halftoning

Halftoning is a printing and imaging technique that simulates continuous tone imagery by using small dots of ink or toner. By varying the size, density, or spacing of the dots, a range of tones is perceived by the human eye, even though each dot has a single color. The method is central to how grayscale and color photographs are reproduced in newspapers, magazines, and packaging where ink or toner layers are limited in tonal levels.

In grayscale halftoning, brightness is encoded by dot size or density within a fixed grid. In color halftoning, a color image is separated into multiple channels (commonly CMYK), and each channel is halftoned with its own dot pattern; the superposition of channels yields a full-color impression. The geometry of the dot patterns and the screen angles are chosen to minimize interference and moiré.

Several techniques have been developed. Thresholding converts an image to a binary dot pattern. Dithering introduces a patterned distribution of dots to simulate intermediate tones; ordered dithering uses a fixed matrix (for example, Bayer matrices). Error diffusion, such as Floyd–Steinberg or Stucki, propagates quantization error to neighboring pixels to preserve detail and smooth tonal transitions.

Historically, halftoning emerged in the 19th century with photoengraving and line screens and became standard for mass print reproduction in the 20th century. With modern digital printers and displays, halftoning remains essential, enabling grayscale or color rendering on devices with limited color depth.

Limitations include visible dot structure (graininess), moiré when misconfigured color screens, and artifacts at high-contrast edges. Nevertheless, halftoning offers efficient reproduction with simple hardware, and it continues to underpin print and display technologies alongside more advanced continuous-tone rendering.