Posterize Image

Two different operations

Posterizing an image can mean reducing the number of distinct colours, which is the common meaning and produces a flat, illustrated look, or reducing the number of tonal steps per channel, which produces visible banding in gradients. They are related but they are not the same, and confusing them leads to a result that looks worse than either intended. Reducing colours affects hue and how many distinct tones exist. Reducing levels affects how smoothly light transitions to dark.

NoteReducing colours changes the hues. Reducing levels only changes the smoothness of the transitions.

Few colours is not a small change

Dropping to eight or sixteen colours removes most of the information in a photograph. Skin becomes a handful of flat tones, a sky becomes a single colour, and gradients disappear entirely. That is exactly what makes the effect useful for a poster or an illustration, and exactly why it destroys a photograph that was meant to look realistic. The number you choose is the difference between a recognisable photograph and a graphic, and there is no middle setting that preserves realism while looking posterised.

• Eight to sixteen colours gives a strong poster or print look.
• Thirty or more starts to look like a slightly odd photograph.
• Skin and gradients are the first things to break down.
• Choose deliberately whether you want a graphic or a photograph.

NoteThere is no setting that posterises while keeping a photograph realistic.

Where the effect genuinely suits an image

A posterised image works when the subject has large flat areas to begin with, because those need very few colours and nothing important is lost. Architecture, graphic design, a logo, a simple illustration, and a product shot on a plain background all fall into that group. A portrait with skin tones, a landscape with a wide sky gradient, and a night scene with subtle shadow detail are the worst cases, because those are exactly the areas that depend on many subtle colour steps.

• Suit posterize: architecture, graphics, logos, simple illustrations.
• Suit posterize: product shots on plain backgrounds.
• Avoid posterize: portraits, where skin needs many subtle tones.
• Avoid posterize: skies and wide gradients, which band badly.
• Avoid posterize: night scenes with subtle shadow detail.

Dithering, which is the fix for banding

Reducing levels creates visible bands, particularly in a smooth gradient, because there are not enough steps to describe the transition. Dithering solves this by adding a small amount of noise to break up the banding, so the eye averages the dithered pixels and perceives a smooth transition. It is the same principle as the dithering in early digital printing, and it is what makes a low level reduction look acceptable rather than broken. Without it, a strong level reduction on a gradient looks like a printing error.

• Level reduction causes visible banding in smooth gradients.
• Dithering adds small noise to break up the bands.
• The eye averages the noise and perceives a smooth transition.
• The same principle as dithering in early digital printing.
• Without dithering, a strong reduction looks like a printing error.