halftone

halftone

In printing, the simulation of a continuous-tone image (shaded drawing, photograph) with dots. All printing processes, except for Cycolor, print dots. In photographically generated halftones, a camera shoots the image through a halftone screen, creating smaller dots for lighter areas and larger dots for darker areas. Digitally composed printing prints only one size of dot.

In order to simulate variable-sized halftone dots in computer printers, dithering is used, which creates clusters of dots in a "halftone cell." The more dots printed in the cell, the darker the gray. As the screen frequency gets higher (more cells per inch), there is less room for dots in the cell, reducing the number of shades of gray or color that can be generated.

In low-resolution printers, there is always a compromise between printer resolution (dpi) and screen frequency (lpi), which is the number of rows of halftone cells per inch. For example, in a 300 dpi printer, the 8x8 halftone cell required to create 64 shades of grays results in a very coarse 38 lines per inch of screen frequency (300 dpi divided by 8). However, a high-resolution, 2400 dpi imagesetter can easily handle 256 shades of gray at 150 lpi (2,400 / 16).

   PRINTER RESOLUTION & MAXIMUM SCREEN FREQUENCY

            Shades
      Cell  of gray  At printer resolutions:
      size  or cols  300 dpi  1200 dpi  2400 dpi

       4x4    16     150 lpi   300 lpi   600 lpi
       8x8    64      38 lpi   130 lpi   300 lpi
     16x16   256      19 lpi    75 lpi   150 lpi

Analog Vs. Digital

The analog world of commercial printing prints dots in varying sizes. The digital world prints in grids of dots. Increasingly, digital printers use techniques that overlap dots to achieve greater variability in dot sizes.

---

halftone

Art a tonal value midway between highlight and dark shading

---

(graphics)

halftone - The reproducion of greyscale images using dots of a single shade but varying size to simulate the different shades of grey. Laser printers that cannot print different sized dots, halftones are produced by varying the numbers of dots in a given area. This process is also used to produce a black and white version of a colour original using shades of grey in place of colours. See also device independent bitmap.