Ahmed Louchene and Ammar Dahmani
mark embedding, image watermarking
In this paper, the authors present a digital watermarking method for still image. The hybrid method is based on the application of discrete wavelet transform (DWT), log-polar mapping (LPM) and phase correlation. To be robust against compression, the watermark is embedded in the discrete Fourier transform (DFT) magnitude of the low pass (LL) sub-band obtained by applying the DWT. Moreover, the inverse log-polar mapping (ILPM) approximation, of the positions chosen in the LPM domain of the low-pass sub-band DFT magnitude, is applied to determine where to insert the watermark to ensure the robustness against geometric transformations rotation, scaling and translation (RST). The phase correlation of the LPM of the original image and the LPM of the watermarked image, allows calculating the displacement caused by the geometric transformations (RST). The DWT is used to take advantage of the human visual system (HVS) characteristics and obtaining the optimal place for embedding the watermark, meanwhile the Fourier–Mellin transform (FMT) provides a transformed domain which is invariant to RST. To avoid computing the ILPM for the whole image, the ILPM approximation is applied. In this case, the image quality is preserved and smaller correlation coefficient for under marked images. Experimental results demonstrate that the proposed method is robust against RST, added noise, JPEG compression and the combination of these attacks.
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