Dithering

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  5

The 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 to the right gets 7/16 of the error, the one below 5/16, and so on. 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 per 4×8 cell + a background shared by all cells)	Essential — multicolour pixels are 2:1 wide so dithering happens at “2 multi-pixels = 1 hi-res” granularity
FLI / IFLI	More per cell	Still used — the converters that put photographs on the C64 dither into these modes

The word reached C64 owners through peripherals before it reached their own screens. Reviewing the Okimate-10 colour printer in 1985, COMPUTE!’s Gazette’s Charles Brannon explained that its screen-dump program “uses a trick known as dithering to create extra colors. Dithering places two colored dots side by side, rather than mixing them. From the right distance, a magenta dot next to a cyan dot looks like a fat purple dot” — and that this was how the printer’s seven ink colours reproduced “the 16 colors used on the Commodore 64”. The same year Ahoy! reported a $39.95 video digitizer kit from Kinney Software that used “an alternate dot pattern technique (dithering)” to show six and eight grey levels on the C64.

Error diffusion arrived on the machine itself with the late-1990s image converters. Adrian Gonzalez’s renderer for Stephen Judd’s C64 JPEG decoder, described in C=Hacking 19 (2000), runs Floyd-Steinberg on the C64 — the error “has to be distributed (diffused) amongst adjacent pixels”, seven sixteenths to the right and five sixteenths below — with a table of RGB values for the sixteen colours to measure the error against. The hard part is not the arithmetic but the cell rule: multicolour bitmap mode allows “up to 3 different colors out of the C64’s 16 colors plus one background color common to all cells”, which Gonzalez calls “a very tough choice for displaying true color images”, so the renderer targets IFLI, where the image alone “takes up half the c64’s memory”.

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

More colours from fewer