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431
Adobe Photoshop Help
Preparing Graphics for the Web
Using Help
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Contents
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Index
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431
Optimizing colors in GIF and PNG-8 images
Decreasing the number of colors in an image is a key factor in optimizing GIF and PNG-8
images. A reduced range of colors will often preserve good image quality while dramati-
cally reducing the file space required to store extra colors.
The color table gives you precise control over the colors in optimized GIF and PNG-8
images (as well as original images in indexed color mode). With a maximum of 256 colors,
you can add and delete colors in the color table, shift selected colors to other colors or to
transparency, and lock selected colors to prevent them from being dropped from the
palette.
Viewing a color table
The color table for a slice appears in the Color Table panel in the Save for Web dialog box
(Photoshop) or the Color Table palette (ImageReady).
Note:
In ImageReady, be careful not to confuse the Color Table palette with the Color
palette or Swatches palette. You use the Color Table palette to optimize colors; you use the
Color palette and Swatches palette to select colors.
To view the color table for an optimized slice:
1
(ImageReady) Choose Window > Color Table.
2
Select a slice that is optimized in GIF or PNG-8 format. (See
“Selecting slices” on
page 373
.) The color table for the selected slice appears in the Color Table palette.
Note:
In ImageReady, the color table for the selected slice will not appear if Auto Regen-
erate is off. For more information, see
“Controlling optimization (ImageReady)” on
page 426
.
If an image has multiple slices, the colors in the color table may vary between slices
(you can link the slices first to prevent this from happening). If you select multiple slices
that use different color tables, the color table is empty and its status bar displays the
message “Mixed.”
Generating a color table
You can change the palette—or set of colors—in the color table by selecting a color
reduction option in the Optimize panel/palette. There are three categories of options:
•
Dynamic options use a color reduction algorithm to build a palette based on the colors
in the image and the number of colors specified in the optimization setting. The colors
in the palette are regenerated every time you change or reoptimize the image.
Perceptual, Selective, and Adaptive are dynamic options.
•
Fixed options use a set palette of colors. In other words, the set of available colors is
constant, but the actual colors in the palette will vary depending on the colors in the
image. Web, Mac OS, Windows, Black & White, and Grayscale tables are fixed options.
•
The Custom option uses a color palette that is created or modified by the user. If you
open an existing GIF or PNG-8 file, it will have a custom color palette.
