Function dither 

pub fn dither(darkness: u8) -> Rows

Bayer ordered dither of a grey darkness from 0 (white) to 255 (black): black where 4 × order + 2 < darkness, so 64 is Base::Dither25, 128 Base::Dither50 and 192 Base::Dither75 exactly, and the levels between fall evenly.