Generating a color table

You can change the palette--or set of colors--in the color table by selecting a color reduction option in the Optimize panel/palette. There are three categories of options:

• Dynamic options use a color reduction algorithm to build a palette based on the colors in the image and the number of colors specified in the optimization setting. The colors in the palette are regenerated every time you change or reoptimize the image. Perceptual, Selective, and Adaptive are dynamic options.
• Fixed options use a set palette of colors. In other words, the set of available colors is constant, but the actual colors in the palette will vary depending on the colors in the image. Web, Mac OS, Windows, Black & White, and Grayscale tables are fixed options.
• The Custom option uses a color palette that is created or modified by the user. If you open an existing GIF or PNG-8 file, it will have a custom color palette.

To select a color reduction algorithm:

Choose an option from the Color Reduction Algorithm pop-up menu (below the file format menu in the Optimize panel/palette):

Perceptual

Creates a custom color table by giving priority to colors for which the human eye has greater sensitivity.

Selective

Creates a color table similar to the Perceptual color table, but favoring broad areas of color and the preservation of Web colors. This color table usually produces images with the greatest color integrity. Selective is the default option.

Adaptive

Creates a custom color table by sampling colors from the spectrum appearing most commonly in the image. For example, an image with only the colors green and blue produces a color table made primarily of greens and blues. Most images concentrate colors in particular areas of the spectrum.

Web

Uses the standard 216-color color table common to the Windows and Mac OS 8-bit (256-color) palettes. This option ensures that no browser dither is applied to colors when the image is displayed using 8-bit color. (This palette is also called the Web-safe palette.) If your image has fewer colors than the total number specified in the color palette, unused colors are removed.

Using the Web palette can create larger files, and is recommended only when avoiding browser dither is a high priority.

Custom

Preserves the current color table as a fixed palette that does not update with changes to the image.

Mac OS

Uses the Mac OS system's default 8-bit (256-color) color table, which is based on a uniform sampling of RGB colors. If your image has fewer colors than the total number specified in the color palette, unused colors are removed.

Windows

Uses the Windows system's default 8-bit (256-color) color table, which is based on a uniform sampling of RGB colors. If your image has fewer colors than the total number specified in the color palette, unused colors are removed.

Other color tables appear in the menu if you have saved them previously. (See Loading and saving color tables.)

You can use an alpha channel to influence the generation of color tables. (See Using masks to modify color reduction.)

To regenerate a color table (ImageReady):

Choose Rebuild Color Table from the Color Table palette menu. Use this command to generate a new color table when the Auto Regenerate option is off. (See Controlling optimization (ImageReady).)