How to create a custom mesh on THREE.JS?

I've asked this and got the answer:

var geom = new THREE.Geometry(); 
var v1 = new THREE.Vector3(0,0,0);
var v2 = new THREE.Vector3(0,500,0);
var v3 = new THREE.Vector3(0,500,500);

geom.vertices.push(new THREE.Vertex(v1));
geom.vertices.push(new THREE.Vertex(v2));
geom.vertices.push(new THREE.Vertex(v3));

var object = new THREE.Mesh( geom, new THREE.MeshNormalMaterial() );
scene.addObject(object);

I expected this to work but it didn't.

You've added vertices, but forgot to put those vertices into a face and add that to the geometry:

geom.faces.push( new THREE.Face3( 0, 1, 2 ) );

so your snippet becomes:

var geom = new THREE.Geometry(); 
var v1 = new THREE.Vector3(0,0,0);
var v2 = new THREE.Vector3(0,500,0);
var v3 = new THREE.Vector3(0,500,500);

geom.vertices.push(v1);
geom.vertices.push(v2);
geom.vertices.push(v3);

geom.faces.push( new THREE.Face3( 0, 1, 2 ) );

var object = new THREE.Mesh( geom, new THREE.MeshNormalMaterial() );
scene.addObject(object);

The idea is that a Face3 instance references 3 vertices(the x,y,z coords you've added previously to the geometry) by using the indices of the vertices in the list/array. Currently you only have 3 vertices and you want to connect them,so your face references index 0,1 and 2 in the vertices array.

Since you're using a mesh normals material, you might want to compute normals for the geometry. Also, make sure your object can be visible (is not to big or to close to the camera to be clipped out, is facing the right direction - towards the camera, etc.) Since you're drawing in the YZ plane, to see your triangle, something like this should work:

var geom = new THREE.Geometry(); 
var v1 = new THREE.Vector3(0,0,0);
var v2 = new THREE.Vector3(0,500,0);
var v3 = new THREE.Vector3(0,500,500);

geom.vertices.push(v1);
geom.vertices.push(v2);
geom.vertices.push(v3);
                
geom.faces.push( new THREE.Face3( 0, 1, 2 ) );
geom.computeFaceNormals();
                
var object = new THREE.Mesh( geom, new THREE.MeshNormalMaterial() );
                
object.position.z = -100;//move a bit back - size of 500 is a bit big
object.rotation.y = -Math.PI * .5;//triangle is pointing in depth, rotate it -90 degrees on Y
                
scene.add(object);

Update: THREE.Geometry and THREE.Face3 are deprecated: THREE.BufferGeometry is recommended instead.

const geometry = new THREE.BufferGeometry();

const positions = [
0,   0, 0,    // v1
0, 500, 0,   // v2
0, 500, 500  // v3
];

geometry.setAttribute( 'position', new THREE.Float32BufferAttribute( positions, 3 ) );
geometry.computeVertexNormals();

const object = new THREE.Mesh( geometry, new THREE.MeshNormalMaterial() );
scene.add(object);

In short, as opposed to providing vertex positions and Face3 objects with 3 vertex indices now you would use a flat array in x1, y1, z1, x2, y2, z2, ..., xn, yn, zn order (every 3 x,y,z triplets in order define a face).

Additionally, it's possible to compute and provide vertex colours and normals. There are plenty of three.js example to start with, including:

• webgl_buffergeometry
• webgl_buffergeometry_indexed

You can automate your triangulation

For big polygons it can be quite a job to manually add the faces. You can automate the process of adding faces to the mesh using the triangulateShape method in THREE.ShapeUtils like this:

var vertices = [your vertices array];
var holes = [];
var triangles, mesh;
var geometry = new THREE.Geometry();
var material = new THREE.MeshBasicMaterial();

geometry.vertices = vertices;

triangles = THREE.ShapeUtils.triangulateShape( vertices, holes );

for( var i = 0; i < triangles.length; i++ ){

    geometry.faces.push( new THREE.Face3( triangles[i][0], triangles[i][1], triangles[i][2] ));

}

mesh = new THREE.Mesh( geometry, material );

Where vertices is your array of vertices and with holes you can define holes in your polygon.

Note: Be careful, if your polygon is self intersecting it will throw an error. Make sure your vertices array is representing a valid (non intersecting) polygon.