Patent US 6,181,802

A method of encoding invisible identification code into an image, which is highly resistant to degradation across communications links, and which does not require the presence of the original image for decoding, comprises analyzing the image and determining strongly featured regions such as edges, and inserting code into such regions by altering the structure of the image in a predictable manner, as for example a concave elliptical insert centered on and aligned with an edge, which alteration is not visible to the eye. When decoding, area of concavity are determined, and a correlation is performed with a predicted insert function to assess whether code has been inserted. A hardware embodiment is described.