Dither an Image to Two Colours

Floyd–Steinberg · ordered Bayer · Atkinson · custom ink and paper

Dithering fakes shades of grey using only two colours, by scattering pixels in a pattern the eye averages out. It's how the original Macintosh displayed photographs, how e-ink screens still work, and one of the few retro effects that's a genuine algorithm rather than a filter.

The three algorithms, and when each is right

Floyd–Steinberg is error diffusion: each pixel is rounded to the nearest available level and the rounding error is pushed into the neighbours that haven't been processed yet, so they compensate. The result has no regular pattern and preserves the most detail. It's the default for good reason.

Atkinson — written by Bill Atkinson for the original Mac — diffuses only three quarters of the error and discards the rest. That loses some tonal accuracy and gains contrast and a distinctive open, sparkly texture. If you're chasing the 1984 QuickDraw look specifically, this is the one.

Ordered Bayer compares each pixel against a fixed 4×4 threshold matrix instead of diffusing anything. It produces a visible cross-hatch pattern, which is either the point — it's the authentic look of old games, GIF banners and print screening — or a distraction. It's also the only one of the three where neighbouring pixels don't influence each other, which makes it the correct choice for dithering frames of an animation, since the pattern stays stable instead of crawling.

Levels and colours

At 2 levels you get pure two-colour output — the true 1-bit look. Raising it to 3 or 4 keeps the dither texture but allows intermediate tones, which reads as a limited-palette illustration rather than a bitmap. Beyond about 6 the dithering becomes hard to see.

The two colour pickers aren't limited to black and white. Amber on dark brown gives an old terminal; dark blue on pale grey gives an e-reader; two flat brand colours give a usable graphic treatment. Boost contrast on the source first — dithering has only two tones to work with, and a flat original gives a flat, mushy dither.

Related: Reduce colors dithers against an adaptive multi-colour palette, Black & white thresholds with no dithering at all, and Halftone simulates print dots rather than pixel diffusion.

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