1. A method of determining a portion of a document corresponding to a captured image, comprising: receiving a captured image of a portion of a document, wherein the document has a pattern that uniquely identifies different positions in the document and wherein the portion of the pattern in the captured image is at least partially obscured; determining obscured areas of the document in which the pattern is at least partially obscured; comparing the captured image with determined obscured areas of the document; and identifying the position of the portion of the document corresponding to the captured image.