Although GIF is restricted to 256 colors, it??™s worth noting that you don??™t have to use the
same 256 colors every time. Most graphics applications provide a number of palette
options, such as perceptual, selective, and Web. The first of those, perceptual, tends to prioritize
colors that the human eye is most sensitive to, thereby providing the best color
integrity. Selective works in a similar fashion, but balances its color choices with web-safe
colors, thereby creating results more likely to be safe across platforms. Web refers to the
216-color web-safe palette discussed earlier. Additionally, you often have the option to
lock colors, which forces your graphics application to use only the colors within the
palette you choose.
Images can also be dithered, which prevents continuous tones from becoming bands of
color. Dithering simulates continuous tones, using the available (restricted) palette. Most
graphics editors allow for three different types of dithering: diffusion, pattern, and noise??”
all of which have markedly different effects on an image. Diffusion applies a random pattern
across adjacent pixels, whereas pattern applies a half-tone pattern rather like that
seen in low-quality print publications. Noise works rather like diffusion, but without diffusing
the pattern across adjacent pixels. Following are four examples of the effects of
dithering on an image that began life as a smooth gradient. The first image (1) has no
dither, and the gradient has been turned into a series of solid, vertical stripes.
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