Glossar

An Alpha value indicates the transparency of a pixel. Besides its Red, Green and Blue values, a pixel has an alpha value. The smaller the alpha value of a pixel, the more visible the colors below it. A pixel with an alpha value of 0 is completely transparent. A pixel with an alpha value of 255 is fully opaque.

With some image file formats, you can only specify that a pixel is completely transparent or completely opaque. Other file formats allow a variable level of transparency.

An alpha channel of a layer is a grayscale image of the same size as the layer representing its transparency. For each pixel the gray level (a value between 0 and 255) represents the pixels's Alpha value. An alpha channel can make areas of the layer to appear partially transparent. That's why the background layer has no alpha channel by default.

The image alpha channel, which is displayed in the channels dialog, can be considered as the alpha channel of the final layer when all layers have been merged.

See also Example for Alpha channel.

Antialiasing is the process of reversing an alias, that is, reducing the „jaggies“. Antialiasing produces smoother curves by adjusting the boundary between the background and the pixel region that is being antialiased. Generally, pixel intensities or opacities are changed so that a smoother transition to the background is achieved. With selections, the opacity of the edge of the selection is appropriately reduced.

A spline is a curve which is defined mathematically and has a set of control points. A Bézier spline is a cubic spline which has four control points, where the first and last control points (knots or anchors) are the endpoints of the curve and the inner two control points (handles) determine the direction of the curve at the endpoints.

In the non-mathematical sense, a spline is a flexible strip of wood or metal used for drawing curves. Using this type of spline for drawing curves dates back to shipbuilding, where weights were hung on splines to bend them. The outer control points of a Bézier spline are similar to the places where the splines are fastened down and the inner control points are where weights are attached to modify the curve.

Bézier splines are only one way of mathematically representing curves. They were developed in the 1960s by Pierre Bézier, who worked for Renault.

Bézier curves are used in GIMP as component parts of Paths.

The image above shows a Bézier curve. Points P0 and P3 are points on the Path, which are created by clicking with the mouse. Points P1 and P2 are handles, which are automatically created by GIMP when you stretch the line.

From The Free Online Dictionary of Computing (13 Mar 01) :

BMP is an uncompressed image file format designed by Microsoft and mainly used in Windows. Colors are typically represented in 1, 4 or 8 bits, although the format also supports more. Because it is not compressed and the files are large, it is not very well suited for use in the internet.

Bump mapping is a technique for displaying extremely detailed objects without increasing the geometrical complexity of the objects. It is especially used in 3-dimensional visualization programs. The trick is to put all the necessary information into a texture, with which shadowing is shown on the surface of the object.

Bump mapping is only one (very effective) way of simulating surface irregularities which are not actually contained in the geometry of the model.

A channel refers to a certain component of an image. For instance, the components of an RGB image are the three primary colors red, green, blue, and sometimes transparency (alpha).

Every channel is a grayscale image of exactly the same size as the image and, consequently, consists of the same number of pixels. Every pixel of this grayscale image can be regarded as a container which can be filled with a value ranging from 0 to 255. The exact meaning of this value depends on the type of channel, e.g. in the RGB color model the value in the R-channel means the amount of red which is added to the color of the different pixels; in the selection channel, the value denotes how strongly the pixels are selected; and in the alpha channel the values denote how opaque the corresponding pixels are. See also Channels.

Channel encoding refers to how fast the intensity (more technically correct for grayscale and RGB images, the relative Luminance) of a channel in a digital image progresses from dark to light as the channel values progress from 0.0 to 1.0 floating point (0 to 255 for 8-bit integer, 0 to 65535 for 16-bit integer).

Other ways of referring to "channel encoding" include "companding curve", "gamma" (which is technically not correct unless the channel encoding is an actual gamma curve), "tone reproduction curve" ("TRC" for short), and "tone response curve" (also "TRC" for short).

The linear light channel encoding reflects the way lightwaves combine there in the real world. The linear light channel encoding is also referred to as "gamma=1.0", "linear gamma" or simply "linear".

Perceptually uniform channel encodings reflects the way our eyes respond to changes in luminance.

In ICC profile color managed workflows, the following channel encodings are commonly used:

The Linear light, sRGB, and LAB channel encodings compared.

Looking at the above image:

In GIMP 2.10 two different channel encodings are used internally for various editing operations, these being "Linear light" and "Perceptually uniform (sRGB)".

The companding-curves-compared.png shown above is a slightly modified version of an image from Completely Painless Programmer's Guide to XYZ, RGB, ICC, xyY, and TRCs , which is licensed as Creative Commons Attribution-ShareAlike 3.0 Unported License.

A channel masks is a special type of mask which determines the transparency of a selection. See Masks for a detailed description.

The Clipboard is a temporary area of memory which is used to transfer data between applications or documents. It is used when you Cut, Copy or Paste data in GIMP.

