polygon reduction algorithm to combine edges and reduce triangle count.

You might want to take a look at Stan Melax's easy mesh simplification technique with edge collapse at http://www.melax.com/polychop (read the full article link) - I've ported the code so it runs cross platform on https://github.com/dougbinks/BunnyLOD

I use a variant of progressive meshing, but constrained to preserve edges, normals and ambient occulusion in Avoyd based on Stan Melax's work: https://github.com/dougbinks/BunnyLOD

My triangle reduction in Avoyd, loosely based on Stan Melax's progressive mesh approach checks that the vertex being removed is internal to a set of triangles by checking that the internal angles add up to roughly 2Pi. This also ensures only flat surfaces are reduced, although I also perform normal checks and that vertex ambient occlusion gradients are preserved.

If you use something like libassimp https://github.com/assimp/assimp it will allow you to import these and access the data to generate your Vertex Array Objects then render to a frame buffer object.

If you just want to load other data in a relatively simple fashion, assimp https://www.assimp.org supports loading a bunch of different formats and is pretty easy to use.

The emscripten interface for the assimp library. It runs entirely in the browser, and allows you to import 40+ 3D file formats and access the result in JSON or glTF format. This is not a full port of assimp, but an easy to use interface to access it's functionality.

I ran into what sounds like a similar problem a few months ago. It turns out this was an assimp bug: https://github.com/assimp/assimp/issues/1974

You could give assimp a go (there is a python version) i've used it for 3DS before then you can export to something else. https://github.com/assimp/assimp/blob/master/port/PyAssimp/README.md

I have checked the source code for GLTF for this and it is not handled by Assimp. I created an issue as in their Github.

Assimp is your friend

Have you read the license? It's quite permissive. https://github.com/assimp/assimp/blob/master/LICENSE

There is probably some library more specific to your use case, but you can use assimp to load 3d models in C++.