Dithering for thermal printers

A thermal print head heats tiny dots — typically 8 per millimetre (203 dpi) — on heat-sensitive paper. Each dot is either on or off. There is no grey ink, so a photo with smooth tones has to be converted into a pattern of black dots whose density imitates brightness. That conversion is called dithering, and the method you pick changes the result more than any other setting.

Threshold: no dithering at all

Every pixel brighter than a cut-off value becomes white, every darker pixel becomes black. Elysano's Threshold slider sets the cut-off (128 is the middle).

• Best for: logos, text, line art, barcodes and QR codes — anything that is already black and white.
• Why: edges stay perfectly sharp and scanners read codes reliably. Dithered QR codes can fail to scan.
• Avoid for: photos. Faces turn into black blobs and skies into white voids.

Floyd–Steinberg: the all-rounder for photos

Floyd–Steinberg error diffusion processes the image pixel by pixel. When a grey pixel is rounded to black or white, the rounding "error" is not thrown away but passed on to the neighbours that haven't been processed yet: 7/16 to the pixel on the right, and 3/16, 5/16 and 1/16 to the three pixels below. Over an area the dot density then matches the original brightness very closely.

• Best for: photographs, gradients, screenshots with shading.
• Character: fine, slightly grainy texture that keeps the most tonal detail.
• Watch out: on thermal paper the result often prints darker than the preview, because neighbouring hot dots bleed together. Add brightness.

Atkinson: lighter and cleaner

Atkinson dithering, made famous by the original Apple Macintosh, also diffuses error — but it spreads only 6/8 of it, 1/8 each to six neighbouring pixels, and simply discards the rest. Highlights and shadows lose a little detail, while midtones look cleaner and the overall image is lighter and higher in contrast.

• Best for: portraits, illustrations, product shots on white backgrounds, anything that looks muddy with Floyd–Steinberg.
• Character: crisper, more "graphic" look with open highlights.

Brightness and contrast matter more than you think

Thermal output almost always comes out darker than a screen preview: dots spread slightly as the paper heats, and cheap paper darkens unevenly. Practical starting points:

Size before you dither

Dithering works on the final printed pixels, so the image should be scaled to the printer's width first — Elysano does this automatically (384 dots for 58 mm, 576 for 80 mm). Very small source images upscaled to that width print soft and blocky; very large ones are fine. For logos, a source at least as wide as the print width gives the cleanest edges.

A quick decision rule

If the image has hard edges and only a few colours, use Threshold. If it is a photo, try Atkinson first and switch to Floyd–Steinberg when you want more shadow detail. Then nudge brightness until the preview looks slightly lighter than you want the print to be.