Posterize & dither

How it works

Posterizing rounds every color to the nearest of a few levels. With four levels per channel a photograph turns into flat patches, like a screen print or a poster, hence the name. Rounding alone makes hard stripes wherever a gradient crosses from one level to the next: the same banding that 8-bit output fights, made huge on purpose.

Dithering turns those stripes back into tones. Before rounding, add a small offset that changes from pixel to pixel. Where a color lies 30% of the way from one level to the next, about 30% of the pixels now round up, and from a normal viewing distance the eye averages them back into the in-between tone. The offset can come from a hash of the pixel position, which looks like grain, or from a small repeating Bayer matrix, which gives the regular cross-hatch of old computer and handheld screens.

Pushed to the limit, two colors and nothing else, the result is the 1-bit look: every pixel is ink or paper, and all the shading lives in the pattern. The Rendering page on banding uses the same trick invisibly, one 8-bit step at a time; here it is the style.