Three Dimensional Concepts and Display Methods 

 Imagine yourself taking a picture by a camera.  What do you normally do?  You specify a viewpoint and view direction and then set up a view window.  Once that is done you take a snap and the image of 3D scene/object is captured in a 2D film.  Basic idea behind this is projection from 3D to 2D.  Projections are broadly classified under two names (i) Parallel (ii) Perspective.  When objects are projected onto the display plane along parallel lines, we call it parallel projection.  In this case a view direction is specified and image of points on the object are obtained by taking lines parallel to the view direction and finding their intersection with the display plane.  If you have seen an architectural drawing of a building plan, you would have noticed that they show three different plots, top view, front view and side view normally.  This actually is an example of a parallel projection of the building to be constructed.  On the other hand, perspective projection corresponds more closely to the way a human eye perceives a 3D scene.  A viewpoint is specified and points from the object are projected to the view plane by considering lines emanating from the points on the object and converging to the viewpoint.  Intersections of these converging lines with the display plane make the image of the object under perspective projection.