title: Getting list of created object instances and change their opacity
body: <p>In THREE.js I am trying to access created instances of an object that I got from GLTFLoader in &quot;update()&quot; function and make them semi-transparent. But I have no idea where those instances are. Do I need to register those in some array or something?</p>
  <pre><code>let grassBigGeometry,
      grassBigMaterial,
      grassBigMesh,
      grassBigInstanceNumber = 50;
  
  //BIG GRASS
  loader.load(
      GRASS_BIG_PATH,
      function(gltf) {
          
          const _grassBigMesh = gltf.scene.getObjectByName( 'Grass' );
          
          grassBigGeometry = _grassBigMesh.geometry.clone();
          
          const defaultTransform = new THREE.Matrix4()
              .multiply( new THREE.Matrix4().makeScale( 1, 1, 1 ) );
          
          grassBigGeometry.applyMatrix4( defaultTransform );
          
          grassBigMaterial = _grassBigMesh.material;
          
          grassBigMesh = new THREE.InstancedMesh( grassBigGeometry, grassBigMaterial, grassBigInstanceNumber * 3 );
          
          grassBigMesh.castShadow = true;
          
          grassBigMesh.instanceMatrix.setUsage( THREE.DynamicDrawUsage );
          
          scene.add(grassBigMesh);
          
          var zPos = -90;
          var xPos = -90;
          
          var gCounter = 0;
          
          while(zPos &lt;= 90) {
  
              zPos += 3 + Math.floor(Math.random() * 6);
              
              for ( let k = -1; k &lt; 2; k++){
                  
                      dummy.scale.setScalar( 0.2 + Math.floor(Math.random() * 4) / 10 );//setting random size 40-100%
                      
                      dummy.rotation.y = Math.random() * Math.PI * 2;//random rotation
                      
                      xPos = 60 * k - 30 + Math.floor(Math.random() * 60);//cell size: 60x60
                      
                      dummy.position.set(
                          xPos,
                          0,
                          zPos + Math.floor(Math.random() * 5) / 10
                      );
  
                      dummy.updateMatrix();
                      grassBigMesh.setMatrixAt( gCounter, dummy.matrix );
                      
                      gCounter++;
              
              }
              
          }
          
      },
      undefined, // We don't need this function
      function(error) {
          console.error(error);
      }
  );
  </code></pre>
  <p>I place grass instances at random positions within 9 cells. Now I need to access each grass instance, see it's position and set it to semi-transparent. I don't know how to do either. I tried     <code>dummy.material.opacity = 0.5;</code> within the loop just for testing purposes but even there it doesn't work.</p>
  <p>I tried to <code>console.log(grassBigMesh)</code>, and look into the object, but I couldn't find the list of instances there. Please, advice</p>