title: Rotating a model in 3D: Suggestions for center of rotation
body: <p>I need to allow the user to pan, rotate, zoom in/out of my scene in 3D which uses a parallel projection. Panning and zooming in/out has been pretty straight forward. However, when using the mouse for rotation, I am not sure what to use as the center of rotation. In particular, I am not sure at what depth inside the screen the center of rotation should fall.</p>
  <p>Here is what I am doing right now: Take the centroid of the model and use that as the center of rotation. However, the centroid can at times fall outside the visible area (as a result of panning and the zoom level) and the user (and I myself) find it counterintuitive to rotate the model around a point we cannot see.</p>
  <p>What is considered the standard for user-interaction in such a case? What could be the most intuitive behaviour here?</p>
  <p>PS: I have only one view port (unlike <a href="http://blender3d.org" rel="nofollow noreferrer">Blender</a> and friends).</p>
  <p><strong>Edit:</strong> It would be great if experienced parties could evaluate current practices in packages such as Google SketchUp and <a href="http://blender3d.org" rel="nofollow noreferrer">Blender 3D</a> and post their opinions.</p>