2.4. Colormap Dialog

2.4. Colormap Dialog
	2. Image Structure Related Dialogs

Revision History
Revision $Revision: 2551 $	2006-09-29	scb

Figure 14.14. An indexed image with 6 colors and its Colormap dialog

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. Activate dialog

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 look-up 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. ^[7] 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 HTML notation
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.

	Note
	If you paint an indexed image with a color which is not in the Colormap, GIMP will use the most similar color of the Colormap.

2.4.4. The Colormap context menu

Right-clicking on a color in the Colormap selects this color and opens a pop-up submenu:

Figure 14.15. The Colormap context menu

Edit color: This command opens a color selector which allows you to modify the color.
Add from FG: This option is enabled if the indexed palette contains less than 256 colors. The new color is appended to the color list.
Add from BG: This command is enabled only if the indexed palette contains less than 256 colors. The background color of the Toolbox is appended to the color list.
Rearrange Colormap: This command opens a dialog which can be understood very intuitively and doesn't need to be explained more.

Figure 14.16. The “Rearrange Colormap” window and its sub-menu

^[7] This is sometimes referred to as Quantization, which is described in the Glossary.