1. A method creating digital watermarks in grayscale images, comprising: receiving the first image to be watermarked; determining the first watermark to be embedded in the first image; embedding the first watermark into the first image by halftoning the first image using a three-dimensional threshold array having a phase shift value as an input and a first halftone structure defined by the spatial vectors V a1 (x a1 , y a1 ) and V b1 (x b1 , y b1 ); outputting a first watermarked grayscale image containing the first digital watermark embedded with the first halftone structure; receiving the second image to be watermarked; determining the second watermark to be embedded in the second image; embedding the second watermark into the second image by halftoning the second image with a three-dimensional threshold array having a phase shift value as an input and a second halftone structure defined by the spatial vectors V a2 (x a2 , y a2 ) and V b2 (x b2 , y b2 ), wherein the interior angle between the two vectors V a1 (x a1 , y a1 ) and V b1 (x b1 , y b1 ) of the first halftone structure is equal to the interior angle between the two vectors V a2 (x a2 , y a2 ) and V b2 (x b2 , y b2 ) of the second halftone structure, the vector amplitude |V a1 (x a1 , y a1 )| of the first vector of the first halftone structure is equal to the vector amplitude |V a2 (x a2 , y a2 )| of the first vector of the second halftone structure, and the vector amplitude |V b1 (x b1 , y b1 )| of the second vector of the first halftone structure is equal to the vector amplitude |V b2 (x a2 , y b2 )| of the second vector of the second halftone structure; and outputting a second watermarked grayscale image containing the second watermark embedded with the second halftone structure.