In this paper, we introduce a LUT based watermarking method for a halftone image. Watermark bits are hidden at pseudo-random locations of halftone image during halftoning process which is based on LUT method. The pixel values of the halftone image are determined from the LUT entry indexed by both the neighborhood halftone pixels and current grayscale value. The LUT is trained by a set of grayscale images and halftone images. The advantage of LUT method is that it can be executed very fast compared with other watermarking method for a halftone image. Therefore LUT watermarking algorithm can be embedded in a printer. Experiments using real scanned images show that the proposed method is feasible method to hide the large amount of data within a halftone image without noticeable distortion and the watermark is robust to cropping and rotation.
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