Use OCR over small part of the screen using MobileVision

I’m trying to build an OCR using Google’s Mobile Vision. However, what I’ve built till now recognizes everything that appears on the screen. I want my OCR to detect only those text that appears in a rectangular box on the UI.
I want the UI to be like like this.

And the run the OCR only on those characters which would lie in the red region.
I’m tired searching online for the solution.