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421
Adobe Photoshop Help
Preparing Graphics for the Web
Using Help
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Contents
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Index
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421
Dithering
Dithering refers to the method of simulating colors not available in the color
display system of your computer. 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.
GIF image with 0% dither, and with 100% dither
Choose a option from the Dither Algorithm menu:
•
No Dither applies no dither to the image.
•
Diffusion applies a random pattern that is usually less noticeable than Pattern dither.
The dither effects are extended across adjacent pixels. If you select this algorithm,
specify a Dither percentage to control 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, but can also increase the file size. You can use masks from type
layers, shape layers, and alpha channels to vary the Dither percentage across an image.
This technique produces higher-quality results in critical image areas without sacri-
ficing file size. (See
“Using weighted optimization” on page 428
.)
Note:
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.
•
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 algorithm, but without
diffusing the pattern across adjacent pixels. No seams appear with the Noise algorithm.
Transparency and Matte
Select a combination of Transparency and Matte options to
specify how transparent pixels in the image are optimized:
•
To make fully transparent pixels transparent and blend partially transparent pixels with
a color, select Transparency and select a matte color.
•
To make all pixels with greater than 50% transparency fully transparent and all pixels
with 50% or less transparency fully opaque, select Transparency and choose None from
the Matte menu.