Lines not parallel to view plane or receding parallel lines
appear to converge at some point called the
*vanishing point* on the view plane.
Lines parallel to principal axes (i.e. the world
coordinate axes) converge to a
*principal* vanishing point. Different types
of perspective projection are named after the finite
number of vanishing points. A perspective projection can
have **1**, **2**, or **3** principal vanishing points.

One point perspective occurs when the projection plane is parallel to two principal axes. Conversely, when the projection plane is perpendicular to one of the principal axis, one point perspective occurs. Receding lines along one of the principal axis converge to a vanishing point.

Even though change in eye position
or tilt of head (i.e. direction of **U** and **V** )
affects the vanishing point , the view is
still one point perspective.

If the projection plane is parallel to one of the principal axes or if the projection plane intersects exactly two principal axes, a two-point perspective projection occurs.

If the projection plane is not parallel to any principal axis, a three-point projection occurs

*From P. Asokarathinam -- see details*
*27.11.1996*