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Chapter Two · Filter (Processors)

Posterize

Reduces the picture to a small number of flat colour steps, like a screen print or a weather map. levels sets how many.

The obvious way to do it, rounding every value to the nearest step, produces a result that looks wrong: the bands bunch up in the shadows and spread out in the highlights. That is because the numbers stored in an image are not proportional to how bright things actually are. They are deliberately bent so that dark tones get more of the available range, which is where our eyes need the precision.

So this effect straightens the numbers out first, does the rounding on values that are proportional to real light, and bends them back afterwards. The bands land evenly. gamma then lets you deliberately push them toward the shadows or the highlights, and antialias softens each band edge by a single pixel so gradients do not come out with jagged contours.

Further reading Posterization Color quantization