Adobe Photoshop CS6 User Manual page 151

Transparency is selected; otherwise, all transparent areas are filled with 100% white. The image must have transparency for the Matte
options to be available.
Dithering Unless you're using the Exact color table option, the color table may not contain all the colors used in the image. To simulate colors not
in the color table, you can dither the colors. Dithering mixes the pixels of the available colors to simulate the missing colors. Choose a dither
option from the menu, and enter a percentage value for the dither amount. A higher amount dithers more colors but may increase file size. You
can choose from the following dither options:
None Does not dither colors but instead uses the color closest to the missing color. This tends to result in sharp transitions between
shades of color in the image, creating a posterized effect.
Diffusion Uses an error-diffusion method that produces a less-structured dither than the Pattern option. To protect colors in the image
that contain entries in the color table from being dithered, select Preserve Exact Colors. This is useful for preserving fine lines and text for
web images.
Pattern Uses a halftone-like square pattern to simulate any colors not in the color table.
Noise Helps to reduce seam patterns along the edges of image slices. Choose this option if you plan to slice the image for placement in
an HTML table.
More Help topics
About color working spaces
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