posterize
Concise Oxford English Dictionary © 2008 Oxford University Press:
posterize /ˈpəʊstərʌɪz/ (or posterise /ˈpəʊstərʌɪz/)
▶verb print or display (an image) using only a small number of different tones.
– derivatives
posterization noun.
posterization noun.
'posterize' also found in these Oxford entries:

