a dither method converts multi - bit grey level pixel values into an image array of binary level pixel values , wherein the image array is arranged as a raster of scan lines . the method includes the following steps : for each line of grey level pixels , deriving an error value for each grey level pixel of the scan line by finding a difference between a variable threshold value and a grey value of the pixel ; combining the error value with a next - in - line grey level pixel value before deriving an error value therefor ; assigning a first binary value to the pixel if the error value is less than the variable threshold or assigning a second binary value to the pixel if the error value is greater than the variable threshold value ; and deriving the variable threshold values that are compared with grey level pixel values in the scan line , by generating a random value to control generation of the variable threshold value at each pixel site , the random value for each one of a plurality of contiguous scan line evidencing a different value .