Convert to Indexed
This mode enables you to convert RGB or grayscale images to indexed images. An indexed image is an image which only has the colors specified in its color palette. The color palette is saved in the image file. The maximum number of colors in an indexed image is 256. If you want to make transparent GIF images, then you can only use a maximum of 255 colors since the last color will be used to determine if the pixel is opaque or transparent.
Palette Options
- Generate optimal palette
For the most part, this option is the best to use when creating an indexed image. GIMP will evaluate your colors and create a color palette suitable for the image. You can specify the number of colors that you want to have in your indexed image, but remember that you can't have more than 255 colors if are about to create an indexed image with transparency.
- Use custom palette
If you want to use a predefined palette, you have to use this option. You then choose your palette from the drop down menu. By default it's Web palette. The Web palette is the palette used by web browsers such as Netscape. This will help you create web-safe indexed images.
There is some debate over indexing against the Web palette.
- Custom Palette Options
Remove unused colors from final palette: If the palette contains colors that aren't used in the indexed image, you can remove the extra colors and make the image file size smaller. This is a good option so keep it enabled.
- Use black/white (1-bit) palette
This option will create a monochrome image only built up of black and white pixels.
Dithering
An indexed image can only be built up of a maximum of 256 colors. Most of the time this is quite limiting and you will not be able to have all the colors in your image represented in this limited color space. The image might lo