In Figure 1 we
have drawn the image plane as though it were orthogonal to the z-axis
indicating that the optical axis of the camera is
(anti) parallel to . In fact, the effective viewpoint
and the axis of symmetry of the mirror profile z(r)
need not necessarily lie on the optical axis. Since perspective
projection is rotationally symmetric with respect to any ray that
passes through the pinhole , the camera could be rotated
about so that the optical axis is not parallel to the
z-axis. Moreover, the image plane can be rotated independently so
that it is no longer
orthogonal to . In this second case, the image plane
would be non-frontal. This does not pose any additional problem
since the mapping from a non-frontal image plane to a frontal image
plane is one-to-one.
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