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

We present a method to include watermarks in printed images. With accurate printer calibration, in theory, the same color under different gray component replacement (GCR) strategies should look the same, under specific viewing conditions. 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 our focus. For that, we estimate the RGB value of each pixel and the CMYK values intended to be put onto 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 concerned only with the practical aspects of producing a spatially varying GCR and of robustly estimating which GCR strategy was used at a region. Promising GCR detection results are shown to illustrate the method's potential to watermark printed images.