Halftoning originated in 19th-century printing as a way to reproduce photographs using ink-on-paper, which inherently supports only two states: ink or no ink. By breaking the image into a grid of dots whose size varies with the desired tone, the human eye blends these dots at viewing distance into the perception of continuous gradation. This same principle underpins every newspaper photograph, magazine page, and inkjet print.
Classical Halftone Screening
In amplitude modulation (AM) screening, dots are placed on a fixed grid and their size varies with the image tone. Light areas get small dots, dark areas get large dots. The grid angle and frequency (measured in lines per inch, LPI) determine the visual quality. Common screen rulings range from 85 LPI (newspaper) to 300+ LPI (fine art printing).
In frequency modulation (FM) screening — also called stochastic screening — dots are a fixed (tiny) size but their density varies. This eliminates the visible screen pattern and moiré artifacts of AM screening, producing a more photographic appearance at the cost of requiring more precise printing equipment.
Halftoning vs. Dithering
While halftoning and dithering both simulate continuous tones from discrete values, they differ in context and approach:
- Halftoning traditionally refers to the physical reproduction domain (print, lithography) and uses structured or stochastic dot patterns.
- Dithering traditionally refers to the digital display domain and uses error-diffusion algorithms to distribute quantization error across neighboring pixels.
In practice, the terms overlap significantly. Floyd-Steinberg error diffusion can be classified as both dithering and digital halftoning. FM screening is essentially stochastic dithering applied to print output.
As a Creative Tool
Beyond its functional origins, halftoning has become a distinctive visual aesthetic. Pop art (Roy Lichtenstein), comic book illustration (Ben-Day dots), and retro print design all use visible halftone patterns as deliberate stylistic choices. In digital art, halftone filters and pattern generators can transform photographs into graphic, high-contrast compositions that reference this print heritage.
