Digital watermarking using phase-shifted stoclustic screens

#1. Field of Invention

#This invention is directed to a system and method for embedding and automatically detecting invisible digital watermarks in halftone images, or any image which uses microstructures to simulate a continuous tone image.

#2. Description of Related Art

#The preferred embodiments of this invention will be described in detail, with reference to the following figures, wherein:

#FIG. 1 is a halftone image containing an embedded invisible or embedded digital watermark according to this invention;

#FIG. 2 shows the embedded watermark retrieved using the method and apparatus according to this invention;

#FIG. 1 shows an image 100 embedded with an invisible watermark in accordance with this invention. After the watermark retrieval process according to this invention, the embedded watermark, here the letter “T” 110, becomes visible, as shown by the image 100′ in FIG. 2.

#FIGS. 3-5 illustrate the effect phase shifting has on halftone patterns. FIG. 3 shows a checkerboard halftone pattern created by a regular 45-degree cluster halftone screen. FIG. 4 shows another checkerboard pattern created by a halftone screen having four slightly stretched columns 150 in the middle section. Due to the pitch difference between the regular columns 130 and the slightly stretched columns 150, the phase, which represents the transition between white and black, has changed from “in phase,” ie., zero phase shift, on the left to “opposite phase,” ie., π phase shift, on the right. FIG. 5 illustrates the result as FIG. 3 is overlaid with FIG. 4. FIG. 5 clearly illustrates the gradual disappearance of the checkerboard pattern, scanning from left to right in the image, which is proportional to the amount of phase difference between FIGS. 3 and 4.

#Therefore, if a portion of an image is encoded with π phase-shifted clusters and immediately surrounding these π phase-shifted clusters are clusters which vary from π phase-shifted to zero phase-shifted, by overlapping a checkerboard pattern of the same spatial frequency, a high contrast watermark defined by the portion with the phase shift is produced.