the third memory 23 the data on the second primary scanning line or after and lying outside of the two straight lines 26 and 27 which originate from the ends of the first primary scanning lines should be moved to the inside thereof whereby a rectangular unit pattern 35 is formed in the third memory 23 as shown in fig9 the positions of the two straight lines which is hereinafter referred to as the joining lines 26 and 27 do not necessarily coincide with the ends of the first primary scanning lines but they can be anywhere as long as they are separate by the repetition length l or n pixels as shown by the double chain line in fig8 the unit pattern producing process is detailed referring to the flowchart of fig2 since the image data is processed by the primary scanning line the variable i representing every primary scanning line is initialized to 0 step s7 1 and incremented by 1 step s7 9 until i gt i max step s7 10 while steps s7 2 to s7 9 are repeated the process executed on every primary scanning line is as follows first image data d i j of the pixels at j p th i to j p th i n on the present object line primary scanning line i are.