[meshes3d] bugs in clipConvexPolyhedronByPlane and intersectPlaneMesh

[joneumann]

matlab version: 23.2.0.2380103 (R2023b) Update 1
matGeom version: bdc8c65

Dear Monsieur Legland,

i tried to revive a code that i wrote ~8 years ago but experienced severals errors, particularly concerning clipConvexPolyhedronByPlane. I tried to go back in history, but since the function has been renamed, the history seems broken somehow.

There, i get an error:

Index exceeds the number of array elements. Index must not exceed 1.

Error in clipConvexPolyhedronByPlane (line 116)
    if minDists(2) < tol

or

Index exceeds the number of array elements. Index must not exceed 1.

Error in clipConvexPolyhedronByPlane (line 99)
        inds(indNews(2)) = Nn + 2;

I looked into this, particulary trying to run the examples, that are in the headers, but they also throw errors.
demo file attached.
I had to change the file extension, because github doesn't let me attach *.m.
I also get 2 errors running the test suite, however, it seems unrelated.

debug_matGeom.txt

Your help is kindly appreciated.

Regards
Johannes Neumann

[oqilipo]

Hi joneumann

Try to replace the function by clipMeshByPlane.

Kind regards

[joneumann]

Dear Oqilipo,

yeah, that does the trick if i triangulate all my polyhedra before i use clipMeshByPlane.
Thank you for the hint.
No general issue with this, i just have to call minConvexHull once more after clipping.
Need to change my code a bit to do that.
But that means support for clipping polyhedra is broken atm, right?

[dlegland]

Hi joneuman,

and thank you for detailed report, this always helps!
The functions clipMeshByPlane.m and clipConvexPolyhedronByPlane.m have different development histories, but also different use cases:

• clipMeshByPlane.m is more recent (and a priori more efficient),
• clipConvexPolyhedronByPlane.m can handle also non-triangle meshes, but is currently limited to convex meshes (hence the name...)

I think it should be possible to have one function that could manage all cases, but we need to implement it!

I have been checking the tests for clipConvexPolyhedronByPlane.m, but could not get any error... Do you have some minimal working example of the problem?

It seems the function intersectPlaneMesh.m has a a problem, I'll investigate.

Best,
David