4.3. Perspective Projection   

True perspective requires dividing by depth or z coordinate for every point.
Note how z affects the height or width.

Note what happens when z approaches  0.

Often the scaling of points is done via
      Xs=D*x/S*z         Ys=D*y/S*z

where S is the size of the screen This makes Xs and Ys dimensionless.

Need our eye anywhere in space,
object anywhere in space,
looking anywhere in space.

Note that the viewing screen creates a viewing pyramid with base at screen corners and apex at eye position.

Note we could have object at z = x = y = 0 and eyes elsewhere.