In halftone imaging using rotated halftone dot cells, the determination of correspondences between microdots and threshold matrix cells is simplified by leaving the threshold matrices which overlie each halftone dot cell unrotated. Threshold decisions for each microdot in a given horizontal row of a grid proceed in sequence. Each increment along a microdot row within a halftone cell is a unit increment along a horizontal axis of the threshold matrix. Hence, within each halftone cell, two integer additions are sufficient to derive an address of the threshold matrix cell associated with a microdot given the address of the threshold matrix cell associated with the previous microdot.