Digital watermarking is a key technique for protecting intellectual property of digital media. As a number of methods have been proposed in recent years to embed watermarks in images for various applications, evaluation of watermarking algorithms becomes more and more important. The degradation of watermarked images, which can be measured by signal to noise ratio (SNR) or peak signal to noise ratio (PSNR), is one of the major performance indexes of watermarking algorithms. In this paper, based on the Laplacian distribution model of DCT coefficients, we deduce a theoretical relationship between the scaling parameter in a typical watermarking algorithm and the degradation of watermarked images. Experimental results show that the estimation error of SNR and PSNR is less than 1 dB. Using this relationship, we design an adaptive insertion strategy that can be employed to embed watermarks in natural images with assigned SNR or PSNR.
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