In this paper, a semi-fragile watermarking algorithm is proposed to prevent images from being altered, based on dither modulation and linearity of discrete cosine transform (DCT). As the algorithm transforms DCT coefficients in spatial domain using dither modulation and the linearity of DCT, it embeds watermarking information very fast without DCT. The robustness of the pro-posed algorithm against compression proved in a serious of experiments. Furthermore, because a semi-fragile watermark can be embedded into DCT coefficients with only additions and subtractions, watermarking information can be embedded fast with simple computations. As a result, semi-fragile watermark-ing information can be added in a real time to prevent forgeries and alteration in portable devices with limited resources such as digital cameras, digital camcorders and cellular phones.
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