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.Watermark visibility

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 AC DCT coefficients

we deduce theoretical relationship between the scaling parameter in some typical watermarking algorithms and the degradation of watermarked images.Experimental results show that the evaluated error of SNR and PSNR is less than 1 dB.