19
Determine true sign of ty and compute
remaining rotation parameters.
7.  Select an object point P whose image coordinates
     (u,v) are far from the image center. 
8.  Use P’s coordinates and the translation and rotation
     parameters so far to estimate the image point
     that corresponds to P. 
     If its coordinates have the same signs as (u,v),
     then keep ty, else negate it.
9.  Use the first 4 rotation parameters to calculate
     the remaining 5.