Hemesh Neighbors Mystery

amnon.owed

in Contributed Library Questions • 1 year ago

Hey everyone,

I'm experimenting with the awesome Hemesh library. I am using the getNeighborFaces() function in the HE_Face class. However it seems this function is not returning the most recent state of affairs. Even when there are no other faces left in the mesh, this method still returns faces. How can I solve this so I get the actual neighbor faces (which in this example - after removing all the other faces - I expected to be none)?

Run this code and the mystery will be printed to your console.

Code Example

import wblut.hemesh.core.*;
import wblut.hemesh.creators.*;
void setup() {
HE_Mesh mesh = new HE_Mesh(new HEC_Cube().setEdge(100));
// put all the faces of the mesh in an arraylist
ArrayList <HE_Face> faces = mesh.getFacesAsArrayList();
println("The mesh has " + faces.size() + " faces.");
// get a single face (the first one)
HE_Face face = faces.get(0);
// put all it's neighbor faces in an arraylist
ArrayList <HE_Face> neighbors = face.getNeighborFaces();
println("The selected face has " + neighbors.size() + " neighbors.");
// remove all the faces except the first from the mesh
for (int i=faces.size()-1; i>0; i--) {
HE_Face f = faces.get(i);
mesh.remove(f);
}
// put all the (remaining) faces of the mesh in an arraylist
faces = mesh.getFacesAsArrayList();
println("The mesh has " + faces.size() + " face.");
// get a single face (the first -and now only!- one)
face = faces.get(0);
// put all it's neighbor faces in an arraylist
neighbors = face.getNeighborFaces();
println("The selected face still has " + neighbors.size() + " neighbors!");
println("How is this possible? Where are these neighbors coming from?");
exit();
}

Replies(3)

frederikw..

Re: Hemesh Neighbors Mystery

1 year ago

Ok, good question.

First off, remove() isn't doing what you're thinking it does. It doesn't remove a face from a mesh, it removes it from the datastructure. This isn't the same since the face is still referred to by its halfedges which you haven't removed. As long as anything is referring to an object, it still exists in Java... So after remove() the face is not directly accessible (like when getting the numberOfFaces(), but it still indirectly accessible through the halfedges (like when getting the neighbors)).
Remove should be used with care because it will invalidate your datastructure ("external faces").

Use deleteFace() instead. This will take care of thi problem. Alternatively you could use remove and then call cleanUnusedElementsByFace() and then capHalfedges() to make the datastructure consistent again...

Cheers,

F.

amnon.owed

Re: Hemesh Neighbors Mystery

1 year ago

Excellent. Clear explanation and easy solutions. I will use deleteFace() instead.

rbrauer

Re: Hemesh Neighbors Mystery

1 year ago

I am iterating through verts in a geodesic like so: for(HE_Vertex cur : test.getVerticesAsArray()) {...} and calling test.removeFaces(cur.getFaceStar()). After, I try calling cleanUnusedElementsByFace(), collapseDegenerateEdges(), and capHalfedges() in that order. I also tried without collapseDegenerateEdges() and using deleteFace() for each face in the star. In all cases it mostly does what I want, but the validate(...) method says it is invalid and after exiting the sketch and changing setLevel(...) on the geodesic creator it hangs on subsequent runs.

I create my mesh like this:

test =
    new HE_Mesh(
      new HEC_Geodesic().setRadius(500).setLevel(3)
    );

Should I be concerned about the validate(...) results.

Edit:
I think the hanging was related to this problem which I believe I solved. It hasn't been hanging today at least. I am still wondering about the validation though.

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