Reducing the processing time of the hierarchical watermark detector when applied to unmarked images

Abstract

In this paper, we improve the performance of the hierarchical detector we proposed in [1] for real-time software or low-cost hardware implementation. Although the original hierarchical detector is faster than sub-sampled brute force-base detector when processing marked images, it unnecessarily continues to process unmarked images looking for a watermark that is not present. This processing is time-consuming; hence, it represents a significant deployment obstacle. The improved detector, however, avoids most of the processing of the unmarked areas of an image by exploiting the presence of a reference signal usually included with the embedded watermark. This reference signal enables the detector to synchronize the image after it has been subjected to a geometric transformation (scaling, rotation, and translation). The improved detector refrains from searching an image area any further whenever the level of the reference signal is very weak or the estimated scale factors and rotation angles associated with this reference signal are not consistent among the processed blocks within the same layer in the hierarchy. The proposed detector has been implemented, and the experimental results indicate that the proposed detector is computationally more efficient with unmarked images, while achieving a detection rate similar to that of the original hierarchical detector.