Disclosed is an original halftone image that is transformed into a linear grid halftone image comprising regularly spaced grid lines (5), the width (y) of which is modulated according to the density step that is to be represented. According to the invention, the width (y) of the grid lines (5) is modulated only on one side such that the grid lines (5) are flat on one side and width-modulated on the other. Disclosed are several examples of embodiments, which demonstrate how the inventive grid line modulation allows effective copy protection.