The clipboard is implemented slightly differently under different operating systems. Under Linux/XFree, GIMP uses the XFree clipboard for text and the GIMP internal image clipboard for transferring images between image documents. Under other operating systems, the clipboard may work somewhat differently. See the GIMP documentation for your operating system for further information.

The basic operations provided by the clipboard are „Cut“, „Copy“, and „Paste“. Cut means that the item is removed from the document and copied to the clipboard. Copy leaves the item in the document and copies it to the clipboard. Paste copies the contents of the clipboard to the document. The GIMP makes an intelligent decision about what to paste depending upon the target. If the target is a canvas, the Paste operation uses the image clipboard. If the target is a text entry box, the paste operation uses the text clipboard.

CMYK is a color model which has components for Cyan, Magenta, Yellow and Black. It is a subtractive color model, and that fact is important when an image is printed. It is complementary to the RGB color model.

The values of the individual colors vary between 0% and 100%, where 0% corresponds to an unprinted color, and 100% corresponds to a completely printed area of color. Colors are formed by mixing the three basic colors.

The last of these values, K (Black), doesn't contribute to the color, but merely serves to darken the other colors. The letter K is used for Black to prevent confusion, since B usually stands for Blue.

Abbildung 1141. Subtractive color model

GIMP does not currently support the CMYK model. (An experimental plug-in providing rudimentary CMYK support can be found [PLUGIN-SEPARATE].)

This is the mode used in printing. These are the colors in the ink cartridges in your printer. It is the mode used in painting and in all the objects around us, where light is reflected, not emitted. Objects absorb part of the light waves and we see only the reflected part. Note that the cones in our eyes see this reflected light in RGB mode. An object appears Red because Green and Blue have been absorbed. Since the combination of Green and Blue is Cyan, Cyan is absorbed when you add Red. Conversely, if you add Cyan, its complementary color, Red, is absorbed. This system is subtractive. If you add Yellow, you decrease Blue, and if you add Magenta, you decrease Green.

It would be logical to think that by mixing Cyan, Magenta and Yellow, you would subtract Red, Green and Blue, and the eye would see no light at all, that is, Black. But the question is more complex. In fact, you would see a dark brown. That is why this mode also has a Black value, and why your printer has a Black cartridge. It is less expensive that way. The printer doesn't have to mix the other three colors to create an imperfect Black, it just has to add Black.

On the one hand, light comes from the sun or other radiant sources, and is refractedby mediums (water, the atmosphere, glass) and diffusely or specularly reflected by surfaces.

On the other hand, color isn't out there in the world in the same tangible way that light is. Rather color is part of how we sense the world around us. Light enters the eyes, is processed by light receptors (cones and rods), and sent via the optic nerves to the brain for further processing and interpretation.

Light varies in wavelengths, which our eyes and brain interpret as varying hues (reds, blues, greens, and so on), and also in intensity (aka "luminance"). So our perception of color is composed of both intensity ("luminance") information and chromaticity information.

The naming of colors carries one out of the narrow realm of color perception, and into the larger realm of cultural and linguistic interpretation and classification of color, and thence into even larger philosophical, aesthetic, theological, and metaphysical considerations.

The above explanation of Color is a slightly modified excerpt from the Completely Painless Programmer's Guide to XYZ, RGB, ICC, xyY, and TRCs , which is licensed as Creative Commons Attribution-ShareAlike 3.0 Unported License.

Color depth is simply the number of bits used to represent a color (bits per pixel : bpp). There are 3 channels for a pixel (for Red, Green and Blue). GIMP can support 8 bits per channel, referred as eight-bit color. So, GIMP color depth is 8 * 3 = 24, which allows 256 * 256 * 256 = 16,777,216 possible colors (8 bits allow 256 colors).

A color model is a way of describing and specifying a color. The term is often used loosely to refer to both a color space system and the color space on which it is based.

A color space is a set of colors which can be displayed or recognized by an input or output device (such as a scanner, monitor, printer, etc.). The colors of a color space are specified as values in a color space system, which is a coordinate system in which the individual colors are described by coordinate values on various axes. Because of the structure of the human eye, there are three axes in color spaces which are intended for human observers. The practical application of that is that colors are specified with three components (with a few exceptions). There are about 30 to 40 color space systems in use. Some important examples are:

The phrase "display-referred" refers to images that can be displayed (either directly or by means of ICC profile color management) on devices. The displaying device might be a monitor, or an image printed on paper, or some other display technology.

Regardless of the technology, when you display an image on a device, that device has a maximum and minimum brightness. The maximum and minimum brightnesses are referred to as display-referred white and display-referred black.

