Posterise a photograph

Posterisation divides each channel into a small number of allowed values and moves every incoming value to its nearest step. With five shared levels, a channel can leave the tool only at five positions from dark to light. The result is deterministic, local and reversible only by returning to the original: no later blur can recover the values discarded between steps.

The level count is therefore the actual specification, not a decorative strength slider. Two levels are severe; four to six usually preserve the subject while making the construction visible; sixteen can look unchanged in a textured frame and still show clear bands in a sky.

Shared levels

One count applied to red, green and blue. Neutral pixels remain neutral because all three channels cross their boundaries together.

Per-channel levels

Separate counts for red, green and blue. Their boundaries diverge, deliberately creating coloured bands rather than only light-and-dark contouring.

Pre-dither

A small ordered displacement before the nearest-level decision. It replaces some large flat boundaries with a visible, repeatable texture.

Banding is not a rendering mistake here; it is the visible consequence of asking a continuum to fit a small set of answers. On skin, broad steps can make shape look bruised or dirty because the channel boundaries do not follow anatomy. On skies and product gradients, the same bands may be exactly the graphic effect being sought.

Pre-dither should be chosen by destination size. At small display sizes the eye averages the pattern and the transition feels smoother. At a large print or a close crop, that pattern becomes part of the image. Compare at the size the result will actually be used before exporting.

How many colours does five levels create?

Five shared levels per RGB channel permit up to 125 channel combinations, though a particular photograph normally occupies far fewer. This is not the same as a five-colour indexed palette.

Why use different levels per channel?

Unequal channel counts make the steps separate at different places, creating coloured contour bands. Equal levels preserve greys and look more like a tonal reduction.

What does pre-dither change?

It adds a small ordered offset before quantisation, trading broad abrupt contour lines for a fine repeating texture. It does not add levels; it distributes the error around their boundaries.

• DitherError diffusion and ordered matrices, at palettes from one bit upwards.
• PixelateSquare or hexagonal blocks, each one flat, with optional palette quantisation.
• DuotoneTwo colours and the ramp between them, rebuilt on every change to either end.
• Effects, and why they are not filtersThe one test that sorts a colour grade from a rebuild, applied to fifteen awkward cases.

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