Adobe PHOTOSHOP 6.0 Manual page 352

CHAPTER 12
342
Optimizing Images for the Web
When optimizing images, keep in mind that two
kinds of dithering can occur:
• Application dither occurs in GIF and PNG-8
images when Photoshop or ImageReady attempts
to simulate colors that appear in the original image
but not in the color palette you specify for the
optimized image. You can control the application
dither that Photoshop or ImageReady applies to an
optimized image.
You can choose a dithering pattern to be applied to
the image. In addition, you can create customized
dither patterns for GIF or PNG-8 images using the
DitherBox filter. (See "Creating and applying
custom dither patterns" on page 344.)
Note: Applying a dithering pattern is not
recommended for JPEG or PNG-24 images.
Browser dither occurs when a Web browser using
•
an 8-bit color display (256-color mode) attempts
to simulate colors that appear in an optimized
image but not in the color palette used by the
browser. Browser dither can occur with GIF, PNG,
or JPEG images and can occur in addition to appli-
cation dither in GIF or PNG-8 images. You can
control the amount of browser dither by shifting
selected colors in the image to Web-safe colors.
Options in the color picker, the Color palette in
Photoshop or ImageReady, and the Color Table
panel (Photoshop) or the Color Table palette
(ImageReady) let you specify Web-safe colors
when choosing a color.
Previewing and controlling application
dither
You can preview application dither in GIF and
PNG-8 images. The Dither Algorithm pop-up
menu lets you choose a dithering method for the
image. Images with primarily solid colors may
work well with Dither set to none. Images with
continuous-tone color (especially color gradients)
may require dithering to prevent color banding.
To control application dither:
Choose an option from the Dither Algorithm
1
pop-up menu:
No Dither applies no application dither to the
•
image.
Pattern applies a halftone-like square pattern to
•
simulate any colors not in the color table.
• Diffusion applies a random pattern that is
usually less noticeable than Pattern dither.
The dither effects are diffused across adjacent
pixels. Diffusion dither may cause detectable
seams to appear across slice boundaries. Linking
slices diffuses the dither pattern across all linked
slices, and eliminates the seams.
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.
If you chose Diffusion as the dithering
2
algorithm, drag the Dither slider or enter a value to
select a dithering percentage.