DITHERING

6/26/2004

Dithering is a method of printing or displaying colors and or color intensities that are not directly accessible. It presents very small dots of different colors to the viewer and depends on the eye to merge them. This technique has many similarities to "halftone" printing where tiny dots of three colors are laid down on paper and the eye merges them into a continuous span of color. In halftone printing, the intensity of each "primary" color can typically be varied by making its dot smaller or larger thus covering less or more of the background color. Some computer technologies permit varying color intensity. Many computer related technologies don't or have only limited intensity choices. They depend on using the number and arrangement of dots of each color to control the perceived color and color saturation. This often is not entirely satisfactory, but it's the best that can be done.

Many color display/printing systems provide several dithering options -- typically, none, ordered and "error diffusion". In ordered dithering, a repeated pattern of colors and background is used to produce a specific shade and intensity. This is not always satisfactory as it can produce patterned artifacts. In error diffusion, the same overall amounts of color and background are present as in ordered dithering, but their arrangement is randomized. Error Diffusion is generally thought usually to yield somewhat more satisfactory results than ordered dithering.

There are a wide variety of dithering algorithms in use besides Error Diffusion and ordered. Which is "best" is likely to depend on the specific image being presented, the capabilities of the medium, and on subjective factors. One possibility is that dithering that looks good on a computer monitor may not be satisfactory on a specific printer and vice versa.

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