Apple II

Color palette

The Apple II has two distinct graphics modes with completely different color systems.

Hi-Res artifact colors

The Hi-Res mode is technically monochrome — it outputs a 1-bit signal at 280 pixels wide. But because the pixel clock is close to the NTSC color subcarrier frequency (~3.58 MHz), adjacent pixels create interference patterns that a color TV decodes as color. This produces the famous artifact colors.

The palette bit (bit 7 of each byte in video memory) selects between two color groups:

Lo-Res 16-color palette

The Lo-Res mode uses large 7x8-pixel blocks at 40x48 resolution, with a full 16-color palette that doesn’t depend on NTSC artifacts:

The key constraint: NTSC artifact color

Hi-Res colors are not freely assignable. They emerge from the interaction between three factors:

1. Pixel position — whether the pixel falls on an even or odd column determines which color from the active group appears. Even columns produce violet or blue; odd columns produce green or orange.
2. Palette bit — bit 7 (the MSB) of each byte in video memory selects between Group 1 (violet/green) and Group 2 (blue/orange). This bit applies to all 7 pixels encoded in that byte.
3. NTSC decoding — the TV’s composite decoder interprets the high-frequency pixel pattern as color information. Two or more adjacent lit pixels blend together to produce white.

This means that within any 7-pixel group (one byte of video memory), you can only use colors from a single group. You cannot mix violet and orange in the same byte. And the specific color that appears depends on horizontal position — you cannot freely place any color at any pixel.

bitmapped’s artifact-color constraint solver models this interaction by evaluating both palette groups for each 7-pixel segment and selecting the group that minimizes overall color error.

Interactive demo

Code example

import { process } from 'bitmapped';
import { getPreset } from 'bitmapped/presets';
 
const apple2 = getPreset('apple-ii-hires')!;
 
// Group 1: Black, Violet, Green, White
const group1 = ['#000000', '#FF44FD', '#14F53C', '#FFFFFF'];
// Group 2: Black, Blue, Orange, White
const group2 = ['#000000', '#14CFFD', '#FF6A3C', '#FFFFFF'];
 
const result = process(imageData, {
  blockSize: 1,
  palette: apple2.palette!,
  dithering: 'atkinson',
  distanceAlgorithm: 'ciede2000',
  constraintType: 'artifact-color',
  artifactConfig: {
    pixelsPerGroup: 7,
    paletteSets: [group1, group2],
  },
});

Presets

bitmapped includes two Apple II presets:

• apple-ii-lores — Lo-Res mode, 40x48 resolution, 16 colors, no constraint
• apple-ii-hires — Hi-Res mode, 280x192 resolution, 6 artifact colors, NTSC artifact color constraint