How to calculate the region of a popup(or dialog or dropdown) without having to store reference images???

Is it there an easy way to calculate the region of a recently open popup or dialog after doing an action?

It is easy to calculate a region using two opossite corners for example, but having the reference images updated or captured can add extra complexity to the automated scripts development, so I need a way to infer the region (for example) of the popup by doing some image diff to the screen before opening the popup compared to the state of the screen after the popup was open.

I wouldn't mind to use any low level API to do this or some Java library, the question is do we have access to some Vision operations through Sikuli to accomplish this?

Cheers,
Raul