How to merge three.js meshes into one Mesh?

Question

Is it possible in Three.js to merge two or more meshes, with different materials?

The solutions I've found, merges geometry only, or just puts the Meshes into one Object3D or Group.

Answer 1

Yes: Kind-of (see the comments attached to the question and this answer post):

var blueMaterial = new THREE.MeshPhongMaterial( {color: 0x0000FF } );
var redMaterial = new THREE.MeshPhongMaterial({ color:0xFF0000 });
var meshFaceMaterial = new THREE.MeshFaceMaterial( [ blueMaterial, redMaterial ] );

var boxGeometry = new THREE.BoxGeometry( 10, 10, 10 );

for ( var face in boxGeometry.faces ) {
    boxGeometry.faces[ face ].materialIndex = 0;
}

var sphereGeometry = new THREE.SphereGeometry( 5, 16, 16 );
sphereGeometry.applyMatrix( new THREE.Matrix4().makeTranslation(0, 5, 0) );

var mergeGeometry = new THREE.Geometry();
mergeGeometry.merge( boxGeometry, boxGeometry.matrix );
mergeGeometry.merge( sphereGeometry, sphereGeometry.matrix, 1 );

var mesh = new THREE.Mesh( mergeGeometry, meshFaceMaterial );
scene.add( mesh );

I went with a cube and a sphere because a box for example wants to know a material id for each of its faces.

http://jsfiddle.net/v49ntxfo/