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 look like it is built up of "bands" or "color areas". To make indexed images look better, you can dither them. This means that two or more colors are mixed to mimic the missing color. The disadvantage is that the image can look like it's built up of "dots".
- No color dithering
Will disable dithering completely.
- Positioned color dithering
Use this option when you are dealing with animations such as GIF animations. The problem with dithering in animations is that the dithering will not be constant. If you choose positioned dithering instead, the dithering in constant areas will remain constant across your frames. It is not as good as Floyd Steinberg dithering, but is better than no dithering at all.
- Floyd Steinberg color dithering (reduced color bleeding)
With normal Floyd Steinberg dithering, you may experience too much color bleeding. This is very visible when you index gradients, causing an unnatural look. If you encounter this effect, it is advisable to use this option (i.e Floyd Steinberg dithering reduced colour bleeding).
- Floyd Steinberg dithering (normal)
This is the best option to use when you are indexing images. It is only in special cases that you will use the other dithering methods available.
- Enable dithering of transparency
Indexed images only have one transparency mode — either it is off (the pixel is totally solid) or on (the pixel is totally transparent). This makes it very difficult to index images with smooth transitions from opaque to transparent. When you enable dithering of transparency, GIMP will try to mimic the smooth transition by dithering pixels on and off.
A good alternative to transparency dithering is the Rightclick > Filters > Colors > Semi-Flatten function.
Additional Information
Default Keyboard Shortcut: Alt-I