But note that other choices for the value of the third component are equally valid. Kanatani [Kanatani 1992] sets the third component to be equal to the focal length of the camera, and this has advantages which he describes.

Ideal points have their third component equal to zero by convention. It is possible to define a coordinate frame in which an ideal point has a non-zero value in its third component, but this is rarely encountered so it is not worth digressing to discuss it in this short appendix.

The definition of which elements are dual to one another depends on the projective space - points are dual to lines on the projective plane. However, points are dual to planes, and lines are dual to lines, in projective 3-space.

Bob Fisher
Fri Nov 7 12:08:26 GMT 1997