ZX Spectrum

Color palette

The Spectrum has two brightness levels with 8 colors each. Black is shared between both levels, giving 15 unique colors.

Non-bright colors use a hardware voltage ratio of 255 × 0.85 = 216 (0xD8). Bright colors use full voltage (0xFF).

The key constraint: attribute clash

Every 8×8 pixel cell has a single attribute byte encoding:

• INK (foreground color): 3 bits → one of 8 colors
• PAPER (background color): 3 bits → one of 8 colors
• BRIGHT: 1 bit → applies to BOTH ink and paper
• FLASH: 1 bit → alternates ink/paper

The critical limitation: the BRIGHT bit applies to the entire cell. You can’t mix bright and non-bright colors within a single 8×8 block. This “attribute clash” is the defining visual characteristic of Spectrum graphics.

When an image has color boundaries that don’t align to the 8×8 grid, the constraint solver must choose which pair of colors best represents each cell — creating the distinctive color bleeding artifacts.

Interactive demo

Code example

import { process } from 'bitmapped';
import { getPreset } from 'bitmapped/presets';
 
const zx = getPreset('zx-spectrum')!;
 
const result = process(imageData, {
  blockSize: 1,
  palette: zx.palette!,
  dithering: 'floyd-steinberg',
  distanceAlgorithm: 'redmean',
  constraintType: 'attribute-block',
  attributeBlockConfig: {
    width: 8,
    height: 8,
    maxColors: 2,
    brightLocked: true,
  },
});

Note brightLocked: true — this tells the constraint solver that colors within a block must all come from the same brightness level.