A number of palette types are available for converting an image to indexed color. For the Perceptual, Selective, and Adaptive options, you can choose between using a local palette based on the current image's colors or a master palette created in ImageReady. (See "Using master palettes (ImageReady)" on page 438.)
Exact Creates a palette using the exact colors appearing in the RGB image—an option available only if the image uses 256 or fewer colors. Because the image's palette contains all colors in the image, there is no dithering.
System (Mac OS) Uses the Mac OS default 8-bit palette, which is based on a uniform sampling of RGB colors.
System (Windows) Uses the Windows system's default 8-bit palette, which is based on a uniform sampling of RGB colors.
Web Uses the 216-color palette that Web browsers, regardless of platform, use to display images on a monitor limited to 256 colors. This palette is a subset of the Mac OS 8-bit palette. Use this option to avoid browser dither when viewing images on a monitor display limited to 256 colors.
Uniform Creates a palette by uniformly sampling colors from the RGB color cube. For example, if Photoshop takes 6 evenly spaced color levels each of red, green, and blue, the combination produces a uniform palette of 216 colors (6 cubed = 6 x 6 x 6 = 216). The total number of colors displayed in an image corresponds to the nearest perfect cube (8, 27, 64, 125, or 216) that is less than the value in the Colors text box.
Perceptual Creates a custom palette by giving priority to colors for which the human eye has greater sensitivity.
Selective Creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of Web colors. This option usually produces images with the greatest color integrity.
Adaptive Creates a palette by sampling the colors from the spectrum appearing most commonly in the image. For example, an RGB image with only the colors green and blue produces a palette made primarily of greens and blues. Most images concentrate colors in particular areas of the spectrum. To control a palette more precisely, first select a part of the image containing the colors you want to emphasize. Photoshop weights the conversion toward these colors.
Custom Creates a custom palette using the Color Table dialog box. Either edit the color table and save it for later use or click Load to load a previously created color table. (See "Customizing indexed color tables (Photoshop)" on page 100.) This option also displays the current adaptive palette, which is useful for previewing the colors most often used in the image.
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