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It is nowadays more probable that a print media is captured and shared with a mobile phone than with a scanner. The reasons for photographing the print range from intention of copying the image to simply sharing an interesting add with friends. Watermarking offers a solution for carrying side information in the images, and if the watermarking method being used is robust to the print-cam process, the information can be read with a mobile phone camera. In this paper, we present a print-cam robust watermarking method that is also implemented on a mobile phone and evaluated with user tests. Especially, the lens focusing problem when the picture is captured in a wide angle with respect to the printout is addressed. The results show that the method is highly robust to capturing the watermark without errors in angles up to 60➦ with processing times that are acceptable for real-life applications.

Abstract. In this paper, we propose a print-scan resilient watermarking method which takes advantage of multiple watermarking. The method presented here consists of three separate watermarks out of which two are used for inverting the geometrical transformations and the third is the multibit message watermark. A circular template watermark is embedded in magnitudes of the Fourier transform to invert rotation and scale after print-scan process and another template watermark is embedded in spatial domain to invert translations. The message watermark is embedded in wavelet domain and watermark robustness in both approximation coefficient and detail coefficients is tested. Blind, cross-correlation based methods are utilized to extract the watermarks. The obtained success ratios were at least 91% with JPEG and JPEG200 quality factors of 80-100 and scanner resolution of 300dpi. The BER obtained with the previous settings was less than 1.5%.

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Thus far the research of print-cam robust watermarking methods has focused on finding new methods for embedding and extracting the watermark. However, the capturing process itself, has been neglected in scientific research. In this paper, we propose a solution for the situation when the watermarked image has been captured in a wide angle and the depth of focus of the camera is not deep enough to capture the whole scene in-focus resulting in unfocused areas. The solution proposed here relies on a subfield of computational photography, namely all-in-focus imaging. All-in-focus images are generated by fusing multiple images from the same scene with different focus distances together, so that the object being photographed is fully in focus. Traditionally, the images to be fused are selected by hand from the focal stack or the whole stack is used for building the all-in-focus image. In mobile phone applications, computational resources are limited and using the full focal stack would result in long processing times and the manual selection of images would not be practical. In addition, we propose a method for optimizing the size of the focal stack and automatically selecting appropriate images for fusion. It is shown here that a watermark can still be recovered from the reconstructed all-in-focus image accurately.