You are building an application to scan documents using a mobile phone camera. When a user captures a document, the edges of the document are not aligned with the horizontal and vertical axis of the phone screen.

Let us assume that user takes precaution to ensure that the bottom edge of the document is parallel to the x$-$axis when photo is taken $($as shown in the agure PO below, A$.$B$.$Cand D are the corners of the document in the captured image$)$