1. a method comprising: (a) providing digital image data that represents an image; (b) applying a digital watermark to the digital image data to produce watermarked digital image data; (c) applying a print-scan distortion transformation wherein the print-scan transformation approximates the effect of printing and scanning on pixel values of the watermarks digital image data to the watermarked digital image data to produce transformed watermarked digital image data that improves the quality of the digital image data to be printed; (d) retrieving a characteristic of the watermark as represented by the transformed watermarked digital image data produced at step (c); (e) printing an image on the basis of the watermarked digital image data produced at step (b); (f) scanning the printed image to produce scanned image data; (g) retrieving a characteristic of the watermark as represented by the scanned image dab produced at step (f); and (h) comparing the characteristic retrieved at step (d) with the characteristic retrieved at step (g).