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Additional file 1. Data declaration.

Additional file 3.

This paper aims at digital watermark which is a new popular research topic recently, presents some methods to embed digital watermark based on modifying frequency coefficients in discrete wavelet transform (DWT) domain. First, the present progress of digital watermark is briefly introduced; after that, starting from Pitas's method and discarding his pseudo random number method, the authors use a digital image scrambling technology as preprocessing for digital watermarking. Then the authors discuss how to embed a l-bit digital image as watermark in frequency domain. Finally another digital watermarking method is given in which 3-D DWT is used to transform a given digital image. Based on the experimental results, it is shown that the proposed methods are robust to a large extent.