1. A method of embedding a digital watermark in a digital image to be printed on an object comprising: generating an image to be printed based on the digital image, including converting the digital image to a halftone image; and embedding an auxiliary signal in the digital image so that the auxiliary signal is substantially imperceptible, yet machine readable; and encoding a message in the auxiliary signal that includes halftone information, the halftone information being automatically readable from a printed version of the image to enable checking whether measured attributes of the printed version of the image correspond to the halftone information.