Adobe PHOTOSHOP ELEMENTS Manual page 244

CHAPTER 11
236
Optimizing Images for the Web
the appearance of a third color. For example, a red
color and a yellow color may dither in a mosaic
pattern to produce the illusion of an orange color
that does not appear in the color palette.
When optimizing images, keep in mind that two
kinds of dithering can occur:
Application dither occurs in GIF and PNG-8
•
images when Photoshop Elements attempts to
simulate colors that appear in the original image
but not in the color lookup table. 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
custom dither patterns" on page 237.)
• 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 let you specify Web-
safe colors when choosing a color.
filter. (See "Creating and applying
™
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 to apply no application dither to
•
the image.
Pattern to apply a halftone-like square pattern to
•
simulate any colors not in the color table.
• Diffusion to apply a random pattern that is
usually less noticeable than Pattern dither.
The dither effects are diffused across
adjacent pixels.
Noise to apply 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.
The Dither percentage controls the amount of
dithering that is applied to the image. A higher
dithering percentage creates the appearance of
more colors and more detail in an image,