This conversion tool will convert non-indexed images to indexed ones. This includes conversion from RGB or grayscale images. An indexed image contains only those colors specified in its color palette. The maximal number of colors supported by The GIMP for indexed images is 256. Transparent GIF images can only have a maximum of 255 colors as the last color is used to describe the transparency.
General Palette Options
This option allows user specified palette depth. Unless a more specific depth is required for some reason, this option is the most useful of the three palette options.
This option provides selection of a predefined palettes to choose from. Conversion to one of these palettes will ensure that only colors used in the named palette will be used. Of the custom palettes, the Web palette is most useful. It provides a palette of colors that are considered to be web safe.
This option will convert the image to one that uses only two colors. Black or white.
Dithering refers to algorithms which are used to allow images with reduced color sets to mimic colors that are not within the palette.
Dither Options
Disables dithering of colors entirely.
Positional dithering is for use with animations. Animations can contain many frames and each frame is converted at the same time. With the more complex dithering systems, the colors in each from may differ slightly and in these situations the animation may look poor when playing. Positional dithering ensures that there is balanced dithering across the frames leading to smoother results for animations.
This option utilizes Floyd-Steinberg dithering to enhance the image. In particular, this option helps to reduce bleeding in high contrast images or images with high levels of color saturation.
This option (as above) utilizes Floyd Steinberg dithering to enhance the image. Unlike the previous type, Normal mode does not make any attempt to adjust the results for color saturation.
Indexed images that have transparency information cannot contain partial transparency information. Transparency can either be on or off. To help in situations where this is not suitable, the Enable Dithering of Transparency option can be enabled so that areas of subtle transparency can be faked by using dithering.
Custom Palette Options
This option is only available when a custom palette is used. If there are any colors in the palette that are not in the image, this option will remove them from the image palette. This will reduce the file size of the indexed image. This option is recommended.
Default Keyboard Shortcut: Alt-I