User Guide

CHAPTER 4. IMAGE 35
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.
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There is some debate over indexing against the Web palette.
Custom Palette Options Remove unused colors from final palette: If the palette con-
tains 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 Stein-
berg dithering, you may experience too much color bleeding. This is very visible
when you index gradients, causing an unnatural look. If you encounter this ef-
fect, 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 in-
dexing 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.