Image Dither

What the three algorithms change

Dithering represents tones with a smaller set of colors by arranging available pixels spatially. Floyd and Steinberg introduced forward error diffusion in their 1976 paper, An Adaptive Algorithm for Spatial Greyscale. Each quantized pixel leaves an error that is distributed to neighboring pixels still waiting to be processed. This usually produces fine, irregular texture and retains gradual changes better than applying one threshold everywhere.

Atkinson is also an error-diffusion method, but its neighbor pattern and retained error differ. Apple lists Floyd-Steinberg and Atkinson as separate vImage dithering methods. Atkinson often gives a crisper, higher-contrast result, but that appearance can lose some smooth tonal variation.

Bayer 4×4 is ordered dithering. It repeats a fixed 4×4 threshold matrix across the image instead of carrying error forward from one pixel to the next. The method comes from B. E. Bayer’s 1973 paper, recorded as An Optimum Method for Two-Level Rendition of Continuous-Tone Pictures. Its visible, regular pattern is useful for retro graphics and predictable texture.

Palette and alpha boundaries

Monochrome reduces visible RGB to two tones. Grayscale keeps several neutral levels, while Limited color quantizes RGB to a small color set. These modes do not identify subjects, rebuild missing detail or infer new colors. The output remains a raster at the processed pixel dimensions; it is not SVG or other vector artwork.

The dither calculation changes RGB values only. It copies the source alpha for each pixel, including partial transparency, and fully transparent pixels stay transparent. The PNG specification defines per-pixel alpha from fully transparent through fully opaque. JPEG has no alpha to preserve, while PNG and some WebP inputs may have it.

Browser processing and PNG export

The browser decodes the selected file, reads pixel data, applies the selected algorithm and builds the download locally. The tool never uploads the image or puts its bytes in Livewire requests or funnel URLs. In funnel mode, it uses temporary, tab-isolated browser storage to carry the local project across full page steps. Reset deletes that project and an automatic expiry removes abandoned data. The HTML Canvas specification defines pixel readback and toBlob() export, with PNG required when a requested type is unsupported.

The 15 MiB compressed-file, 20-megapixel source and 8,192-pixel-per-side limits protect browser memory. A small file can still decode into a large pixel buffer. Processing automatically scales the output down when needed so it does not exceed 4 megapixels, which keeps the interface responsive. Resize an input first if it exceeds a source limit. PNG export preserves processed pixels and alpha losslessly, but it does not preserve source metadata, layers or vector objects.