Error with CalcPrincipalMomentsAndMaybeAxesOfInertia in Simple Example

[cohnt]

What happened?

I have a single obj model that I'm trying to load into the world, but I'm getting the following error message when I process the associated model directives:

    models = ProcessModelDirectives(directives, plant, parser)
RuntimeError: CalcPrincipalMomentsAndMaybeAxesOfInertia(): Unable to calculate the eigenvalues or eigenvectors of the 3x3 matrix associated with a RotationalInertia.

I tried printing out the matrix whose eigenvalues were being computed, and all entries were nan.

This gist has a simple reproduction of the bug.

Version

No response

What operating system are you using?

No response

What installation option are you using?

No response

Relevant log output

No response

[mitiguy]

FYI: I checked the geometry in the file test.obj with one of my CAD programs.
It displayed something strange -- picture attached. It also displays NAN for inertia properties. If the geometry is poorly formed, perhaps we could issue a better error message?

[cohnt]

I've confirmed that fixing the direction of the faces no longer leads to an error. New test.obj attached below.

Having a better error message would be very helpful in this case -- I would have never thought that having faces in the wrong direction would lead to inertia errors.

g test

v 0.000000 0.000000 0.500000
v 0.000000 0.000000 -0.500000
v 0.000000 1.000000 0.500000
v 0.000000 1.000000 -0.500000
v 1.000000 0.000000 0.500000
v 1.000000 0.000000 -0.500000

f 1 5 3
f 2 4 6
f 1 3 2
f 2 3 4
f 3 5 4
f 4 5 6
f 5 1 6
f 6 1 2

[mitiguy]

@cohnt PR #21929 is meant to produce a better error message. Feel free to provide guidance or comments here or there. As of now, the error message is something like: "CalcSpatialInertia(): The calculated volume of a triangle surface mesh is 0 whereas a reasonable positive value was expected. The mesh may have bad geometry, e.g., the winding (order of the vertices) of some faces do not produce outward normals."

[cohnt]

Yes, that error message is very helpful. Thank you!