Banding reduction in incremental printing, by spacing-apart of swath edges and randomly selected print-medium advance

Abstract

A printhead makes passes over a printing medium, each pass forming a swath of marks on the medium. In one aspect of the invention, between passes the medium steps by a nonzero distance that varies from step to step. In another aspect, swath edges are spaced away (ideally well away) from each other. In yet another aspect a printer has a reciprocating carriage—to carry a printhead for forming, in each certain multiple of a half-reciprocation, a swath of marks on the medium. Each head includes multiple printing elements, a number of combinations of groups of which are used to print each region of each swath; the invention increases the number of combinations used to print each region. In still another aspect, the step distance is random or randomized. Ideally these aspects are all used together. Preferably (1) step distance varies at every step—e.g. alternating between two values, such as a sixth and a half of swath height, for three-passes; (2) the number N of passes is odd, and the distance varies among values of form (2n−1)/2N, n ranging from 1 through N; (3) banding with the method has twice the spatial frequency of banding with nonvarying step distance; (4) no two swath edges coincide, and the distance is random or randomized; (5) an installed algorithm for accommodating print-medium-advance-directionality error is adapted for step control; and (6) the certain multiple is one half or one full reciprocation, or two full reciprocations.