A method of generating a secure image for a security document, comprising: providing a multi-color input image consisting of a plurality of pixels, separating the multi-color input image into a plurality of mono-color base images (14, 15, 17), each pixel in each base image having an intensity value I(x, y) between a minimum intensity value I min and a maximum intensity value I max; for each base image (14, 15, 17), dividing the base image into a plurality of cells, providing at least two two-dimensional threshold functions T 1 (x, y), T 2 (x, y),..., T n (x, y) for each cell, wherein the threshold functions do not intersect each other and define a plurality of intervals between I max and I min, and generating a binary image by, for each cell, comparing the intensity value I(x, y) of each pixel to the at least two threshold functions to determine an interval including the intensity value of the pixel, and setting the intensity value of each pixel to one of a first value and a second value associated with the interval, the first value and the second value being alternatingly associated with adjacent intervals; and generating the secure image by combining all the binary images generated for all the base images (14, 15, 17). providing a multi-color input image consisting of a plurality of pixels, separating the multi-color input image into a plurality of mono-color base images (14, 15, 17), each pixel in each base image having an intensity value I(x, y) between a minimum intensity value I min and a maximum intensity value I max; for each base image (14, 15, 17), dividing the base image into a plurality of cells, providing at least two two-dimensional threshold functions T 1 (x, y), T 2 (x, y),..., T n (x, y) for each cell, wherein the threshold functions do not intersect each other and define a plurality of intervals between I max and I min, and generating a binary image by, for each cell, comparing the intensity value I(x, y) of each pixel to the at least two threshold functions to determine an interval including the intensity value of the pixel, and setting the intensity value of each pixel to one of a first value and a second value associated with the interval, the first value and the second value being alternatingly associated with adjacent intervals; and dividing the base image into a plurality of cells, providing at least two two-dimensional threshold functions T 1 (x, y), T 2 (x, y),..., T n (x, y) for each cell, wherein the threshold functions do not intersect each other and define a plurality of intervals between I max and I min, and generating a binary image by, for each cell, comparing the intensity value I(x, y) of each pixel to the at least two threshold functions to determine an interval including the intensity value of the pixel, and setting the intensity value of each pixel to one of a first value and a second value associated with the interval, the first value and the second value being alternatingly associated with adjacent intervals; and comparing the intensity value I(x, y) of each pixel to the at least two threshold functions to determine an interval including the intensity value of the pixel, and setting the intensity value of each pixel to one of a first value and a second value associated with the interval, the first value and the second value being alternatingly associated with adjacent intervals; and generating the secure image by combining all the binary images generated for all the base images (14, 15, 17).