Dither, quantise and trap in Paint by Zaweba

Reducing an image to a handful of colours is part craft, part maths. Done well it gives you crisp retro graphics, tiny files and prints that hold together. Done badly it gives you blotches. Paint by Zaweba has a dedicated command for it, Image > Dither and Quantise, and a companion print tool, Image > Trap, which deals with a different problem that appears when inks don't line up perfectly.

Quantising: choosing the colours

Quantising is picking the limited set of colours the image will use. Paint by Zaweba offers adaptive, web, uniform, greys, black and white, and your own swatches as the source.

Adaptive looks at the image and builds a palette to suit it, which is usually best for photos. Web and uniform give fixed palettes that don't depend on the picture. Greys and black and white are for monochrome output. Swatches is the interesting one for designers: it restricts the image to colours you already keep, so a set of illustrations can share a house palette exactly.

Dithering: hiding the steps

Once colours are limited, smooth gradients turn into bands. Dithering scatters pixels of neighbouring colours so the eye blends them into in-between shades. Paint by Zaweba offers Floyd-Steinberg, Atkinson, Jarvis-Judice-Ninke, Bayer and blue-noise dithering, and they have quite different characters.

Floyd-Steinberg is the classic error diffusion method, a good all-rounder that keeps detail. Jarvis-Judice-Ninke spreads the error further, giving smoother results with a softer look. Atkinson diffuses only part of the error, so highlights and shadows stay clean and contrast stays high, which is why it is associated with early Mac graphics. Bayer uses an ordered pattern, producing the regular crosshatch texture of old games. Blue noise scatters dots without visible patterns, and tends to look the most natural at small sizes.

Try more than one at 100 percent zoom. The differences are obvious up close and disappear when zoomed out, so judging at the wrong scale leads to the wrong choice.

When to use it

Pixel art, game sprites and retro posters are obvious uses. So are small web graphics where a limited palette cuts the file size, and images bound for e-paper or thermal printers that can only show a few tones. It is also a creative effect in its own right: a portrait reduced to eight colours with Atkinson dithering has a distinctive look no filter quite matches.

Work on a copy. Quantising throws colour away permanently, so keep the full image as your .ptx master and quantise a duplicate.

Trapping for print

Trap solves a separate problem. On a printing press, the separate ink plates never line up quite perfectly, so thin white gaps can appear where two colours meet. Image > Trap spreads the lighter colours' inks slightly under the darker ones, so a small misregistration shows overlap instead of paper.

It matters most for files with hard edges between solid colours, such as logos, labels and flat illustration printed in CMYK. Photographs with soft transitions rarely need it. If you are unsure, ask the printer: some apply trapping themselves in their workflow, and trapping twice is not helpful.

Putting them together

The two tools often meet in screen and spot printing. You quantise to the few inks a job allows, dither to keep tone, and trap so the colours register cleanly. Paint by Zaweba keeps both in the Image menu, offline and free, which makes it a handy place to prepare small print runs as well as pixel art.