Posterize an image — reduce it to a few flat colour levels

How to use Image Posterize

1. Choose one or more images.
2. Set Levels per channel from 2 to 32. The default is 4.
3. Leave Save as on PNG, or choose another format.
4. Press Apply and download the posterized image.

What it does and when to use it

Posterizing gives the bold, flat look of a printed poster, a screen print or a pop-art portrait. It is also a handy way to plan a painting or a paper-cut piece: the result shows where each tone area starts and ends.

At 2 levels each channel is either full or empty, so a photo turns into strong primary and secondary colours. At 4 to 6 levels it still looks like the photo but flatter. At 16 or more the change is subtle and mainly shows in smooth gradients.

For two tones only, black and white, use Image Threshold. To shift the colours of the bands afterwards, try Image Hue Rotate, or map them to two colours of your choice with Image Duotone.

How it works

Every channel value v (0–255) is replaced using a 256-entry lookup table:

output = round(v ÷ 255 × (n − 1)) × 255 ÷ (n − 1)

where n is the number of levels. With n = 4 the only allowed values are 0, 85, 170 and 255. Red, green and blue are each snapped separately; transparency is not changed. The image is processed in a background worker in your browser and never leaves your device.

Worked examples

These values come from the tool’s own formula.

• 4 levels, a warm brown pixel (200, 100, 50). 200 snaps to 170, 100 to 85 and 50 to 85, giving (170, 85, 85), a dusty red.
• 4 levels, a sky-blue pixel (100, 150, 200). It becomes (85, 170, 170), a flat teal.
• 2 levels, the same sky blue. 100 is below the halfway point so it becomes 0, while 150 and 200 become 255: (0, 255, 255), pure cyan. Middle grey (128, 128, 128) becomes white at 2 levels, because 128 is just past halfway.

Limits and tips

• There is no dithering, so bands have hard edges. That is the intended effect.
• Noise in dark areas can turn into speckles. A light blur first gives smoother bands.
• The same level count applies to all three channels.