Decentering distortion is often not modeled because its contribution is much smaller than radial lens distortion. It is necessary only for the highest accuracy measurements. Many metric cameras come with the radial distortion parameters only and the P1 and P2 (in mm^-2) decentering parameters can be set to zero.
The formulas used by PhotoModeler for decentering distortion are:
dpx = P1 * ( r^2 + 2 * x^2 ) + 2 * P2 * x * y
dpy = P2 * ( r^2 + 2 * y^2 ) + 2 * P1 * x * y
Where P1 and P2 are in mm^-2, r is in mm from the principal point, and x and y are in mm from the principal point.
These corrections are applied to the x,y marked point as shown above.