Rectify a rectangle to get area
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I need to get the area of a known object inside a scene to get the distance from that. The problem is rectifying it so that the area is independent from the angle. 
I'm using opencv (on Android) with some java code that is equivalent to this:

http://docs.opencv.org/doc/tutorials/features2d/feature_homography/feature_homography.html#feature-homography

In other words: how do i get the area of the object observed perpendicularly from that distance given the H matrix.

Thank you in advance and sorry for my poor english... :)