Abstract: Aiming at the existent algorithms’ defects in transparency and robustness,a new robust image watermarking algorithm based on fractional Fourier transform and spread transform dither modulation was presented in this paper.First,lifting wavelet transform is performed on the original image.The lowpass subband is segmented into non-overlapping blocks,and each block is transformed by the fractional Fourier transform.Then,the low-frequency coefficients of the amplitude information are chosen to construct the host vectors depending on frequency.In the process of embedding watermark,spread transform is performed on the host vectors to obtain the projected host vectors,and then the binary watermark is embedded using dither modulation.A minimum distance decoder is used to decode the watermark.The experimental results show that the proposed algorithm can obtain better transparency as well as robustness against common image attacks such as JPEG compression,filtering,noisy,cropping and so on.Compared with previous similar schemes,the presented method achieves higher practicality and reliability.

Key words: Digital watermarking,Fractional Fourier transform,Spread transform dither modulation,Robustness