1. A method of modeling print dots and generating a threshold value matrix for producing a screen form for printing image data, which comprises: determining values of the dots of the screen form by comparing tone values of the image data to threshold values of the threshold value matrix, and generating the threshold value matrix by: (a) setting dots in a screen data field; (b) determining a first position by low-pass filtering the screen data field with at least two low-pass filters having mutually different widths; (c) searching for certain partial patterns of dots at the first position or in a neighborhood of the first position, determining at that location a second position, and setting a next dot at the second position; and (d) entering a next free threshold value at the second position in the threshold value matrix; wherein the print dots are model points with a base surface and a density gradient over the base surface, where the model points are built up from computed points, and a density value according to a function g(x,y) is assigned to a computed point.