Thin prism distortion coefficients added to the calibration model. 

Dear Sir or Madam, 

I added the thin prism distortion coefficients to the calibration model, being the distortion model: 
Xd = Xu *(1+K1*r2+K2*r4+K3*r6)/(1+K4*r2+K5*r4+K6*r6) +  
(2*P1*Xu*Yu+P2*(r2+2*Xu^2))     +S1*r2+S2*r4;
Yd =Yu *(1+K1*r2+K2*r4+K3*r6)/(1+K4*r2+K5*r4+K6*r6) + (P1*(r2+2*Yu^2)+2*P2*Xu*Yu)    +S3*r2+S4*r4;

I put a new flag CV_CALIB_THIN_PRISM_MODEL, to use the thin prism distortion parameters in the minimization, and also the flag CV_CALIB_FIX_S1_S2_S3_S4 to fix them. 

I have done it because for my camera the results are improved by using the thin prism coefficients.
Hoping to be interesting for you, 
Yours sincerely, 
Aritz.