Goldberg Polyhedra

Goldberg Polyhedron

Within Babylon.js a Goldberg polyhedron (GBP) is formed as a dual of an icosahedron based geodesic polyhedron and its vertices mapped onto a sphere. The two parameters m and n used in forming the geodesic base also determine the arrangement of faces for the GBP.

Fig 1 - The red grid is the dual of the blue grid and vice-versa.

On this page we explain how to create a GBP directly. More information about GBP Mathematics and the first development stage of coding a GBP are available in the documentation workshop.

A GBP can only be created using MeshBuilder

A GBP consists of 12 pentagonal faces and a number of hexagonal faces determined by m and n. The option parameter when creating a GBP has different properties than those for CreateGeodesic or for CreatePolyhedron. However additional properties and methods, to those of a standard mesh, are available for a Goldberg mesh. These allow individual, or groups of pentagonal and hexagonal faces to be colored, textured or built upon giving a hexagon-grid planet shaped world.

The world consists of 12 poles, the pentagonal faces plus a number of hexagonal faces. Around each of the poles there is a group of hexagonal faces that are closer to one pole than the others. These are the unshared faces of the world. Depending on the value of the second parameter, n, there may be zero or more faces that are equidistant from two or even three poles. These are the shared faces.

Fig 2 - Poles are black, unshared faces are colored and shared faces are white.

The faces are stored in the following order

• the poles 0 to 11
• the unshared faces for pole 0, then for pole 1, pole 2 etc
• the shared faces come at the end

MeshBuilder

usage :

const goldbergPoly = BABYLON.MeshBuilder.CreateGoldberg("goldberg", options, scene); //scene is optional and defaults to the current scene

Additional Properties and Methods

Properties

Over and above the features of a standard mesh there are a number of read only features stored in the property goldbergData object.

Access to each follows the following example patterns

const goldbergPoly = BABYLON.MeshBuilder.CreateGoldberg("goldberg", options);
const nbFaces = goldbergPoly.goldbergData.nbFaces;
const centerOfFace32 = goldbergPoly.goldbergData.faceCenters[32];

The following are all the properties of goldbergData

The vector properties faceCenters[face], and faceXaxis[face], faceYaxis[face] and faceZaxis[face] can be used as a frame of reference to place meshes on the given face.

Methods

Colors

Groups of faces can be colored using

goldbergPoly.setGoldbergFaceColors(colorArray); //resets face colors whether the mesh is updatable or not
//or
goldbergPoly.updateGoldbergFaceColors(colorArray); //resets face colors only when the mesh is updatable

where colorArray is an array of elements of the form [start face, end face, Color4]

For example

const colorArray = [
  [18, 18, new BABYLON.Color4(1, 0, 0, 1)], // color face 18 red
  [26, 37, new BABYLON.Color4(0, 1, 0, 1)], //color faces 26 to 37 inclusive green
];

PG: Color Faces

PG: Pick and Color

PG: Adjacent Faces

PG: Random Adjacent Faces

Textures

Groups of faces can have their texture changed by using

goldbergPoly.setGoldbergFaceUVs(uvArray); //resets face UVs whether the mesh is updatable or not
//or
goldbergPoly.updateGoldbergFaceUVs(uvArray); //resets face UVs only when the mesh is updatable

where uvArray is an array of elements of the form [start face, end face, center, radius, angle]

center is a Vector2 with 0 ≤ x, ≤ 1 and 0 ≤ y ≤ 1 giving the relative center of a pentagon/hexagon, within a image, of given radius and set at the given angle. The image used should be square to prevent distortion of the texture.

For example

const uvArray = [
  [18, 18, new BABYLON.Vector2(0.25, 0.75), 0.25, 0], // updates the uvs for face 18 to match the vertices of the green hexagon in Fig 3
  [26, 37, new BABYLON.Vector2(0.625, 0.37), 0.37, Math.PI / 2], //updates the uvs for faces 26 to 37 to match the vertices of the red hexagon in Fig 3
];

For the poles pentagons are used to match the uvs.

Fig 3 - Areas of Image to Use as Face Textures

Fig 4 - Texture Map to Apply to Different Faces

In the following playground different areas of the texture map from Fig 4 are applied to the 12 polar regions and to the shared faces

PG: Different Textures

Fig 5 - Same Texture for the Hexagonal Faces In the following playground the same area is applied to all the hexagonal faces and a blank area to the poles.

PG: Angling Textures

Placing Meshes

A mesh can be placed on a face using

goldbergPoly.placeOnGoldbergFaceAt(mesh, face, position); //position is a Vector3

The position is relative to the center of the face and the axes, faceXaxis, faceYaxis and faceZaxis

For example placing a box mesh on face 32

const height = 2;
const width = 0.1;
const depth = 0.08;
const box = BABYLON.MeshBuilder.CreateBox("box", { width: width, depth: depth, height: height });
const position = new BABYLON.Vector3(0.53, height / 2, 0.34);
goldbergPoly.placeOnGoldbergFaceAt(box, 32, position);

Meshes should be sized accoring to the size of the face. To keep a mesh within a face values for position.x and position.z should be between around ±radius of face * √3

PG: Placing Meshes

Faces Related to a Pole

Since the faces are stored in the following order, the poles 0 to 11, the unshared faces closest to pole 0, then to pole 1, pole 2 etc and finally the shared faces, those equidistant from two or more poles, this can be used to find face numbers related to a particular pole.

For pole 0 ≤ k &lt 12 you can determine the face number for the unshared faces closest to pole k using

const faceNumb = goldbergPoly.relatedGoldBergFace(k, s); // 0 <= s < nbFacesAtPole

The unshared faces related to a pole are numbered in an anti-clockwise spiral around the pole.

For the shared faces you use

const faceNumb = goldbergPoly.relatedGoldBergFace(n); //0 <= n < nbSharedFaces

missing out the second parameter. For n = 0 the face number returned is that of the first shared face, n = 1 that of the second shared face and so on.

Importing An Exported Goldberg Mesh

Although a Goldberg mesh is a very specialized mesh Babylon.js supports the export and import of a Goldberg mesh using standard export and import methods.

Example

A Goldberg mesh is exported to file.babylon.

BABYLON.SceneLoader.ImportMeshAsync("", "PATH TO FOLDER", "file.babylon").then((result) => {
  const goldbergPoly = result.meshes[0];
});

PG: Import as a Goldberg Mesh

Further reading

Icospheres

Learn how to create icospheres in Babylon.js.

Geodesic and Goldberg Polyhedra Code Design

The Math used to build Geodesic and Goldberg Polyhedra

Geodesic and Goldberg Polyhedra Mathematics

The Math used to build Geodesic and Goldberg Polyhedra