REBOL [ Title: "Posterize" Date: 31-Jan-2013 File: %oneliner-posterize.r Purpose: "A short function for 'posterization' effects on images" Version: 1.0.0 Author: "Vincent Ecuyer" Usage: { 'posterize applies a type of color reduction on images, ignoring the less significative bits of each color channel, the result having flat colors zones instead of gradual transitions. result: posterize value (image!) depth (integer!) where depth is between 1 and 7, with 1 keeping only 8 colors, and 7 with the less noticeable effect. The result is darker so a brightness correction would be appropriate: for the strongest setting, with depth = 1 (-> 8 colors), half the brightness is lost, so applying a VID 'effect like “multiply 255” would do the trick. ; save a posterized version of a picture save/png %img-dest.png posterize load %img-src.png 3 ; directly used in a display view layout [image posterize load http://www.rebol.com/view/demos/nyc.jpg 1 effect [multiply 255]] } Comment: { depth = 0 or less gives a black result (0 bit/color channel), depth = 8 does nothing (8 bits/color channel as the original), but depth > 8 gives a false color effect (but darker so one should compensate with 'multiply or similar) } Library: [ level: 'intermediate platform: 'all type: [tool one-liner function] domain: [graphics] tested-under: [ view 2.7.8.2.5 on [Macintosh osx-x86] ] support: none license: 'public-domain see-also: none ] ] posterize: func [value [image!] depth [integer!] /local t][value and make image! reduce [value/size 255.255.255 / (t: 2 ** (8 - depth)) * t]]