The Posterize command (in the Adjust section of the Image menu) is used to reduce the number of tonal levels in an image. This is useful in creating a silk-screen effect, as in some of Andy Warhol’s pictures.
In the "Posterize" dialog set the new number of levels in the Levels box (the default is 4, the maximum 255). The number you enter is used to give an average for the 256 levels making up each of the color channels. With a low number of levels the effect of posterization is to display conspicuous bands or areas of color, while with a high number the changes are much less obvious. At 255 the image remains the same.
Any changes you make are immediately applied to the image, so you should click on Cancel if you want to abandon the changes and exit the dialog. Otherwise click on OK to keep the changes.
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