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CHAPTER 11
234
Optimizing Images for the Web
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Select Transparency.
For GIF and PNG-8 format, decide how you
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want to treat partially transparent pixels in the
original image. You can blend partially transparent
pixels with a matte color, or you can create hard-
edged transparency. (See "Creating background
matting in GIF and PNG images" on page 234 and
"Creating hard-edged transparency in GIF and
PNG-8 images" on page 234.)
Creating background matting in GIF and
PNG images
When you know the Web page background color
on which an image will be displayed, you can use
the matting feature to fill or blend transparent
pixels with a matte color that matches the Web
page background.
The results of matting GIF and PNG-8 images
depend on the Transparency option. If you select
Transparency, only the partially transparent pixels,
such as those at the edge of an anti-aliased image,
are matted. When the image is placed on a Web
page, the Web background shows through the
transparent pixels, and the edges of the image
blend with the background. This feature prevents
the halo effect that results when an anti-aliased
image is placed on a background color that differs
from the image's original background. This feature
also prevents the jagged edges that result with GIF
hard-edged transparency.
If you deselect Transparency, fully transparent
pixels are filled with the matte color, and partially
transparent pixels are blended with the
matte color.
GIF with Transparency option selected, and with Trans-
parency option deselected
To create a matted GIF or PNG image:
Open or create an image that contains
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transparency, and choose File > Save for Web.
In the Save For Web dialog box, select GIF,
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PNG-8, or PNG-24 as the optimization format.
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For GIF and PNG-8 format, select or deselect
the Transparency option as desired.
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Select a color from the Matte pop-up menu:
Eyedropper (to use the color in the eyedropper
sample box), White, Black, or Other (to select a
color using the color picker).
Creating hard-edged transparency in GIF
and PNG-8 images
When working with GIF or PNG-8 files, you can
create hard-edged transparency, in which all pixels
that are more than 50% transparent in the original
image are fully transparent in the optimized
image, and all pixels that are more than 50%
opaque in the original image are fully opaque in
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