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ADOBE ILLUSTRATOR CS2
User Guide
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.
You can select one of the following dithering methods:
•
Diffusion
Applies a random pattern that is usually less noticeable than Pattern dither. The dither effects are
diffused across adjacent pixels.
•
Pattern
Applies a halftone-like square pattern to simulate any colors not in the color table.
•
Noise
Applies a random pattern similar to the Diffusion dither method, but without diffusing the pattern across
adjacent pixels. No seams appear with the Noise dither method.
Transparency and Matte
Determines how transparent pixels in the image are optimized. (See “To optimize trans
parency in GIF and PNG images” on page 458.)
Interlace
Creates an image that displays as a low-resolution version in a browser while the full image file is
downloading. Interlacing can make downloading time seem shorter and can assure viewers that downloading is in
progress. However, interlacing also increases file size.
Web Snap
Specifies a tolerance level for shifting colors to the closest web palette equivalents (and prevents the colors
from dithering in a browser). A higher value shifts more colors.
See also
“About the Save For Web dialog box” on page 452
“To optimize and save web graphics” on page 452
To 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 dialog box to specify how transparency pixels in GIF
and PNG images are optimized.
•
To make fully transparent pixels transparent and blend partially transparent pixels with a color, select Trans
parency 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, select Transparency and select None from the Matte
menu.