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Grokking The Gimp
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9.5.3 Converting to Indexed Color

For display on 8-bit (low-color) systems, the best way to control the color in an image is to convert it to Indexed format. Figure  9.27(a)

Figure 9.27: The Indexed Color Conversion Dialog
Figure 9.27

illustrates the Indexed Color Conversion dialog which is invoked by the Indexed  function found in the Image:Image/Mode menu.

The dialog allows the choice of three types of palettes. The first option, Generate Optimal Palette, is based on the colors actually in the image. The number of colors used in this palette can be specified up to a maximum of 256.

The second option, Use Custom Palette, allows you to select from a large collection of palettes including user-defined ones. The names of the GIMP's standard palettes are shown in Figure  9.27(b) and (c). The default custom palette, Web, is optimized for display on low-color systems using browsers such as Netscape Navigator or Internet Explorer. This consists of the 216 color, web-safe color palette illustrated in Figure  9.25. Choosing this option guarantees that the resulting indexed image will display with the same colors on almost all systems.

The final palette option choice, shown in Figure  9.27, is the Use Black/White (1-bit) Palette. This option converts the image to a true black and white (no grays) Indexed format.

The second area in the Indexed Color Conversion dialog is for dithering options. The following are the choices:

  • No colour dithering
  • Positioned colour dithering
  • Floyd-Steinberg colour dithering (reduced colour bleeding)
  • Floyd-Steinberg colour dithering (normal)
Normal Floyd-Steinberg dithering is the default. Some experimentation might be necessary to find the best conversion to Indexed, especially if the image consists of smooth color variations and large solid color regions. Read on!

Grokking The Gimp
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  Published under the terms of the Open Publication License Design by Interspire