[ptx] Calculate Optimal Size

[Michal]

How exactly works this calculation?

I tried this on dummy image (400px x 400px). I set all parameters to 
zero (except v of course), output projection to rectilinear and FOV of 
result to the same value as FOV of input image. Then I expected optimal 
size to be 400x400. But optimal size varies for different values of v 
parameter.

v=100   optimal size=293x293
v=50    optimal size=374x374
v=20    optimal size=396x396
v=10    optimal size=399x399
v=5	optimal size=400x400

With v=5 I got also wrong optimal FOV of output image (6 horizontal, 4 
vertical).

I think this is a bug or maybe I missed something?

Michal