As we have seen previously, given an infinite homography 
between the images of the same camera, it is possible to calculate its real Jordan decomposition and hence a matrix 
such that 
. Then, it was shown that 
should satisfy eq:kkt. The calibration process consists of solving this equation which can be written as a system of 6 equations with 7 unknowns (a, k, 
, 
, r, 
and 
) :
It is clear that this system cannot be solved just as it is (there is one unknown too many). We must either consider a constraint on one unknown, or add equations from several motions (with non parallel rotation axes).
If we want to calibrate with a single motion or with planar motions (where all rotation axes are parallel), a constraint on the internal parameters must be imposed. Commonly used constraints are either 
or r=0.
