1. A method for aligning a modified document and an original document for emphasizing differences, the method comprising: receiving a first bitmap representative of the modified document including a first anchor, and a second bitmap representative of the original document including a second anchor, the first and second bitmaps each being defined by a plurality of pixels arranged in ordered rows and columns from a left side to a right side, each of the pixels having pixel coordinates associated therewith; deriving a set of first vertex coordinates of the first anchor and a set of second vertex coordinates of the second anchor; transforming the first bitmap to a common reference based upon the first set of vertex coordinates; and transforming the second bitmap to the common reference based upon the second set of vertex coordinates; wherein deriving the set of first vertex coordinates further includes: storing in a first set of comprehensive coordinates lists the pixel coordinates of a leftmost pixel matching a trigger value and a rightmost pixel matching the trigger value in every row of the first bitmap; storing in a sample coordinates list a subset of the pixel coordinates in the first set of comprehensive coordinates lists; and assigning average pixel coordinates of the sample coordinates list to a one of the set of first vertex coordinates when attributes of the sample coordinates list satisfy a predefined condition.