The default setting for SIP order is 2. Assuming the distortion can be modeled by typical barrel or pincushion distortion an order of 3 gives much better accuracy. Practical tests prove this.
So I would suggest to set the default SIP order at 3. Mathematically this is also easy to prove. See below.
Han
Standard distortion modelling:
{Xdelta:=Xideal*(k1*r1^2 + k2*r1^4 + ....)
simplify by taking only the first factor
Xdelta:=Xideal* k1*r1^2
Xdelta:=Xideal* k1*(X^2+Y^2)
Xdelta:=k1*X^3+ k1* X*Y^2 X^3 is factor AP_3_0 and X*Y^2 is factor AP_1_2
The default setting for SIP order is 2. Assuming the distortion can be modeled by typical barrel or pincushion distortion an order of 3 gives much better accuracy. Practical tests prove this.
So I would suggest to set the default SIP order at 3. Mathematically this is also easy to prove. See below.
Han
Standard distortion modelling: