Incremental printing using masks

Apparatus defines a dither mask (DM) and printmask (PM) with corresponding dimensions. In one invention form the DM dimension is not an integral factor or multiple of the PM dimension. The corresponding dimensions differ by at least three pixels, and by a multiple of two pixels. Another invention form has a scanning printhead making multiple passes across a print medium to form swaths of marks, a mechanism to define an offset smaller than at least one of the two dimensions, and a unit to index one mask by that offset between forming of successive swaths. This is valuable if DM and PM are established by preprogrammed circuits (e. g. an ASIC) or algorithms and one dimension is fixed at an integral multiple or factor of the other, with the system being practically incapable of small (or any) changes in the dimensions. Another invention form is a print method that establishes and operates a rendition stage using a DM - and a PM stage using a PM. The two masks have corresponding dimensions, one an integral multiple or factor of the other - and also have an interaction that defines a spatial periodicity within a range to which human vision is sensitive. The method includes extending the period well outside that range, by causing one dimension to act larger, as regards the interaction, preferably by a factor of two or more, ideally eight. o do this one mask is indexed by an offset under half the mask dimensions; the factor is the ratio of one mask dimension and the offset. The period is the minimum common multiple of the enlarged dimension and the other.