Color menu

In color images, each pixel value is a number that represents the color of that pixel. In indexed-color (8-bit/pixel) images, on the other hand, the pixel value has no fixed meaning. The image uses a colormap which defines the relationship between pixel value and color. If an 8-bit (256-color) image which lacks a colormap of its own is loaded and converted to color, the most recently-selected colormap will be used to perform the conversion.

There are several different ways of changing the colors of an image:

Change colors

The brightness of each red, green, or blue value is adjusted by dragging control points that define a brightness curve. This is the most powerful method, but it is also more difficult to change multiple images in precisely the same way because it is interactive.

Brightness

Changes the red, green, blue, hue, saturation, or value component of each pixel by adding or subtracting some user-specified number. The change is easier to reproduce and can also be done in `image math'.

Gamma

Changes the gamma value of an image. This makes dark areas lighter without affecting normal areas. Ideal for repairing underexposed photographs.

Contrast

Similar to `Brightness', except that instead of adding a number, the pixels are decomposed into their RGB or HSV values and the R,G,B,H,S, or V components are multiplied by separate, constant factors. For example, you could increase the `contrast' of the red pixels while decreasing the contrast of blue.

Colormap/False color

Allows selection of colormaps for indexed-color or grayscale images. User-defined colormaps are also supported.

Remap colors

Switches colors. For example, if some other program created an image in which `red' appeared `blue', this option could repair the image.

Change pixel value

The most basic operation, adds or subtracts a raw number from the image. This is useful for grayscale images, which can be made lighter or darker by a precisely-defined amount.

Each of these options is described in greater detail below.