The Colormap (Indexed Palette is a better name) dialog allows you to
edit the colormap of an indexed image. (If the mode of the active image
is RGB or Grayscale instead of Indexed, the dialog is empty and
unusable.)
This is a dockable dialog; see the section on Dialogs and Docking for help on
manipulating it. It can be activated in two ways:
2.4.1.
Dialog call
From the Toolbox menu:
File->Dialogs->Colormap
From the image menu:
Dialogs->Colormap
2.4.2.
Colormaps and Indexed Images
In an Indexed image, colors are assigned to pixels by an indirect
method, using a lookup table called a colormap.
In GIMP, the maximum number of entries in a colormap is 256. For a
maximum-sized colormap, each index from 0 to 255 is assigned an
arbitrary RGB color. There are no rules restricting the colors that can
be assigned to an index or the order they appear in: any index can be
assigned any color.
In an Indexed image, instead of being assigned a color directly (as
happens in RGB and Grayscale images), each pixel is assigned an index.
To determine the color that should be shown for that pixel, GIMP looks
up the index in the image's colormap. Each indexed image has its own
private colormap.
It is important to realize that the colors in the colormap are the
only colors available
for an indexed image (that is, unless you add new colors to the
colormap). This has a major effect on many GIMP operations: for example,
in a pattern fill, GIMP will usually not be able to find exactly the
right colors in the colormap, so it will approximate them by
using the nearest color available.
[2]
If the colormap is too limited or poorly chosen, this can easily
produce very poor image quality.
The Colormap dialog allows you to alter the colormap for an image,
either by creating new entries, or by changing the colors for the
existing entries. If you change the color associated with a given index,
you will see the changes reflected throughout the image, as a color
shift for all pixels that are assigned that index. The entries are
numbered with 0 in the upper left corner, 1 to its right, etc.
2.4.3.
Using the Colormap dialog
Here are the operations you can perform using this dialog:
Click on a color entry
This sets GIMP's foreground color to the color you click on, as
shown in the Toolbox color area. As a result, this color will be
used for the next painting operation you do.
Ctrl-click on a color entry
This sets GIMP's background color to the color you Ctrl-click on,
as shown in the Toolbox color area.
Double-click on a color entry
This sets GIMP's foreground color to the color you click on, and
also brings up a Color Editor that allows you to change that
colormap entry to a new color.
Color index
You can select a different colormap entry by typing its index
here, or clicking the spinbutton to the right.
Hex triplet
This area shows a hex-code representation (such as is used in
HTML) for the color assigned to the currently selected colormap
entry. You can edit the color here, instead of using a Color
Editor, if you want to. See
HexTriplet
Edit color
This button (in the lower left corner of the dialog) brings up a
Color Editor that allows you to change the color for the currently
selected colormap entry. The effect is similar to double-clicking
on the entry, except that it does not set GIMP's foreground color.
Add color
This button (in the lower right corner of the dialog) allows you
to add new colors to the colormap. If you click on the button, the
current foreground color, as shown in the Toolbox, will be tacked
on to the end of the colormap. If instead you hold down
Ctrl and click, the background color from the
Toolbox will be added. (If the colormap contains 256 entries, it
is full, and trying to add more will have no effect.)
Tip
If you make a mistake, you can undo it by focusing the pointer in
the image whose colormap you have changed, and then pressing
Ctrl-Z or choosing
Edit->Undo
in the image menu.
Note
This dialog provides the most commonly used methods for altering the
colormap for an indexed image. The color tools, such as
Brightness/Contrast, Hue/Saturation, etc, do not operate on indexed
images. There are a few plug-ins that do so, including the
"Normalize", "Color Enhance", and "Stretch Contrast" operations, and
it is possible to create others as well.
[2]
This is sometimes referred to as
Quantization, which
is described in the Glossary.
Published under the terms of the GNU General Public License
![[Tip]](../images/tip.png)
![[Note]](../images/note.png)
