1. A method for generating a dither mask using a local stochastic technique for use for the halftoning of grey scale images by a pixel by pixel comparison of the images against the dither mask, the method comprising the step of: generating the dither mask by producing a sequence of dot profiles, each dot profile of the sequence corresponding to a grey scale level, wherein the dither mask is generated using a stacking constraint with an incremental grey scale level such that for each dot profile of the sequence the same pixels are selected compared to the preceding dot profile and the increment in the or each dot profile is realized by selecting at least one additional pixel, a position of each additional pixel being determined dependent on positions of previously selected pixels using the local stochastic technique taking account of an aspect ratio between a dot pitch in a predetermined direction and a direction perpendicular thereto when rendering images.