Reduce Image Colors

Reducing the number of colours gives an image a flat, poster-like look and makes PNG and GIF files much smaller. Screen printing, embroidery, vinyl cutting and paint-by-number designs also need a small, fixed set of colours. Choose "best N colours" to let the tool pick the palette that fits your picture, or posterize to equal steps per channel, add dithering if gradients band, and get a palette PNG plus a list of every colour with its share of the pixels.

How to use this tool

1. Choose one or more images.
2. Pick "best N colours" and the number of colours, or posterize and the levels per channel.
3. Turn on Floyd–Steinberg dithering for photos with smooth gradients; leave it off for flat artwork.
4. Download the PNG and the palette swatch, and copy the hex codes from the table.

Formula

posterize: value′ = round(value ÷ s) × s with s = 255 ÷ (levels − 1); best N: the palette comes from clustering the image's colours, and each pixel takes its nearest palette colour

The best-N palette is built by gifenc's pairwise-nearest-neighbour quantiser, which groups colours into 5-6-5 bit bins (4-4-4 bins for very colourful images, which keeps it fast; 4-4-4-4 when the image has transparency, so semi-transparent pixels keep an alpha value). Each palette entry is the average of the real colours in its group. Dithering passes each pixel's rounding error to its neighbours with weights 7/16, 3/16, 5/16 and 1/16. Results with up to 256 colours are written as palette PNGs at 1, 2, 4 or 8 bits per pixel.

Example

Posterizing with 2 levels keeps each channel at 0 or 255, so a blue pixel (38, 99, 234) becomes pure blue (0, 0, 255) and an orange (244, 158, 12) becomes yellow (255, 255, 0) — at most 2³ = 8 colours. With 4 levels the steps are 0, 85, 170 and 255, so the same blue becomes (0, 85, 255).