title: Transparent GeometryModel3D
body: <p>I need to create a transparent GeometryModel3D.<br />
  Like this:<br /><img src="https://i.stack.imgur.com/t7nXs.png" alt="Like this:"></p>
  
  <p><br />
  I tried this code:</p>
  
  <pre><code>var m = new Model3DGroup();
  var panelsGeometry = MyMeshGeometry3D;
          Brush br = new SolidColorBrush(Colors.Red);
          br.Opacity = 0.5;
          m.Children.Add(new GeometryModel3D(panelsGeometry, new DiffuseMaterial(br)) { BackMaterial= new DiffuseMaterial(br) });
  </code></pre>
  
  <p><br />
  But as a result I get this:<br />
  <img src="https://i.stack.imgur.com/fZ1LJ.png" alt="enter image description here"></p>
  
  <p><br />
  What am I doing wrong? Thanks.</p>