Halftone screen tiles

An apparatus comprising processing circuitry, the processing circuitry configured to:

• obtain a first halftone screen tile having a first halftone pattern, wherein the first halftone pattern includes amplitude modulated line screens comprising a plurality of dot centers around which elliptical dots are formed according to a matrix of threshold values, the elliptical dots having a first orientation such that an axis of the elliptical dots are at an angle of α relative to a reference direction;
• obtain a second halftone screen tile having a second halftone pattern, wherein the second halftone pattern includes amplitude modulated line screens comprising a plurality of dot centers around which elliptical dots are formed according to a matrix of threshold values, the elliptical dots having a second orientation such that an axis of the elliptical dots are at an angle β relative to the reference direction, wherein β=(90°−α);
• and
• determine a halftone screen having a predetermined spatial arrangement of the first and second halftone screen tiles, wherein the relative placement of the first and second halftone screen tiles in the predetermined spatial arrangement provides a signature for the halftone screen which when applied to image data provides a printed output in which the signature is discernible,
• wherein the angle of the axis of elliptical dots determines the angle of the lines in the amplitude modulated line screen and α and β have values causing the lines in the amplitude modulated line screen of the first halftone screen tile and the lines in the amplitude modulated line screen of the second halftone screen tile to form continuous lines across adjacent halftone screen tiles, and
• wherein α is 37° and β is 53°.