Principle Vanishing point - Perspective Projections
Assume that line 1 and l2 be two straight lines parallel to each other that are also parallel to x-axis. If the projection of line 1 and l2 here call them l'1 and l'2 are appears to meet at a point (point at infinity), then the point is termed as a Principle vanishing point with respect to the x-axis. As the same we have Principle vanishing point w.r.t. the y-axis and z-axis.