Screening techniques by identification of constant grey components

Abstract

Identification of constant grey components in electronic screening by first determining from the continuous-tone input, the maximum value of grey, the minimum value of grey and the average value. The average value is used to determine how many white bits are to be turned on in the halftone dot generated. Since "Moire" problems only occur only from the fluctuating part of the input, the three grey ranges corresponding to the minimum, the maximum and the fluctuating portions of the grey found in the area are then treated separately. The minimum and maximum grey levels identify constant levels within the grey area. That is, every point is at least as light as the minimum and every point is no lighter than the maximum. A template black dot corresponding to the amount of black in the maximum light grey area is generated and correspondingly a template white dot corresponding in size to that for the minimum grey level is generated, and compared to those actual dots created by ordinary halftoning. The ordinary halftone process produces black in at least the area of the template black dot and white in at least the area of the template white dot. The fluctuating grey levels between these two control the remaining dot area as to which is black and which is white.

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