Adobe Illustrator CS4 - Manual (Page 401)

394 USING ADOBE ILLUSTRATOR CS4
Web graphics
Web Snap Specifies a tolerance level for shifting colors to the closest web palette equivalents (and prevent the colors
from dithering in a browser). A higher value shifts more colors.
See also
“ Optimize an image for the web ” on page 386
Optimize transparency in GIF and PNG images
Transparency makes it possible to create nonrectangular images for the web. Background transparency preserves
transparent pixels in the image. This allows the background of the web page to show through the transparent areas of
your image. Background matting simulates transparency by filling or blending transparent pixels with a matte color
that can match the web page background. Background matting works best if the web page background is a solid color
and if you know what that color is.
Use the Transparency and Matte options in the Save For Web & Devices dialog box to specify how transparent pixels
in GIF and PNG images are optimized.
• (GIF and PNG-8) To make fully transparent pixels transparent and blend partially transparent pixels with a color,
select Transparency and select a matte color.
• To fill fully transparent pixels with a color and blend partially transparent pixels with the same color, select a matte
color and deselect Transparency.
• (GIF and PNG-8) To make all pixels with greater than 50% transparency fully transparent and all pixels with 50%
or less transparency fully opaque, select Transparency and select None from the Matte menu.
• (PNG-24) To save an image with multilevel transparency (up to 256 levels), select Transparency. The Matte option
is disabled since multilevel transparency allows an image to blend with any background color.
Note: In browsers that do not support PNG-24 transparency, transparent pixels may be displayed against a default
background color, such as gray.
To select a matte color, click the Matte color swatch and select a color in the color picker. Alternatively, select an option
from the Matte menu: Eyedropper Color (to use the color in the eyedropper sample box), Foreground Color,
Background Color, White, Black, or Other (to use the color picker).
Note: The Foreground Color and Background Color options are only available in Photoshop.
View the color table for an optimized slice
The color table for a slice appears in the Color Table panel in the Save For Web & Devices dialog box.
❖ Select a slice that is optimized in GIF or PNG-8 format. The color table for the selected slice appears in the Save For
Web & Devices color table.
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.”
Customize the color table for GIF and PNG-8 images
You use the color table in the Save For Web & Devices dialog box to customize the colors in optimized GIF and PNG-8
images. Reducing the number of colors often preserves image quality while reducing the file size of the image.
You can add and delete colors in the color table, shift selected colors to web-safe colors, and lock selected colors to
prevent them from being dropped from the palette.