Adobe 13101332 - Photoshop - Mac User Manual page 99

Adobe Photoshop Help
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Previous Uses the custom palette from the previous conversion, making it easy to
convert several images with the same custom palette.
Number of colors
For the Uniform, Perceptual, Selective, or Adaptive palette, you can specify the exact
number of colors to be displayed (up to 256) by entering a value for Colors. The Colors text
box controls only how the indexed color table is created. Adobe Photoshop still treats the
image as an 8-bit, 256-color image.
Color inclusion and transparency
To specify colors to be included in the indexed color table or to specify transparency in the
image, choose from the following options:
Forced Provides options to force the inclusion of certain colors in the color table. Black
and White adds a pure black and a pure white to 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.
Transparency Specifies whether to preserve transparent areas of the image during
conversion. Selecting this option adds a special index entry in the color table for trans-
parent colors. Deselecting this option fills transparent areas with the matte color, or with
white if no matte color is chosen.
Matte Specifies the background color used to fill anti-aliased edges that lie adjacent to
transparent areas of the image. With Transparency selected, the matte is applied to edge
areas to help blend the edges with a Web background of the same color. With Trans-
parency deselected, the matte is applied to transparent areas. Choosing None for the
matte creates hard-edged transparency if Transparency is selected; otherwise, all trans-
parent areas are filled with 100% white.
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 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.
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Working with Color
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