Posterization can also be applied on top of an existing render through the BPR filters. This allows the posterization effect to be modulated by all the filter settings like depth, masking, cavity, etc.
ZBrush has five different posterization filters based on their intensity. Each has its own Step (posterize) slider and opacity.
Because a BPR filter is applied to the final render, its effect can also be applied to a 2D background image or even the panorama image