Projective camera can be described by a linear-fractional function f : < 3 ! < 2 as: f(x) = 1 ct x + d (ax + b); dom f = fx : ct x + d > 0g; with a 2 < 23; b 2 < 2 and rank a ct = 3: essentially, the domain of f consists of points in front of the camera. the 34matrix p = a b ct d ; is called the camera matrix and has rank 3. since f is invariant with respect to scaling, i. e., it does not change if we were to replace a; b; c; d with ka; kb; kc; kd for any constant k 6= 0, we can normalize the parameters and assume that kck2 = 1. the number ct x+d is then the distance of the point x from the plane fz : ct z + d = 0g. this plane is called the principal plane. the point xc = a ct b d