Snap a photo to a handful of flat colours for a screen-print or pop-art look. Choose how many levels, optionally go black and white, and save as a tiny PNG.
Each colour channel normally has 256 possible values. Posterizing rounds every value to the nearest of a small number of evenly spaced levels, so 4 levels means each channel can only be 0, 85, 170 or 255. Smooth gradients turn into stepped bands, subtle shading disappears, and the picture starts to look like a silkscreen print made with a few inks. The tool applies the same lookup to red, green and blue, or to brightness alone when Black and white is on.
A 2400 × 1600 portrait against a plain wall. At 2 levels the face is made of just eight colours and reads like a stencil; at 4 levels it looks like a 1960s screen print with visible tonal steps in the cheeks; at 8 levels it is close to the original with slightly flattened skin. Saved as PNG the 4-level version is 310 KB, down from a 1.9 MB JPEG original, because almost every pixel shares its colour with its neighbours. With Black and white on at 3 levels you get a three-tone stencil ready for Threshold or a vinyl cutter.
Reach for posterize when you want a graphic, printed look: gig posters, t-shirt artwork, stickers, social graphics that need to stand out at small sizes. It is also a cheap way to reduce a photo to a palette before tracing it. It is the wrong tool for making a picture smaller in bytes while keeping it realistic; Image Compressor does that without the bands. For a blocky rather than banded look use Pixelate Effect, and for pure two-tone line art go straight to Threshold.
Levels per channel from 2 to 32; the quick chips cover the useful range. Below 6 the effect is obvious, 8 to 12 is a subtle stylisation, above 16 it is hard to see. Black and white converts to brightness first, then posterizes that, which gives cleaner stencils than posterizing colour and desaturating afterwards. In Output, PNG is recommended; if you must use JPEG, set quality to 95 to limit the ringing along the hard edges.
How many colours will the result have? Up to the number of levels cubed: 2 levels gives 8 colours, 3 gives 27, 4 gives 64. With Black and white on it is simply the number of levels. The section header shows the count as you move the slider.
Why do I see bands in the sky? That is the effect working: a smooth gradient is being snapped to a few steps. Raise the levels to 8 or more to make the steps finer, or accept the bands as part of the poster look.
Should I save as PNG or JPEG? PNG. A posterized image has large flat areas and hard edges, which PNG stores losslessly and very small, while JPEG smears the edges and adds noise back into the flat colours.
Is the photo sent to a server? No. The quantization is a lookup table applied in your browser; nothing is uploaded.
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