Amiga

Color palette

The Amiga’s Original Chip Set (OCS) uses 12-bit RGB — 4 bits per channel — providing a total gamut of 4,096 colors. In standard mode, 5 bitplanes allow up to 32 simultaneously displayed colors chosen freely from that gamut.

This is an RGB bit-depth system rather than a fixed lookup table. Use enumerateColorSpace() from bitmapped/presets to generate the full 4,096-color palette at runtime:

import { enumerateColorSpace } from 'bitmapped/presets';
 
// Generate all 4,096 OCS colors (4 bits per channel)
const fullPalette = enumerateColorSpace({ r: 4, g: 4, b: 4 });

The key constraint: three display modes

The Amiga’s custom chipset supports three distinct display modes, each with different color capabilities and trade-offs:

Standard (OCS) — 32 colors

The default 5-bitplane mode. 32 colors are freely chosen from the 4,096-color gamut and can be placed anywhere on screen with no spatial constraint. This is the simplest mode and the one most Amiga games and applications used.

Extra Half-Brite (EHB) — 64 colors

A 6th bitplane acts as a “half-brightness” flag. When set, the pixel’s color is the half-brightness version of the corresponding 5-bitplane color. This gives 64 simultaneous colors — 32 base colors plus 32 automatic copies at 50% brightness — at the cost of one extra bitplane of memory. The half-brightness colors cannot be independently programmed.

Hold-And-Modify (HAM6) — 4,096 colors

The most distinctive Amiga mode. Each pixel encodes a 2-bit operation code and a 4-bit data value:

• Set — select one of 16 base palette colors
• Modify R — keep G and B from the previous pixel, replace R with the 4-bit value
• Modify G — keep R and B from the previous pixel, replace G with the 4-bit value
• Modify B — keep R and G from the previous pixel, replace B with the 4-bit value

This allows every pixel to potentially display any of the 4,096 OCS colors, enabling photorealistic images. However, because each pixel can only change one channel at a time, sharp color transitions produce characteristic horizontal “fringing” artifacts — the algorithm needs multiple pixels to converge on the target color.

bitmapped’s HAM constraint solver simulates this per-pixel channel modification, choosing the optimal set-or-modify decision for each pixel to minimize color error while reproducing authentic fringing behavior.

Interactive demo

Code example

import { process } from 'bitmapped';
import { getPreset, enumerateColorSpace } from 'bitmapped/presets';
 
const ham = getPreset('amiga-ham6')!;
 
const result = process(imageData, {
  blockSize: 1,
  palette: enumerateColorSpace({ r: 4, g: 4, b: 4 })
    .map((color) => ({ color })),
  distanceAlgorithm: 'ciede2000',
  dithering: 'none',
  constraintType: 'ham',
  hamConfig: {
    basePaletteSize: 16,
    modifyBits: 4,
  },
});

Presets

bitmapped includes three Amiga presets:

• amiga-ocs — Standard 5-bitplane mode, 32 simultaneous colors from 4,096
• amiga-ham6 — Hold-And-Modify mode, all 4,096 colors with per-pixel channel modification
• amiga-ehb — Extra Half-Brite mode, 64 colors (32 base + 32 half-brightness)