Adobe Photoshop CC 2014 v.14.xx - Manual (Page 214)

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Transparency
Matte
Dithering
None
Diffusion
Pattern
Noise
the color table; Primaries adds red, green, blue, cyan, magenta, yellow, black, and white; Web adds the 216 web-safe colors; and Custom
lets you define custom colors to add.
Specifies whether to preserve transparent areas of the image during conversion. Selecting this option adds a special index
entry in the color table for transparent colors. Deselecting this option fills transparent areas with the matte color, or with white if no matte
color is chosen.
Specifies the background color used to fill anti-aliased edges that lie adjacent to transparent areas of the image. When
Transparency is selected, the matte is applied to edge areas to help blend the edges with a web background of the same color. When
Transparency is deselected, the matte is applied to transparent areas. Choosing None for the matte creates hard-edged transparency if
Transparency is selected; otherwise, all transparent areas are filled with 100% white. The image must have transparency for the Matte
options to be available.
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:
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.
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.
Uses a halftone-like square pattern to simulate any colors not in the color table.
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.
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About color working spaces
207