Adobe Illustrator CS4 User Manual
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USING ADOBE ILLUSTRATOR CS4
Web graphics
In addition to the following options, you can also adjust the number of colors in the image’s color table. See
“
Customize the color table for GIF and PNG-8 images
Optimization settings for GIF
A. File Format menu B. Color Reduction Algorithm menu C. Dithering Algorithm menu D. Optimize menu
Lossy (GIF only)
Reduces file size by selectively discarding data. A higher Lossy setting results in more data being
discarded. You
can often apply a Lossy value of 5–10, and sometimes up to 50, without degrading the image. The Lossy
option can reduce file size by 5% to
40%.
Note: You cannot use the Lossy option with the Interlaced option or with Noise or Pattern Dither algorithms.
Color Reduction Method and Colors
Specifies a method for generating the color lookup table and the number of colors
you want in the color lookup table. You can select one of the following color reduction methods:
•
Perceptual
Creates a custom color table 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 color table usually produces images with the greatest color integrity. Selective is the
default option.
•
Adaptive
Creates a custom color table by sampling colors from the predominant spectrum in the image. For
example, an image with only the colors green and blue produces a color table made primarily of greens and blues. Most
images concentrate colors in particular areas of the spectrum.
•
Restrictive (Web)
Uses the standard 216-color color table common to the Windows and Mac
OS 8-bit (256-color)
palettes. This option ensures that no browser dither is applied to colors when the image is displayed using 8-bit color.
(This palette is also called the web-safe palette.) Using the web palette can create larger files, and is recommended only
when avoiding browser dither is a high priority.
•
Custom
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.
Use the Color Table palette in the Save For Web & Devices dialog box to customize the color lookup table.
•
Black and White, Grayscale, Mac
OS, Windows
Use a set palette of colors.
Dithering Method and Dither
Determines the method and amount of application dithering. Dithering refers to the
method of simulating colors not available in the color display system of your computer. A higher dithering percentage
creates the appearance of more colors and more detail in an image, but can also increase the file size. For optimal
compression, use the lowest percentage of dither that provides the color detail you require. Images with primarily solid
colors may work well with no dither. Images with continuous-tone color (especially color gradients) may require
dithering to prevent color banding.
D
A
B
C