Conversion options for indexed-color images (Photoshop)
converting an RGB image to indexed
can specify a number of conversion
options in the Indexed Color dialog box.
number of palette types are available
an image to indexed color.
For the Perceptual, Selective,
and Adaptive options, you
can choose between using a local
palette based on the current images colors
or a master palette created in ImageReady.
Using master palettes
(ImageReady) on section 438.)
a palette using the exact colors appearing in the RGB imagean option
available only if the image uses 256
or fewer colors.
Because the images palette
contains all colors in the image, there
is no dithering.
(Mac OS) Uses the Mac OS default 8-bit palette, which is based on a uniform sampling of RGB colors.
(Windows) Uses the Windows systems 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.
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 =
The total number
of colors displayed in an image corresponds
to the nearest perfect cube
64, 125, or 216) that is less than the value
in the Colors text box.
Creates a custom palette by giving priority
to colors for which
the human eye
has greater sensitivity.
a color table similar to the Perceptual color
table, but favoring broad
areas of color and the preservation
colors. This option usually produces
images with the greatest color
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
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.
Customizing indexed color tables (Photoshop)
on section 100.) This option also displays the current adaptive palette, which is useful for previewing the colors most often used in the image.
Previous Uses the custom
palette from the previous conversion,
making it easy to convert several images with the same custom palette.
Number of colors
For the Uniform, Perceptual, Selective,
or Adaptive palette,
you can specify the exact number of colors to be displayed (up to 256) by entering a value for Colors. The Colors text
box controls only how the indexed color table is created.
Adobe Photoshop still treats the
image as an 8-bit, 256-color
Color inclusion and transparency
To specify colors to be included in the indexed color table or to specify
transparency in the image, choose from
the following options:
Forced Provides options to force
the inclusion of certain colors in the color table. Black and White adds a pure black
and a pure white to the color table; Primaries
adds red, green, blue, cyan, magenta,
and white; Web adds the 216 web-safe
colors; and Custom lets you
deﬁne custom colors to
whether to preserve transparent
areas of the image during conversion.
Selecting this option adds a special index entry in the color table for trans- parent
colors. Deselecting this
option ﬁlls transparent areas
with the matte color, or with white if no matte color is chosen.
the background color used to ﬁll anti-aliased edges that lie adjacent to
transparent areas of the image. With Transparency
selected, the matte is applied to edge areas to help blend the edges with a Web
background of the same color. With Trans-
parency deselected, the matte
is applied to transparent
areas. Choosing None for the matte creates
hard-edged transparency if Transparency
is selected; otherwise, all trans-
parent areas are ﬁlled
with 100% white.
using the Exact color table option, the color
table may not contain all the colors used in the image. To
simulate colors not in the color
table, you can dither the colors. Dithering mixes the pixels of the available colors
to simulate the missing colors. Choose a dither option from the menu, and enter a value
for the dither amount. A higher amount dithers more colors,
but may increase ﬁle size. You can choose from the following dither options:
not dither colors but instead uses the color closest to the
missing color. This tends
in sharp transitions between shades of color in the image, creating
a posterized effect.
Uses an error-diffusion method that produces a less structured dither than the Pattern
option. To protect colors
in the image that contain entries in
the color table from being
Colors. This is useful
for preserving ﬁne lines and text
Pattern Uses a halftone-like
square pattern to simulate any
colors not in the color table.
Noise Helps to reduce seam patterns along the edges of image slices. Choose this option
if you plan to slice the image for placement
in an HTML table.