title: Clipping in clipping coordinate system and normalized device coordinate system (OpenGL)
body: <p>I heard clipping should be done in clipping coordinate system.
  The book suggests a situation that a line is laid from behind camera to in viewing volume. (We define this line as PQ, P is behind camera point)
  I cannot understand why it can be a problem.</p>
  
  <p>(The book says after finishing normalizing transformation, the P will be laid in front of camera.)</p>
  
  <p>I think before making clipping coordinate system, the camera is on original point (0, 0, 0, 1) because we did viewing transformation.
  However, in NDCS, I cannot think about camera's location.</p>
  
  <p>And I have second question.
  In vertex shader, we do model-view transformation and then projection transformation. Finally, we output these vertices to rasterizer.
  (some vertices's w is not equal to '1')</p>
  
  <p>Here, I have curiosity. The rendering pipeline will automatically do division procedure (by w)? after finishing clipping.  </p>