Image Posterize
Reduce a photo to N levels per channel for that flat poster-art look, with optional Floyd-Steinberg dithering and per-channel level control.
How to use
- Drop or pick an image.
- Slide Levels down — 4 is a good starting point for the poster look.
- Toggle dithering on if the bands look too harsh.
- Unlock channels for asymmetric, duotone-style results.
About this tool
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Quantises each RGB channel independently into a chosen number of evenly-spaced levels — 2 levels gives a screen-print look, 8 a vintage photo print, 16 a barely-noticeable smoothing. Floyd-Steinberg error diffusion is optional: leave it off for hard flat banding, turn it on for retro-game style stippling that hides the banding. Unlocking per-channel levels lets you produce duotone-style effects (e.g., 2 red levels, 8 green, 2 blue).
The channels are quantised separately, which is why the number of colours multiplies rather than adds. Two levels per channel is eight colours, not six, and four levels per channel is sixty-four. That also means the palette is a fixed grid of points in the colour cube rather than a selection of colours from your image, so a photo of a sunset and a photo of a forest posterised to the same setting land on exactly the same set of colours.
Error diffusion trades one artefact for another rather than removing it. Floyd-Steinberg pushes each pixel's rounding error onto its not-yet-processed neighbours, so the banding becomes a fine noise that the eye averages back into the original tone. The noise is real detail as far as a compressor is concerned, so a dithered PNG is usually several times larger than the flat version, and a dithered image resized later smears the pattern into visible mush.
Flat banding is not always the defect. Screen printing, vinyl cutting and any process with a fixed ink count wants hard edges between levels, and dithering there produces speckle the process cannot reproduce. Turn diffusion off when the output is headed somewhere physical.
Gradients are the hardest input. A smooth sky is exactly where evenly-spaced levels are most visible, because the bands land as wide stripes rather than following anything in the picture. Raising the level count fixes it and costs the effect you were after.
Frequently asked questions
What's the difference vs the Image Pixelate tool?
Pixelate reduces spatial resolution (chunky pixels), Posterize reduces tonal resolution (fewer distinct shades). You can stack them — pixelate first, then posterize — for a true low-bit look.
Why does dithering make my file bigger?
PNG compression works by finding runs of identical bytes. Dithering breaks those runs into pseudo-random noise, which compresses much worse. For the smallest file, leave dithering off; for the best perceived quality at low levels, leave it on.
How many levels should I pick?
Two per channel gives the hard screen-print look, four to six keeps recognisable shading while still reading as poster art, and above about twelve the effect stops being visible on a photograph. The useful thing to know is that levels are per channel, so four levels each on red, green and blue is up to sixty-four colours in the result, not four.
The result looks worse after saving as JPEG. Why?
Because posterising produces exactly what JPEG handles worst: large flat areas separated by hard edges. JPEG works in blocks and smooths within them, so it adds a faint ripple along every boundary it was supposed to keep sharp. Save posterised images as PNG — the flat areas also compress extremely well there, so the file is usually smaller than the JPEG as well.
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