title: Three.js transparency / disparition
body: <p>I'm new to threejs and I'm trying to make a simple 3d model.
  Nevertheless, I've got some transparency / disparition issue since I've started to play with opacity.</p>
  
  <p>the important part of my code, is here:</p>
  
  <pre><code>    var cylJaun = new THREE.MeshNormalMaterial({color: 0xFFFF00, opacity: 1});
      var cylBleu = new THREE.MeshNormalMaterial({color: 0x0000FF, opacity: 0.5 });
  
      var cylJaun1 = new THREE.Mesh(new THREE.CylinderGeometry(50,50,50,100,1,false),cylJaun);
      var cylJaun2 = new THREE.Mesh(new THREE.CylinderGeometry(50,50,50,100,1,false),cylJaun);
      var cylJaun3 = new THREE.Mesh(new THREE.CylinderGeometry(50,50,50,100,1,false),cylJaun);
  
      var cylBleu1 = new THREE.Mesh(new THREE.CylinderGeometry(70,70,200,100,1,false),cylBleu);
  
      cylJaun1.position.y -= 60; 
      cylJaun3.position.y += 60; 
  
      group.add(cylBleu1);
  
      group.add(cylJaun1);
      group.add(cylJaun2);
      group.add(cylJaun3);
  
      scene.add(group);
  </code></pre>
  
  <p>As you can see, I try to put 3 cylinders into a fourth.The problem is that some of those 3 cylinders disappear when my object is rotated within a specific range.</p>