Tonal compensation for graphic security features

摘要

A printed image includes a plurality of sample gray scale regions. A method of determining authenticity of the printed image includes scanning the printed image to produce pixel data that corresponds to the printed image. A gray scale level is detected for each of a plurality of subsets of the pixel data. Each subset corresponds to a respective one of the sample gray scale regions. The detected gray scale levels are used to define an inverse transformation, which is applied to the pixel data. The inverse-transformed pixel data is processed to detect a security feature of the printed image.