Detecting spatially varying gray component replacement with application in watermarking printed images

We present a method to include watermarks in printed images. Under specific viewing conditions and accurate printer calibration, the same color printed using different gray component replacement strategies (GCR) will look the same. We spatially vary the GCR along the image in a manner that is not perceptible, and we employ an estimation method to detect such changes. The choice of GCR for a given pixel (or region) comprises an additional information channel that embeds a watermark or hidden information. The challenge is how to detect which GCR was used and that is the focus of this paper. For that, we estimate the RGB values of each pixel and the CMYK values on the paper by scanning the printed page. With that information, we can estimate which GCR strategy was used in a given region and retrieve the watermark message. Instead of focusing on a particular watermarking scheme, we are only concerned with the practical aspects of producing a spatially varying GCR and of robustly estimating which GCR strategy was used in a region. Promising GCR detection results illustrate the potential of the method to watermark printed images.