Dithering Topics

• web safe color palette
• pixel colors
• speckled appearance
• visual artifacts
• dithered image
• colored pixels
• poor representation
• color bands
• approach results
• web safe color
• close proximity
• color web
• flat areas
• 256 colors
• color vision
• flatness
• two colors
• human eye
• photographic image
• video hardware

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Dithering, Dithering

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In a dithered image, colors not available in the palette are approximated by a diffusion of colored pixels from within the available palette. The human eye perceives the diffusion as a mixture of the colors within it (see color vision). Dithering is analogous to the halftone technique used in printing. Dithered images, particularly those with relatively few colors, can often be distinguished by a characteristic graininess, or speckled appearance.

For example, an original image ( Figure 1 ) may be reduced to the 216-color "web-safe" color palette. If the original pixel colors are simply translated into the closest available color from the palette, no dithering occurs ( Figure 2 ). Typically, this approach results in flat areas (contours) and a loss of detail, and may produce patches of color that are significantly different from the original. Shaded or gradient areas may appear as color bands , which may be distracting. The application of dithering can help to minimize such visual artifacts, and usually results in a better representation of the original ( Figure 3 ). Dithering helps to reduce color banding and flatness.

If, for example, the palette is limited to only 16 colors, the resulting image could suffer from additional loss of detail, and even more pronounced problems with flatness and color banding ( Figure 5 ). Once again, dithering can help to minimize such artifacts ( Figure 6 ).

For example, dithering might be used in order to display a photographic image containing millions of colors on video hardware that is only capable of showing 256 colors at a time. The 256 available colors would be used to generate a dithered approximation of the original image. Without dithering, the colors in the original image might simply be "rounded off" to the closest available color, resulting in a new image that is a poor representation of the original. Dithering takes advantage of the human eye's tendency to "mix" two colors in close proximity to one another.

Since a web browser may be retrieving graphical elements from an external source, it may be necessary for the browser to perform dithering on images with too many colors for the available display. It was due to problems with dithering that a color palette known as the "web-safe color palette" was identified, for use in choosing colors that would not be dithered on displays with only 256 colors available.

Dithering the 32 or 64 RGB levels will result in a pretty good "pseudo truecolor" display approximation, which the eye cannot resolve as grainy . Furthermore, images displayed on 24-bit RGB hardware (8 bits per RGB primary) can be dithered to simulate somewhat higher bit depth, and/or to minimize the loss of hues available after a gamma correction. High-end still image processing software, as Adobe Photoshop, commonly uses these techniques for improved display.

In particular, the commonly-used GIF format is restricted to the use of 256 or fewer colors in many graphics editing programs. Images in other file formats, such as PNG, may also have such a restriction imposed on them for the sake of a reduction in file size. Images such as these have a fixed color palette defining all the colors that the image may use. For such situations, graphical editing software may be responsible for dithering images prior to saving them in such restrictive formats.

One of the earliest, and still one of the most popular, is the Floyd-Steinberg dithering algorithm, developed in 1975. One of the strengths of this algorithm is that it minimizes visual artifacts through an error-diffusion process; error-diffusion algorithms typically produce images that more closely represent the original than simpler dithering algorithms.

Source: Wikipedia > Dither