As I am unable to understand formula/theory/details of this function details::focalsFromHomography () ? Can someone help me understanding or getting focals form Homography ?

Hello, I am unable to understand formula used in this function **details::focalsFromHomography ()** [here]. (http://docs.opencv.org/2.4/modules/stitching/doc/autocalib.html) Can someone help me in getting proper theory behind formula used?