•Suppose l and l’ are parallel. We can write l=(a,b,c), l’ =
(a,b,c’). l x l’ =
(c’-c)(b,-a,0). This equivalent to (b,-a,0).
•This point corresponds to a line through the focal point that doesn’t intersect the image plane.
•We can think of the real plane as points (a,b,c) where c isn’t equal to 0.
When c = 0, we say these points lie on the ideal line at infinity.
•Note that a projective transformation can map this to another line, the horizon, which we see.
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