A number of novel watermarking applications, and improvements to watermarking methods, are disclosed. Included are techniques for encoding printed circuit boards and street signs with watermarks, deterring credit card fraud and controlling software licensing using watermarks, registering collectibles via watermarks, encoding the margins of printed pages with watermarks, and using watermarks to convey extra information in video by which fidelity of the rendered video may be improved.One particular arrangement is a method in which a PDA, wristwatch, or other portable device with a display screen presents an image that depicts a proprietor of the device, and also includes a machine-readable identifier (e.g., a watermark or barcode). This image is sensed by a separate device, such as a webcam or a camera-equipped cell phone. The sensing device can then take an action based on the identifier (e.g., linking to an email account or to a web site that corresponds to the person depicted on the display screen).