What is the radius of a polyhedron ? (PolyhedronGeometry)

Questions
,
1

Hello,

I want to reproduce this animation with three.js:

gfycat

So I need some tetrahedra. I saw the PolyhedronGeometry which allows to set vertices, but there is a parameter radius. What is it? A polyhedron is entirely defined by its vertices and its faces so I don’t see what is the radius. Is it a scaling?

If this is not adapted I’ll use a BufferGeometry to do the tetrahedra.

2

Ỳ̶̺ḙ̷̎ś̴̱.

1 Like
3

I’ve been able to reproduce the picture except the tetrahedra (i.e. I got the edges fine).

I do:

......
    // an empty tetrahedron
    const fourVertices = [
      0, 0, 0,
      0, 0, 0,
      0, 0, 0,
      0, 0, 0
    ];
    const faces = [
      2, 1, 0,
      0, 3, 2,
      1, 3, 0,
      2, 3, 1
    ];
    let cell = new THREE.PolyhedronGeometry(fourVertices, faces);
    cell.setAttribute(
      "position",
      new THREE.Float32BufferAttribute(fourVertices, 3)
    );

......

and:

......
    for(let i = 0; i < ntetrahedra; i++) {
      let mesh = new THREE.Mesh(cell, cellMaterial);
      mesh.geometry.attributes.position.needsUpdate = true;
      Meshes_tetrahedra[i] = mesh;
      group.add(mesh);
    }
......

and in the animate function:

......
    for(let t = 0; t < ntetrahedra; t++) {
      let mesh = Meshes_tetrahedra[t];
      let tetrahedron = tetrahedra[t];
      let cellVertices = new Array(0);
      for(let i = 0; i < 4; i++) {
        let vi = vertices3d[tetrahedron[i]];
        cellVertices.push(vi.x, vi.y, vi.z);
      }
      mesh.geometry.setAttribute(
        "position", new THREE.Float32BufferAttribute(cellVertices, 3)
      );
      mesh.geometry.computeBoundingBox();
      mesh.geometry.computeBoundingSphere();
    }

Only one face of one tetrahedron appears. What am I missing?

4

I think I have to update the vertices, not the position. But how to do that?

5

I finally switched to BufferGeometry, it works now.

6

Here is the result.

1 Like
7

yes, the idea of PolyhedronGeometry is to help you, but in your case it seems like it was in the way, so there’s no reason to stick with it then