Dithering
More colours from fewer
Dithering created the illusion of additional colours by interleaving pixels of different shades, expanding limited palettes through optical mixing.
Overview
When your palette has 16 colours but your image needs thousands, dithering provides the illusion. By carefully placing pixels of different colours adjacent to each other, artists created apparent gradients and shades that didn’t exist in the palette. The eye blends neighbouring pixels — at the right viewing distance, on the right display — perceiving colours the hardware cannot literally display.
Dithering is the optical equivalent of a pointillist painting: dots of pure pigment that read as a smooth tone when stepped back from.
The principle
Two factors govern perceived colour:
| Factor | Effect |
|---|---|
| Pattern density | More of colour A in the mix → output reads more like A |
| Pattern size | Smaller, finer dots → smoother apparent blend |
| Viewing distance | Further away → better blending; up close, the dots are visible |
| Display blur | CRT phosphor bleed helps; sharp LCDs make dots visible |
Common patterns
Ordered dithering
Regular, repeating patterns. Cheap to compute, predictable artistic results.
Bayer 2×2: Bayer 4×4:
0 2 0 8 2 10
3 1 12 4 14 6
3 11 1 9
15 7 13 5The Bayer matrix gives threshold values: when rendering pixel (x, y) in colour c, compare to bayer[x % 4][y % 4]. Bayer 4×4 is the workhorse of late-80s / early-90s computer graphics — visible in Deluxe Paint tutorials, MicroProse simulators, and most Amiga / Atari ST artwork.
Error diffusion
Spread quantisation error to neighbouring pixels — when picking the closest palette colour, the difference between exact and picked colour is added to surrounding pixels:
X 7
1 5 3 (Floyd-Steinberg distribution, fractions of 16)The pixel below-right gets 7/16 of the error, etc. Result: smoother gradients, more “natural” look.
| Algorithm | Year | Distribution | Notes |
|---|---|---|---|
| Floyd-Steinberg | 1976 | 7/3/5/1 over 4 neighbours | The classic; default in most image editors |
| Stucki | 1981 | Spread over 12 neighbours | Smoother, slightly more expensive |
| Atkinson | ~1986 | 1/8 over 6 neighbours | Bill Atkinson’s original Mac dithering; high contrast |
| Sierra | 1990 | Multiple variants | Speed/quality tradeoffs |
Error diffusion costs more per pixel than ordered, but the result is harder to spot.
Platform applications
Commodore 64
| Mode | Colours per cell | Dithering use |
|---|---|---|
| Hi-res | 2 (1 BG + 1 FG per 8×8 cell) | Common — alternating-pixel dithering creates 50% blends |
| Multicolour | 4 (3 fixed + background per 8×8 cell) | Essential — multicolour pixels are 2:1 wide so dithering happens at “2 multi-pixels = 1 hi-res” granularity |
| FLI | 8+ per cell | Less needed — VIC-II tricks expose more colours per cell |
The C64’s 16-colour palette has limited gradients (especially blues/greens), so dithering between adjacent palette entries was an artist’s stock-in-trade.
ZX Spectrum
The Spectrum’s attribute system (8×8 cells, two colours per cell from a 15-colour palette) makes traditional pixel dithering hard. What Spectrum artists do instead:
- Dither within the cell between the two attribute colours.
- Choose attribute pairs that visually blend (alternate hot/cold colours in the cell to imply midpoints).
- Rely on CRT blur to soften the cell boundaries.
- Skip dithering entirely for game graphics — colour-clash means dithering between cells doesn’t work cleanly.
Amiga
| Mode | Native colours | Dithering use |
|---|---|---|
| OCS lores | 32 from 4096 | Common — 32 colours leaves big gaps in any gradient |
| OCS hires | 16 from 4096 | Heavier dithering — fewer palette entries |
| EHB (Extra Half-Brite) | 64 (32 + 32 half-bright copies) | Less needed for shadows; still useful for warm-cool gradients |
| HAM6 / HAM8 | 4096 / 262144 simulated | Different game entirely — HAM lets each pixel modify one R/G/B channel from the previous, costing horizontal neighbour constraints |
HAM mode complements rather than replaces dithering — HAM’s per-pixel colour-shift constraints often produce streak artefacts that dithering helps mask.
IBM PC (CGA / EGA / VGA)
| Mode | Colours | Dithering use |
|---|---|---|
| CGA | 4 from 16 (limited subset modes) | Essential — CGA’s notorious cyan/magenta/white/black palette is mostly dithered for any complex art |
| EGA | 16 from 64 | Common, especially for skin tones and skies |
| VGA 13h | 256 from 262144 | Less needed but still used for smooth gradients |
Artist techniques
Faking gradients
Five-step blend between colour A and colour B:
| Step | Pattern |
|---|---|
| 1 | 100% A |
| 2 | 75% A / 25% B (3 dots A, 1 dot B per 4-pixel block) |
| 3 | 50% A / 50% B (checkerboard) |
| 4 | 25% A / 75% B |
| 5 | 100% B |
Tightening the dot pattern (smaller blocks) gives smoother apparent gradients at the cost of computation or pre-drawn brush authoring.
Texture simulation
Dithering reads as different surfaces:
- Coarse checkerboard → fabric, mesh
- Random scatter → noise, dust, water spray
- Diagonal stripes → metal, machined surface
- Bayer 2×2 → smooth shading, “computer-y” look
The Bitmap Brothers (Speedball 2, The Chaos Engine) made aggressive dithering a stylistic signature on the Amiga — metallic surfaces and angular shading that only read correctly with their dithering.
Tools
| Software | Platform | Notable for |
|---|---|---|
| Deluxe Paint | Amiga, PC | The canonical dithering toolset; “dither paint” mode and stencil-based dither brushes |
| Brilliance | Amiga | Better gradient tools; Amiga rival to DPaint |
| Art Studio / OCP Art Studio | C64 / Spectrum | Pixel-level art tools with dither pattern support |
| PI / GrafX2 (modern) | PC, modern | DPaint-inspired tools used by current pixel artists |
| Photoshop | Modern | “Bitmap mode” with diffusion-dither output; standard since 90s |
Display considerations
| Display | Dithering appearance |
|---|---|
| CRT | Phosphor bleed naturally blurs adjacent pixels; dithering reads as a smooth blend |
| LCD | Pixels are sharper; dither pattern visible at native resolution; needs higher-frequency dithering |
| Emulator | Depends on filtering — bilinear smooths it; nearest-neighbour preserves the artistic intent |
| Modern 4K display | Original-resolution art is tiny; integer scaling preserves dithering, blurry scaling destroys it |
CRTs’ natural blur was a critical part of how dithering worked. Modern emulators add CRT shaders specifically to restore this blending.
Famous examples
| Game / artwork | Dithering use |
|---|---|
| Speedball 2 (Bitmap Brothers, Amiga) | Heavy dithering on metallic player armour |
| Another World / Out of This World | Vector art with hand-tuned dithering for shadows |
| Sierra adventures (KQ4 onward, EGA) | Background gradients via 16-colour dithering |
| Deluxe Paint art galleries | Showcase pieces with virtuoso dithering |
| Beneath a Steel Sky (Revolution, 1994) | Comic-book backgrounds dithered for cel-shaded look |
| Day of the Tentacle (LucasArts, 1993) | Selective dithering for cartoon shading |