A fast hierarchical watermark detector for real-time software or low-cost hardware implementation

In this paper, we develop a spread spectrum-based watermark algorithm for real-time software or low-cost hardware implementation. The developed detector is suitable for devices such as stand-alone watermark readers, cellular phones, and PDAs. These devices have primitive operating systems with limited processing power, memory, and system bandwidth. Our embedder tiles the watermark over the host image to let the watermark be detected from any region in the digital or printed watermarked image as the data is streamed through the device. It also adapts the watermark strength locally to maximize detection and minimize watermark visibility. Consequently, the watermark may be detectable only in few regions of the image that are not necessarily aligned with the original tile boundaries. To avoid a brute-force search, our detector uses a hierarchical search algorithm to quickly zoom into the region with the strongest watermark. This approach permits a real-time software implementation of the detector and reduces the necessary gate count, on-chip memory, and system bandwidth for a hardware implementation. Software simulation results of the developed algorithm indicate that the algorithm is very efficient and the detection results are better than those obtained using a sub-sampled brute-force search.