Assessment of camera phone distortion and implications for watermarking

In this paper, the distortion introduced by cameras of smart phones such as iPhone 3G and iPhone 3GS is measured and quantified. In particular, the spatial frequency response (SFR) of a mobile camera phone is measured [1]. A new robustness metric that relates camera phone quality to watermark robustness is defined. Experiments were conducted using watermarked images to verify the predictions made by the robustness metric. The main advantage of defining a robustness metric based on camera quality distortion is that it enables fast and effective modeling of watermark detector performance for a wide range of mobile phones. This is important because ideally the watermarked printed image should be readable by a variety of consumer smart phones. By simulating the watermark detector behavior for a range of mobile devices, important feedback can be provided to the chrominance watermark embedding process. In particular, the SFRbased robustness model provides information on the expected performance of watermark detector for images marked with different watermark density and strengths. Thus, the use of robustness models saves time and cost by reducing the need for several watermarked print runs before finding the optimal balance between watermark visibility and robustness to distortion. In addition, new devices can be quickly qualified for their use in watermark applications.