Canon Pixma iP90 User Manual page 114

This control appears when you select Detailed Setting from Print
Mode.
Set halftoning method.
Select either Dither or Diffusion ink dot configurations.
Dither is a suitable method to print pictures and graphs where you
wish to clearly define the difference between colors.
Diffusion is a suitable method to print graphics and photographs
where there are subtle color gradations.
(5) Grayscale Printing
Select to print a grayscale version (black and white) of a color
document.