Adobe PHOTOSHOP 5.0 User Manual page 81

To convert an RGB image to an indexed-color image:
Choose Image > Mode > Indexed Color.
1
Note: The image must be flattened first, or you will
lose layers.
Select Preview to display a preview of the
2
changes.
Select a palette, color depth (bit depth), and
3
dither method as described in the following
sections.
Click OK.
4
Specifying a palette
Eight palette types are available for converting an
image to indexed color.
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.
Uses the Windows system's
System (Windows)
default 8-bit palette, which is based on a uniform
sampling of RGB colors.
Uses the Macintosh system's
System (Macintosh)
default 8-bit palette, which is based on a uniform
sampling of RGB colors.
Uses the palette most often used by Web
Web
browsers to display 8-bit images. This palette is a
subset of the Windows and Macintosh systems'
palettes.
Uniform Creates a palette by uniformly sampling
colors from the spectrum. For example, if you
choose an 8-bit color depth, Photoshop takes 6
evenly spaced color levels each of red, green, and
blue and calculates the combinations of these
colors to produce a uniform palette of 216 colors
(6 x 6 x 6 = 216). A lower bit depth creates a
uniform palette composed of fewer colors. The
total number of colors displayed in the image
corresponds to the cube nearest the chosen bit
depth (8, 27, 64, 125, or 216).
Creates a palette by sampling the colors
Adaptive
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.
Creates a custom palette using the Color
Custom
Table dialog box. Either edit the color table and
save it for later use or click Load to load a previ-
ously created color table. (See "Specifying color
matching options" on page 74.)
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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User Guide