Modified spread-transform dither modulation

According to the correlation between adjacent blocks of the carrier image, two algorithms, called STDM-CO-MW and STDM-PCO-MW, are proposed. By using the quantization step calculated from the previous secondary block to modulate the latter secondary block, STDM-CO-MW solves the problem of the difference of quantization step between embedding and decoding. In order to raise the correlation between adjacent secondary blocks, STDM-PCO-MW exchanges the pixels between the previous secondary block and the latter one before embedding.As a result STDM-PCO-MW is more robust. Thanks to the quantization step calculated above based on modified Watson model, we find that the quantization step of our algorithms varies linearly with the brightness of the carrier image adaptively .From the results of numerical simulation, it is obvious that our proposed algorithms are robust to JPEG compression, Gaussian noise and gain attack. Compared with AQIM and general STDM based on Watson model, our algorithms have a great improvement in performance.