Sega Genesis / Mega Drive

Color palette

The Genesis VDP uses 9-bit RGB (3 bits per channel), giving 512 possible colors. Unlike most systems with even bit-depth spacing, the Genesis DAC (digital-to-analog converter) is nonlinear — the 8 voltage levels per channel are not evenly spaced. The hardware-measured values are:

DAC levels: [0, 52, 87, 116, 144, 172, 206, 255]

This means the steps between adjacent levels are 52, 35, 29, 28, 28, 34, 49 — larger at the extremes (darks and brights) and compressed in the midtones. The result is a distinctive color curve that slightly brightens midrange values compared to a linear 3-bit encoding.

bitmapped uses the genesisQuantize() function to match this curve, snapping each 8-bit channel value to the nearest hardware DAC level rather than using uniform 3-bit quantization.

The key constraint: per-tile palette with nonlinear DAC

Colors are organized into 4 palettes of 16 colors each. One color per palette is reserved as transparent, leaving 15 unique colors per palette plus a single shared backdrop color, for a maximum of 61 simultaneous colors (4 x 15 + 1).

Each 8x8 tile selects one of the four palettes. This means color choices are locked at the tile level — every pixel within a tile must draw from the same 16-color subpalette. Games work around this by carefully assigning tiles to palettes, often reserving one palette for the player character, one for enemies, and two for backgrounds.

The nonlinear DAC compounds this constraint: the 512-color gamut is unevenly distributed through perceptual color space. Two colors that are adjacent in the 3-bit digital domain may be perceptually closer or further apart than you’d expect, which affects palette selection and dithering quality.

Interactive demo

Code example

import { process } from 'bitmapped';
import { getPreset, enumerateColorSpace } from 'bitmapped/presets';
 
const genesis = getPreset('genesis')!;
 
// The full 512-color palette, quantized through the nonlinear DAC
const fullPalette = enumerateColorSpace(genesis.colorSpace!);
 
const result = process(imageData, {
  blockSize: 4,
  palette: genesis.palette!,
  dithering: 'floyd-steinberg',
  distanceAlgorithm: 'ciede2000',
  constraintType: 'per-tile-palette',
  tilePaletteConfig: {
    tileWidth: 8,
    tileHeight: 8,
    subpaletteCount: 4,
    colorsPerSubpalette: 16,
    sharedTransparent: true,
  },
});

Presets

bitmapped includes two Genesis presets:

• genesis — NTSC timing (320x224), nonlinear DAC palette, composite dither enabled
• genesis-pal — PAL timing (320x240), same palette and DAC curve

Both presets also support the alternative H32 mode (256 pixels wide) as documented in the resolution metadata.