title: Usage of "Don't Clear" in "Clear Flags" property of Camera
body: <p>In Unity's <a href="http://docs.unity3d.com/Manual/class-Camera.html" rel="noreferrer">Camera</a> component, there is a property <em>Clear Flags</em> which allows to choose from four options: Skybox, Solid Color, Depth Only and Don't Clear.</p>
  
  <p>As documentation says: </p>
  
  <blockquote>
    <p><strong>Don’t clear</strong></p>
    
    <p>This mode does not clear either the color or the depth buffer. The
    result is that each frame is drawn over the next, resulting in a
    smear-looking effect. This isn’t typically used in games, and would
    more likely be used with a custom shader.</p>
    
    <p>Note that on some GPUs (mostly mobile GPUs), not clearing the screen
    might result in the contents of it being undefined in the next frame.
    On some systems, the screen may contain the previous frame image, a
    solid black screen, or random colored pixels.</p>
  </blockquote>
  
  <hr>
  
  <p><em>"This isn't typically used in games and would more likely be used with a custom shader"</em> </p>
  
  <p>So my question is :</p>
  
  <p>How to use it in a custom shader and What effects can be achieved by using it?</p>
  
  <p>Has anyone ever used it or has a good explanation about the basic concept.</p>
  
  <p>Thanks </p>