How to debug a manifold that is not "manifold".

[briansturgill]

In ways it's great that you check everything... but what happens when it fails?
There's no real information provided by status.
Worse, you get returned an empty mesh to look at.
Is there some sort of diagnostic that will explain?

Currently what I do in Python is:

    # import trimesh
    # mesh_output = trimesh.Trimesh(vertices=vertex_list, faces=triangles)
    # trimesh.exchange.export.export_mesh(mesh_output, "/home/brian/Downloads/mesh.stl", "stl")

(I uncomment these out as needed)

Both trimesh and meshlab can detect manifold issues... and in mesh lab you can even see where the problem is,
which is a big help.

In at least one instance trimesh and meshlab both said the mesh was perfectly manifold... but Manifold() was still
returning empty.
I was experimenting with MidPoint triangulation... and manifold seems to have an issue with the point being in the middle
like that. (Fan works fine.)
Is there at least a good definition of manifold (specific to Manifold) running around?

I ask because I'm about to release a polyhedron function to the world... and I'm sure it will be ugly!

Sadly, I can't find an easy way for people to do this.

[pca006132]

Is there at least a good definition of manifold running around?

Yes. https://github.com/elalish/manifold/wiki/Manifold-Library#manifoldness-definition

Note that we only care about the topology, i.e. how the triangles are connected, ignoring the point coordinates, for the manifoldness definition. Also, stl does not preserve manifoldness because you cannot distinguish points with the same coordinate, so you should use formats such as obj to check the mesh.

In ways it's great that you check everything... but what happens when it fails?
There's no real information provided by status.
Worse, you get returned an empty mesh to look at.
Is there some sort of diagnostic that will explain?

The only case this can happen is when users import a mesh into manifold, and only the input can be faulty, so there is no need to return the faulty mesh. We have some internal checks, e.g. https://github.com/elalish/manifold/blob/master/src/manifold/src/properties.cpp#L286, maybe we can expand them a bit to provide more useful diagnostics, but this may not be very helpful as we are not a GUI application...

I was experimenting with MidPoint triangulation... and manifold seems to have an issue with the point being in the middle

Not sure how you implement that, if you are modifying the halfedge data structure, e.g. adding some points, you need to make sure it is consistent after the modification. And I don't think you need to experiment with other triangulation methods, it is much better to focus on improving the general algorithm. #723 shows a way to improve the triangulator algorithmic complexity.

[briansturgill]

@pca006132 Thanks! I, made the stl to obj change in polyhedron comments and in other places in my code.
Fortunately meshlab and my slicer can use obj.
As to the midpoint, non-manifold issue... trimesh wasn't looking at an 'stl', but rather the exact same arrays that manifold was.

As to triangulate. It might not be important to you... but to normal users, one always tries to do as much as
possible in 2D, then extrude and only then does one use 3D. Extrude is the most critical path routine on your system
as that is how people get to 3D initially.

In my case I've written my own extruder, and want a fast triangulation. Fan save me like 40% of the time over triangulate.
Most things that get extruded can use Fan or MidPoint and it is much faster.

I'll take another look at midpoint... maybe the obj format will show me something in MeshLab.

BTW, are there alternatives to MeshLab I should know about?

[elalish]

Netfabb used to be great, but I think it got rolled into an autodesk product now. To find the problems, you just need to pair up all the halfedges and then spit out any that don't have a pair (by index, not position). You might also try 3MF - since it requires our manifoldness definition, there might be some useful tooling out there for checking them.