title: Lines represented as long thin triangles are not visble in 3D?
body: <p>I am trying to draw a 3D line in wpf and I have this xaml code:</p>
  
  <pre><code>&lt;Grid&gt;
      &lt;Viewport3D x:Name="ViewerViewport"
                  RenderOptions.BitmapScalingMode="HighQuality"
                  Focusable="True" Grid.RowSpan="2"&gt;
  
          &lt;ModelVisual3D x:Name="Model"&gt;
  
          &lt;/ModelVisual3D&gt;
  
          &lt;!-- Camera --&gt;
          &lt;Viewport3D.Camera&gt;
              &lt;PerspectiveCamera x:Name="Camera"
                                 Position="0,0,0"
                                 LookDirection="0,1,0"
                                 UpDirection="0,0,1"
                                 FieldOfView="100"
                                 FarPlaneDistance="10"
                                 NearPlaneDistance="0.1"/&gt;
          &lt;/Viewport3D.Camera&gt;
  
      &lt;/Viewport3D&gt;
  
  &lt;/Grid&gt;
  </code></pre>
  
  <p>and this c# code:</p>
  
  <pre><code>public MainWindow()
  {
          InitializeComponent();
          var ModelsGroup = new Model3DGroup();
           ModelsGroup.Children.Add(this.AddLine(new Point3D(0, 0, 100), new Point3D(0, 100, 100),"line 1)"));
           ModelsGroup.Children.Add(new AmbientLight(Colors.White));
          Model.Content = ModelsGroup;
  }
  </code></pre>
  
  <p>and line creation code:</p>
  
  <pre><code>  private Model3D AddLine(Point3D startPoint, Point3D EndPoint, string name)
      {
          SolidColorBrush brush = new SolidColorBrush(Colors.Black);
          var material = new DiffuseMaterial(brush);
          var mesh = new MeshGeometry3D();
          mesh.Positions.Add(startPoint);
          mesh.Positions.Add(EndPoint);
          mesh.TriangleIndices.Add(0);
          mesh.TriangleIndices.Add(1);
          mesh.TriangleIndices.Add(0);
          return new GeometryModel3D(mesh, material);
      }
  </code></pre>
  
  <p>but it doesn't show any line in output? </p>
  
  <p>What is wrong with this?</p>
  
  <p>I know that there are some 3d libraries that can do this easily, but I like to learn how to do it in WPF and then investigate how to do this using libraries (such as helix3d)</p>