Posterize an image, fewer colours, flat bands.

To posterize an image is to cut the number of colours it holds, so smooth gradients turn into flat bands, the way a screen print or a concert poster looks. This tool does it in two ways. Levels cuts each of red, green and blue to a set number of steps, from 2 to 16. Palette finds the 2 to 32 colours that fit your picture best and gives every pixel the nearest one. Both show the result live, report how many colours are left, and save a PNG.

How to posterize a photo

1. Drop one or more pictures on the box, click it to choose them, or paste one with Command V or Ctrl V.
2. Pick a start: Stencil (2 levels), Poster (4), Soft (8) or 5 colours. Or choose Levels or Palette and move the slider.
3. Press and drag anywhere on the picture to compare it with the original.
4. Turn on Dithering if the flat bands look too harsh, or Black and white for a grey version.
5. Press a colour chip under the picture to copy its hex code, then press Download the PNG, or Download all as ZIP for many pictures.

Levels: what the numbers mean

With n levels, each colour channel can only take n values, spread evenly from 0 to 255. Every value is moved to the nearest step. Because a pixel has three channels, the most colours that can survive is n times n times n.

Here is one pixel worked through at 4 levels. Take red 200, green 100 and blue 30. Divide each by 255 and multiply by 3: that gives 2.35, 1.18 and 0.35. Round to 2, 1 and 0, then multiply by 85. The pixel becomes red 170, green 85 and blue 0.

Palette: the colours come from the picture

Levels treats every picture the same, so a mostly blue photo wastes steps on colours it does not have. Palette looks at the picture, picks the N colours that represent it best, and uses only those. At 5 colours the palette is made of five colours the picture really holds. The chips under the picture show the exact colours, so you can copy them into a design. Each picture in a batch gets its own palette.

Dithering and black and white

Dithering is off by default. When it is on, the error each pixel makes when it is snapped to a colour is passed on to the pixels beside it (the Floyd-Steinberg method). The result shows a fine dotted pattern that fakes the colours in between. It makes a low colour count look smoother, but it also makes the PNG larger. For halftone dots and ordered patterns, use the dither generator or the halftone generator. Black and white converts to grey first, then cuts the grey to the same number of levels or palette colours.

Which setting for which picture

• Portraits and faces: 4 to 6 levels, or a palette of 6 to 8. Skin keeps its form.
• Logos, screenshots and drawings: a palette. These already hold few colours, so the palette matches them closely.
• A bold poster look: 2 or 3 levels. Expect strong, even harsh, colours: red, green, blue, cyan, magenta, yellow, black and white at 2 levels.
• Smaller files: fewer colours often give a smaller PNG, but a photo with dithering can grow.

Common problems

• The picture looks blotchy. That is how posterizing works. Raise the levels, or switch on dithering.
• The saved file is bigger than the JPG. A PNG keeps every pixel exactly. A flat, few-colour picture still packs well; a dithered photo does not.
• Transparent parts. See-through areas stay see-through in the PNG.
• A GIF only gives one frame. Moving pictures are read as a single picture here.

To make big blocky pixels instead, try the pixel art converter.