title: How to update geometry of mesh which divides it into two equal parts and also both part have different materials?
body: <p>I have loaded .obj using OBJLoader2 and also with its .mtl , now when user click on one of Mesh, then i want to change mesh geometry such like that it divides into two equal parts and also have different material for them.</p>
  
  <pre><code>//this.currentobj represents the user clicked mesh.
  let geometry = this.currentobj.geometry;
  geometry.clearGroups();
  geometry.addGroup( 0, Infinity, 0 );
  geometry.addGroup( 0, Infinity, 1 );
  geometry.addGroup( 0, Infinity, 2 );
  geometry.addGroup( 0, Infinity, 3 );
  let material0 = new THREE.MeshBasicMaterial({color: 0xff0000});
  let material1 = new THREE.MeshBasicMaterial({color: 0x444444});
  let material2 = new THREE.MeshBasicMaterial({color: 0x111111});
  let material3 = new THREE.MeshBasicMaterial({color: 0x555555});
  var materials = [ material0, material1, material2, material3 ];
  let mesh = new THREE.Mesh(geometry, materials);
  this.scene.add(mesh);
  </code></pre>