Investigate the best approach for refining the interrogation grid

[MattEdwards94]

How should we refine the interrogation grid each iteration?

For a starting grid with spacing 2^n, we require n refinements to get to a minimum window spacing of 2px. (possibly n-1 if we enforce 4px as the minimum spacing).

For n=7, i.e. 128px, even 7 iterations is quite a lot. Furthermore, we have seen that actually we can get very close to the final solution in just 2 iterations.

In this case, how do we get close to the minimum resolution as quickly as possible without 'oversampling'.

A standard 50% reduction would take the WS from 256->128->64->32 over 4 iterations which seems very gradual.
It would still be reasonable to go 256->96->32->16 - just over doubling the number of samples in x and y each time
More aggressive would be 256->64->16 - i.e. quadrupling the number of samples in both x and y each time.

This tells me that I think it would be reasonable to split by 2 refinements each iteration if need be.
This will result in some 'oversampling' in certain areas, but I think this would actually be countered by the fact that certain areas accelerate analysis. Furthermore, I think some areas that might 'unnecessarily' be sampled, might end up being sampled anyway and so isn't completely a waste.