Only scan image at the center camera ?

Hello,

I am developing an application which use Vuforia library to scan AR image.

My customer want the app only scan AR image if we put the image inside the red square as the picture below. If we put AR image outside the red square, it will not scan.

Any ideal how to do this? Can we set the scan area to center of the camera view or make it only scan if we put the position of the target image at center screen.

Thank you