title: Why can't a perspective camera's LookDirection be straight down?
body: <p>I'm attempting to position my perspective camera 30 units above the origin and pointing straight down.</p>
  
  <p>If I set the <code>LookDirection</code> of the camera to "0,0,-1", however, everything disappears. I have to make it "0.01,0.01,-1" for it to work. Why?</p>
  
  <pre><code>&lt;Window
      x:Class="ThreeDeeTester.Window1"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      Title="Window1"
      Height="300"
      Width="300"&gt;
      &lt;Grid&gt;
          &lt;Viewport3D&gt;
              &lt;Viewport3D.Camera&gt;
                  &lt;PerspectiveCamera
                      Position="0,0,30"
                      LookDirection="0.01,0.01,-1" 
                      UpDirection="0,0,1" /&gt;
                      &lt;!-- LookDirection="0,0,-1" doesn't work...why? --&gt;
              &lt;/Viewport3D.Camera&gt;
              &lt;ModelVisual3D&gt;
                  &lt;ModelVisual3D.Content&gt;
                      &lt;Model3DGroup&gt;
                          &lt;DirectionalLight
                              Color="White"
                              Direction="1,-1,-1" /&gt;
                          &lt;GeometryModel3D&gt;
                              &lt;GeometryModel3D.Geometry&gt;
                                  &lt;MeshGeometry3D
                                      Positions="0,0,10 -5,-5,0 -5,5,0 5,5,0 5,-5,0"
                                      TriangleIndices="2 1 0  2 0 3  4 3 0  1 4 0" /&gt;
                              &lt;/GeometryModel3D.Geometry&gt;
                              &lt;GeometryModel3D.Material&gt;
                                  &lt;DiffuseMaterial Brush="Red" /&gt;
                              &lt;/GeometryModel3D.Material&gt;
                          &lt;/GeometryModel3D&gt;
                      &lt;/Model3DGroup&gt;
                  &lt;/ModelVisual3D.Content&gt;
              &lt;/ModelVisual3D&gt;
          &lt;/Viewport3D&gt;
      &lt;/Grid&gt;
  &lt;/Window&gt;
  </code></pre>