Meshinspector thickenMesh vs. Python thickenMesh | Achieve same results

[emil-peters]

Hello,

I'm experimenting with offsetting an isosurface mesh and found that the Hollowing/Thickening operation in MeshInspector works perfectly for my use case (see Video A).

However, when I try to reproduce the same operation using the Python bindings, specifically mrmeshpy.thickenMesh, the results differ noticeably. The generated mesh appears to be only an isosurface, and the outer boundary of the mesh isn't preserved as cleanly as with the MeshInspector operation (see Image B).

Am I missing something in my implementation, or is there another function that more closely matches MeshInspector's behavior?

I also have two additional questions about the thickening process, listed below.

I've attached the code I used to reproduce the result, along with the input mesh.

Thank you!

---

Additional Questions

1. When using the mapper in thickenMesh, is it possible to obtain the FaceBitSet corresponding to the inner and outer surfaces separately?
2. Would it be possible to apply a positive offset to both the inner and outer surfaces, but with different magnitudes (e.g., the inner surface offset slightly less than the outer shell)?

---

References

Video A:

Screencast.from.23-10-25.16_02_28.webm

Image B:

Code to Reproduce

shell.zip

import meshlib.mrmeshnumpy as mrn import meshlib.mrmeshpy as mr import trimesh mesh = mr.loadMesh("shell.stl") params = mr.GeneralOffsetParameters() params.signDetectionMode = mr.SignDetectionMode.Unsigned params.voxelSize = 0.15 mesh2 = mr.thickenMesh(mesh, 2, params) mr.saveMesh(mesh2, "thicken_mesh.stl") mesh3 = trimesh.load("thicken_mesh.stl") scene = trimesh.Scene([mesh3]) scene.show(): 

[Grantim]

Hello @emil-peters !

First thing is that "Hollowing/Thickening" tool in MeshInspector actually uses different function that is not voxel based:

mm.thickenMesh is older voxel-based function which is more robust in terms of generating clean result surface and mm.makeThickMesh is new mesh-based function, which is much faster and might be preferable in simple cases but also can potentially generate mesh with self-intersections (this new function is used in Hollow/Thicken tool) https://meshlib.io/documentation/Py/namespacemrmeshpy.html#ac4eec9c588d468891eafe22bb7bc1b77

---

As for additional questions:

1. thickenMesh output map is build like this map[inputmesh_id]->outputmesh_id so offset surface is not mentioned there.
   

   MeshLib/source/MRVoxels/MROffset.cpp

   Lines 315 to 335 in 654e90a

   	Expected<Mesh> thickenMesh( const Mesh& mesh, float offset, const GeneralOffsetParameters& params, const PartMapping & map )
   	{
   	MR_TIMER;
   	auto res = offsetOneDirection( mesh, offset, params );
   	if ( !res )
   	return res;
   	auto & resMesh = res.value();
   	if ( offset >= 0 )
   	{
   	// add original mesh to the result with flipping
   	resMesh.addMeshPart( mesh, true, {}, {}, map ); // true = with flipping
   	}
   	else
   	{
   	resMesh.topology.flipOrientation(); // flip to have inversed offset
   	// add original mesh to the result without flipping
   	resMesh.addMesh( mesh, map );
   	}
   	return res;

outMap = mr.FaceMapOrHashMap() map = mr.PartMapping() map.src2tgtFaces = outMap mesh2 = mr.thickenMesh(mesh, 2, params, map) thickenFaces = mesh2.topology.getValidFaces() for ifId in mesh.topology.getValidFaces(): if map.src2tgtFaces.var.current_type() == "Vector_Id_FaceTag_Id_FaceTag": thickenFaces.set(map.src2tgtFaces.getMap()[ifId],False) else: thickenFaces.set(map.src2tgtFaces.getHashMap()[ifId],False) innerFaces = mesh2.topology.getValidFaces() - thickenFaces print (innerFaces.count(),"+",thickenFaces.count(),"=",mesh2.topology.getValidFaces().count())

As for makeThickMesh function

params = mr.ThickenParams() params.insideOffset = 0.1 params.outsideOffset = 0.2 mesh2 = mr.makeThickMesh(mesh,params) outerFaces = mesh2.topology.getValidFaces() - mesh.topology.getValidFaces() innerFaces = mesh2.topology.getValidFaces() - outerFaces # this block is need if input mesh is not closed and we want to eliminate connector faces numFace = 0 for f in mesh2.topology.getValidFaces(): if numFace == 2*mesh.topology.numValidFaces(): outerFaces.reset(f,mesh2.topology.numValidFaces() - numFace ) break numFace = numFace+1

2. Voxel-based thickenMesh function applies offset in one direction only, but mesh-based makeThickMesh can offset on different amount for inside and outside.

---

Suggestion:
If you need offset in both direction but with shift in one side - most reliable way is using Shell offset mode instead of thickening. While it has uniform offset in both directions you can perform something like this:

params = mr.GeneralOffsetParameters() params.signDetectionMode = mr.SignDetectionMode.Unsigned params.voxelSize = 0.05 positive_offset = 0.1 negative_offset = 0.05 mid_offset = 0.5*positive_offset+0.5*negative_offset single_offset = positive_offset - mid_offset oneSideMesh = mr.offsetOneDirection(mesh, single_offset,params) shell = mr.generalOffsetMesh(oneSideMesh,mid_offset,params)

It will surely provide clean mesh