Image Posterize Tool
Reduce an image to a chosen number of tonal levels or colours, with and without dithering, and see the file size saving that colour reduction produces.
Last reviewed by the Radiatus Cloud team
Need this done properly for your business?
Radiatus delivers secure cloud, DevOps & compliance engineering.
Fewer colours, smaller files, visible banding
Reducing the number of distinct colours in an image is how PNG and GIF achieve their compression: a palette of 16 colours needs 4 bits per pixel instead of 24. The cost is banding, where a smooth gradient becomes a series of visible steps, and it is most obvious in skies, skin tones and any large soft area. The number of levels is a direct trade between file size and how visible those steps become.
Dithering trades banding for noise
Dithering scatters the quantisation error into neighbouring pixels, so a boundary between two levels becomes a mixture rather than a hard line. The eye blends it at normal viewing distance and the banding disappears, replaced by a fine texture. On a photograph that texture is usually preferable to the bands; on flat graphics it adds noise to areas that were genuinely flat, and makes the file larger rather than smaller because the noise does not compress.
Levels per channel is not the same as total colours
Posterising to 4 levels per channel gives 4 cubed, or 64, possible colours rather than 4. This confuses people constantly, because the control says four and the result has sixty-four. Reducing to a genuine palette of N colours is a different operation requiring colour quantisation, which chooses the N most useful colours for that particular image rather than dividing each channel evenly.
Related tools
- EXIF Viewer — Extract hidden EXIF data (Camera, GPS, Settings) from images.
- Favicon Generator — Generate favicons and app icons at every required size from one source image, with the exact HTML to reference them.
- Placeholder Image Generator — Generate placeholder images at any size with custom colours and label text. Produced in your browser as downloadable files, no external hotlinking.
- Image Resizer — Resize images in your browser with aspect ratio control and format conversion. Files are processed locally and never uploaded.
Frequently Asked Questions
How many levels do I need?
For a photograph, 6 to 8 per channel is usually indistinguishable from the original at normal viewing sizes. Below 4 the banding becomes a visible style rather than a compression artefact.
Does posterising reduce file size?
For PNG and GIF substantially, since fewer distinct colours compress better and can use a smaller palette. For JPEG it can make files larger, because the hard edges between levels are exactly what JPEG compresses badly.
Should I use dithering?
On photographs, usually yes: the fine texture is less noticeable than the bands. On flat graphics and illustrations, no, since it adds noise to areas that were genuinely flat and prevents them compressing.
Why does 4 levels give 64 colours?
Because the levels apply to each channel separately, so four levels of red times four of green times four of blue is sixty-four combinations. Reducing to a genuine palette of four colours is a different operation entirely.
What is the difference from colour quantisation?
Posterising divides each channel into even steps regardless of the image. Quantisation chooses the most useful colours for that specific image, so 16 quantised colours look far better than the 8 that even division would produce.
Privacy & Security
Everything runs in your browser; nothing is uploaded.
How to Use
Upload an image and choose how many levels to reduce it to.
Disclaimer: This tool is provided "as is" without warranty of any kind. Results are for educational and utility purposes.
Related Tools
EXIF Viewer
ImageExtract hidden EXIF data (Camera, GPS, Settings) from images.
Favicon Generator
ImageGenerate favicons and app icons at every required size from one source image, with the exact HTML to reference them.
Placeholder Image Generator
ImageGenerate placeholder images at any size with custom colours and label text. Produced in your browser as downloadable files, no external hotlinking.