If the matrix 
does not have one of the forms expressed in 3.1.2, the system can then be solved :
if the constraint r=0 is used, the intrinsic parameters are uniquely defined.
if the constraint 
is chosen, there are 2 sets of solutions : they correspond to the 2 solutions 
of a second degree equation. We keep only the one with smallest |r|.
Now consider the degenerate cases.