Application of blurring filters to improve detection of invisible image watermarks

Abstract

Earlier, we presented a highly robust invisible watermarking method having a payload of one bit - indicating the presence or absence of the watermark. Other invisible watermarking methods also possess this property. Our method, by design, places a major portion of the scant energy of the watermark into the low spatial frequencies of the watermarked image while leaving the higher spatial frequencies, contributed by the original image, largely unaltered. In this paper we will show that application of blurring filters to a presumed watermarked image before attempting watermark detection can improve the probability of detection. Blurring filters, because they suppress high spatial frequencies, are generally quite destructive of image quality. However, for a watermark which has dominant low frequency content, the application of a blurring filter can serve to improve the statistical environment for watermark detection and thereby improves detection probability. This is especially true for detections attempted from scans of printed images. The halftone screening process used in preparing images for printing adds significant high frequency noise to the printed image, and that noise is detrimental to watermark detection. Application of a rudimental blurring filter to a watermarked image before detection will be demonstrated showing that increased amounts of blurring improve the probability of watermark detection. However, the amount of blurring can not be increased without limit, and the amount of beneficial blurring will be shown to be related to the amount of low-pass filtering used in constructing the watermark.