Method intended for preventing forgery or alteration of a printed or engraved surface

摘要

A 'fragile' signature in the form of a numerical watermark is incorporated in a part or whole of the document. The watermark has asymmetric modulation of amplitude of a visible or invisible component. To detect the presence or absence of the signature the surface has to be of uniform color. Pre-treatment of the image is carried out in order to eliminate visible printed or engraved data which does not contain the watermark. During searching for the signature the image defining the original surface is predicted from the image of the signed surface. The signature can also be printed so that it is visible on the surface. During searching for the signature the numerical watermark is digitized by a scanner, or portable device. The non visible property of the ink is used to form the asymmetric modulation. Different signatures can be hidden in different regions of the surface.