Importance

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Abstract

Halftone dots are formed pixel by pixel onto a photosensitive film of a size P.times.P in an improved manner by using a screen memory storing thresholds, each of which corresponds to each of m.times.n sub-cells of a halftone dot area of the size S.times.S. In accordance with the invention, four table memories are prepared by respectively storing values ##EQU1## with y.sub.n being the main scanning direction addresses and x.sub.n being the sub-scanning direction addresses. Values read from the first table memory and values read from the second table memory are summed to obtain the sub-scanning reading addresses of the screen memory, while the values read from the third table memory and the values read from the fourth table memory are summed to form the main scanning reading addresses of the screen memory. At this junction, the values (1) to (4) to be stored in each of the table memories are permitted to be residues, taking advantage of the characteristic that the values are periodic. Additionally, the memory capacity required for the operation can be permitted to be smaller by taking advantage of the characteristic that the values of interest form an arithmetic series.

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First Claim