Atkinson Dithering

Bill Atkinson's Original Algorithm

Developed by Bill Atkinson at Apple (1983-1984) for MacPaint and the original Macintosh. The algorithm was designed for the Mac's 512×342 monochrome display and 8MHz processor.

How It Works:

1. Convert to grayscale: Transform RGB pixels using luminance weighting
2. Quantize: Compare each pixel to threshold 128—below becomes black (0), above becomes white (255)
3. Calculate error: error = original_value - quantized_value
4. Distribute error: Spread 1/8 of error to each of 6 neighboring pixels
5. Discard remainder: 25% of error is intentionally lost

Error Distribution Pattern:

	P	1/8	1/8
1/8	1/8	1/8
	1/8

Processing moves left-to-right, top-to-bottom. Error spreads to 6 future neighbors: right (x+1,y), far-right (x+2,y), down-left (x-1,y+1), down (x,y+1), down-right (x+1,y+1), and two-down (x,y+2) positions. Pattern extends +2 pixels right and +2 pixels down, but never extends left or up from current position.

Why It's Different:

Unlike Floyd-Steinberg (which distributes all error via coefficients 7⁄16+3⁄16+5⁄16+1⁄16=16⁄16=100%^†), Atkinson deliberately discards 25% of error (distributing only 6⁄8≈75%). This prevents "muddy" midtones and creates the crisp, high-contrast aesthetic perfect for Mac's interface elements and line art.

The algorithm prioritized visual clarity over smooth gradations—exactly what was needed for desktop publishing and the Mac's revolutionary graphical interface.