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gray scale

A series of shades from white to black. The more shades, or levels, the more realistic an image can be recorded and displayed, especially a scanned photo. Scanners differentiate typically from 16 to 256 gray levels, and high-resolution gray scale is widely used for medical x-rays.

At a resolution of 300 dpi, each square inch comprises 90K pixels (300x300). At one byte per pixel (256 levels), a 10x10" image would take up 9MB of storage (90K x 100). See 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 of
      Gray or  --At printer resolutions--
 Cell  Colors  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.






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