Atari 2600

Color palette

The Atari 2600’s TIA chip uses a 7-bit color register to generate color via NTSC composite encoding. The register format is CCCC_LLL0 — 4 bits select one of 16 hues, 3 bits select one of 8 luminance levels, and bit 0 is unused. This gives 16 hues x 8 luminances = 128 unique colors in NTSC mode. The PAL version uses a different color encoding that produces only 104 colors (13 hues x 8 luminances).

Here are the first 8 hue rows (64 of 128 NTSC colors), showing the luminance ramp for each hue:

The key constraint: scanline color limits

The TIA has no framebuffer — it renders one scanline at a time from a handful of hardware registers. Each scanline can display at most 4 colors: the background, the playfield, and two player/missile objects. There is no way to exceed this limit on a single line of pixels.

Games achieve more color variety by reprogramming the TIA’s color registers between scanlines, a technique called “racing the beam”. The CPU must update registers in the narrow window between the end of one scanline and the start of the next. This is why Atari 2600 games often feature horizontal bands of color — each row can have a completely different set of 4 colors, but within a row the limit is strict.

bitmapped’s per-scanline constraint solver enforces the 4-color-per-scanline limit, selecting the optimal set of colors for each row to minimize overall color error.

Interactive demo

Code example

import { process } from 'bitmapped';
import { getPreset } from 'bitmapped/presets';
 
const atari = getPreset('atari-2600-ntsc')!;
 
const result = process(imageData, {
  blockSize: 4,
  palette: atari.palette!,
  dithering: 'bayer',
  distanceAlgorithm: 'redmean',
  constraintType: 'per-scanline',
  scanlineConfig: {
    maxColorsPerLine: 4,
  },
});

Presets

bitmapped includes two Atari 2600 presets:

• atari-2600-ntsc — 128-color NTSC TIA palette (16 hues x 8 luminances), 160x192 resolution
• atari-2600-pal — 104-color PAL TIA palette (13 hues x 8 luminances), 160x228 resolution