User Guide

GNU Image Manipulation Program
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specific fractions of these differences are dispersed among several adjacent pixels which haven’t yet been visited (below
and to the right of the original pixel). Because of the order of processing, the procedure can be done in a single pass over
the image.
When you convert an image to Indexed mode, you can choose between two variants of Floyd-Steinberg dithering.
G
Gamma
Gamma or gamma correction is a non-linear operation which is used to encode and decode luminance or color values in
video or still image systems. It is used in many types of imaging systems to straighten out a curved signal-to-light or
intensity-to-signal response. For example, the light emitted by a CRT is not linear with regard to its input voltage, and the
voltage from an electric camera is not linear with regard to the intensity (power) of the light in the scene. Gamma encoding
helps to map the data into a perceptually linear domain, so that the limited signal range (the limited number of bits in each
RGB signal) is better optimized perceptually.
Gamma is used as an exponent (power) in the correction equation. Gamma compression (where gamma < 1) is used to
encode linear luminance or RGB values into color signals or digital file values, and gamma expansion (where gamma > 1)
is the decoding process, and usually occurs where the current-to-voltage function for a CRT is non-linear.
For PC video, images are encoded with a gamma of about 0.45 and decoded with a gamma of 2.2. For Mac systems, images
are typically encoded with a gamma of about 0.55 and decoded with a gamma of 1.8. The sRGB color space standard used
for most cameras, PCs and printers does not use a simple exponential equation, but has a decoding gamma value near 2.2
over much of its range.
In GIMP, gamma is an option used in the brush tab of the
GIMPressionist filter and in the Flame filter. The display filters
also include a Gamma filter. Also see the Levels Tool, where you can use the middle slider to change the gamma value.
GIF
GIF™ stands for Graphics Interchange Format. It is a file format with good, lossless compression for images with low
color depth (up to 256 different colors per image). Since GIF was developed, a new format called Portable Network
Graphics (PNG) has been developed, which is better than GIF in all respects, with the exception of animations and some
rarely-used features.
GIF was introduced by CompuServe in 1987. It became popular mostly because of its efficient, LZW compression. The
size of the image files required clearly less disk space than other usual graphics formats of the time, such as PCX or
MacPaint. Even large images could be transmitted in a reasonable time, even with slow modems. In addition, the open
licensing policy of CompuServe made it possible for any programmer to implement the GIF format for his own applications
free of charge, as long as the CompuServe copyright notice was attached to them.
Colors in GIF are stored in a color table which can hold up to 256 different entries, chosen from 16.7 million different color
values. When the image format was introduced, this was not a much of a limitation, since only a few people had hardware
which could display more colors than that. For typical drawings, cartoons, black-and-white photographs and similar uses,
256 colors are quite sufficient as a rule, even today. For more complex images, such as color photgraphs, however, a huge
loss of quality is apparent, which is why the format is not considered to be suitable for those purposes.
One color entry in the palette can be defined to be transparent. With transparency, the GIF image can look like it is non-
rectangular in shape. However, semi-transparency, as in PNG, is not possible. A pixel can only be either entirely visible or
completely transparent.
The first version of GIF was 87a. In 1989, CompuServe published an expanded version, called 89a. Among other things,
this made it possible to save several images in one GIF file, which is especially used for simple animation. The version
number can be distinguished from the first six bytes of a GIF file. Interpreted as ASCII symbols, they are ‘GIF87a’ or
‘GIF89a’.
GNU
The GNU project was started in 1983 by Richard Stallman with the goal of developing a completely free operating system.
It is especially well-known from the GNU General Public License (GPL) and GNU/Linux, a GNU-variant with a Linux
kernel.