Projections - Viewing Transformation
Specified 3-D object in a space, Projection can be explained as a mapping of 3-D object into 2-D viewing screen. Now, 2-D screen is termed as Plane of projection or view plane that constitutes the display surface. The mapping is resolved by projection rays termed as the projectors. Geometric projections of objects are formed via the intersection of lines termed as projectors with a plane termed as plane of projection or view plane. Projectors are lines from an arbitrary point, termed as the centre of projection i.e. COP, by each point in an object. Following figure 1 demonstrates a mapping of point P(x,y,z) on its image P′(x',y',z') in the view plane.
Figure: 1
If the COP that is center of projection is located at finite point in the 3-space, the effect is a perspective projection. If the COP i.e. center of projection is located at infinity, all the projectors are parallel and the consequence is a parallel projection. Following figure 2(a)-(b) demonstrates the diversity between perspective and parallel projections. In following figure 2(a): ABCD is projected to A'B'C'D' on the plane of projection and O is a COP. In the condition of parallel projection the rays by an object converge at infinity, the rays from the object turn into parallel and will have a direction termed as "direction of projection".
Figure 2(b): Parallel projection