Digital halftoning with spiral dots

A method of transforming a continuous-tone image to the halftone raster image comprising:

• a plurality of halftone dots including:
• image pixels defining a first arc or a plurality of first arcs that together define a first spiral;
• and
• non-image pixels defining a second arc or a plurality of second arcs that together define a second spiral,
• wherein the halftone raster image creates an illusion of a continuous-tone image on a printed copy, the plurality of halftone dots are regularly tiled at a distance between centers of neighboring halftone dots from 50 μm to 400 μm, and
• the first arc, the first spiral, the second arc, and the second spiral each have a length and/or thickness which is determined by local densities of the continuous-tone image, the method comprising:
• transforming the continuous-tone image to the halftone raster image using at least one threshold tile such that:
• in highlights and midtones of the halftone raster image, a number of image pixels increases as a result of increasing the length and/or thickness of the first arc or the first spiral;
• and
• in shadows of the halftone raster image, a number of image pixels increases as a result of decreasing the length and/or thickness of the second arc or the second spiral.