The above explanation is a slightly modified excerpt from Models for image editing: Display-referred and scene-referred. The modified excerpt was written and quoted by permission of the author, who has licensed the modified excerpt under the Creative Commons Attribution-ShareAlike 3.0 Unported License.

"Display-referred black" (or for simplicity, "black") means the floating point RGB color (0.0, 0.0, 0.0) and its integer equivalents. This color has the very special significance that there's no such thing as "less bright than black". So in display-referred image editing, all RGB channel values are greater than or equal to 0.0 and no color is less bright than "black", (0.0, 0.0, 0.0).

The above explanation is a slightly modified excerpt from Models for image editing: Display-referred and scene-referred. The modified excerpt was written and quoted by permission of the author, who has licensed the modified excerpt under the Creative Commons Attribution-ShareAlike 3.0 Unported License.

"Display-referred white" (or for simplicity, "white") means the floating point RGB color (1.0, 1.0, 1.0) and the integer equivalents (255,255,255),(65535,65535,65535), etc, for 8-bit integer, 16-bit integer, etc.

"Display-referred white" has the very special significance that in display-referred editing there's no such thing as "brighter than white". So in display-referred image editing, all RGB channel values are less than or equal to 1.0 and no color is brighter than "white", (1.0, 1.0, 1.0).

The above explanation is a slightly modified excerpt from Models for image editing: Display-referred and scene-referred. The modified excerpt was written and quoted by permission of the author, who has licensed the modified excerpt under the Creative Commons Attribution-ShareAlike 3.0 Unported License.

Dithering is a technique used in computer graphics to create the illusion of more colors when displaying an image which has a low color depth. In a dithered image, the missing colors are reproduced by a certain arrangement of pixels in the available colors. The human eye perceives this as a mixture of the individual colors.

The Gradient tool uses dithering. You may also choose to use dithering when you convert an image to Indexed format. If you are working on an image with indexed colors, some tools (such as the pattern fill tool) may also use dithering, if the correct color is not available in the colormap.

The Newsprint filter uses dithering as well. You can use the NL Filter (Non Linear filter) to remove unwanted dithering noise from your image.

Also note that although GIMP itself uses 24-bit colors, your system may not actually be able to display that many colors. If it doesn't, then the software in between GIMP and your system may also dither colors while displaying them.

See also the glossary entry on Floyd-Steinberg dithering, which is used in GIMP.

Exchangeable image file format (official abbreviation Exif, not EXIF) is a specification for the image file format used by digital cameras. It was created by the Japan Electronic Industry Development Association (JEIDA). The specification uses the existing JPEG, TIFF Rev. 6.0, and RIFF WAVE file formats, with the addition of specific metadata tags. It is not supported in JPEG 2000 or PNG. Version 2.1 of the specification is dated June 12, 1998 and version 2.2 is dated April 2002. The Exif tag structure is taken from that of TIFF files. There is a large overlap between the tags defined in the TIFF, Exif, TIFF/EP and DCF standards [WKPD-EXIF].

The process of Feathering makes a smooth transition between a region and the background by softly blending the edges of the region.

In GIMP, you can feather the edges of a selection. Brushes can also have feathered edges.

A file format or file type is the form in which computer data is stored. Since a file is stored by an operating system as a linear series of bytes, which cannot describe many kinds of real data in an obvious way, conventions have been developed for interpreting the information as representations of complex data. All of the conventions for a particular „kind“ of file constitute a file format.

Some typical file formats for saving images are JPEG, TIFF, PNG and GIF. The best file format for saving an image depends upon how the image is intended to be used. For example, if the image is intended for the internet, file size is a very important factor, and if the image is intended to be printed, high resolution and quality have greater significance. See Format types.

A floating selection (sometimes called a „floating layer“) is a type of temporary layer which is similar in function to a normal layer, except that a floating selection must be anchored before you can resume working on any other layers in the image.

Floating selections are described in Abschnitt 4.5, „Float“.

In early versions of GIMP, when GIMP did not use layers, floating selections were used for performing operations on a limited part of an image (you can do that more easily now with layers). Now floating selections have no practical use, but you must know what you have to do with them.

Floyd-Steinberg dithering is a method of dithering which was first published in 1976 by Robert W. Floyd and Louis Steinberg. The dithering process begins in the upper left corner of the image. For each pixel, the closest available color in the palette is chosen and the difference between that color and the original color is computed in each RGB channel. Then 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.

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.

In color reproduction, including computer graphics and photography, the gamut, or color gamut (pronounced /ˈgæmət/), is a certain complete subset of colors. The most common usage refers to the subset of colors which can be accurately represented in a given circumstance, such as within a given color space or by a certain output device. Another sense, less frequently used but not less correct, refers to the complete set of colors found within an image at a given time. In this context, digitizing a photograph, converting a digitized image to a different color space, or outputting it to a given medium using a certain output device generally alters its gamut, in the sense that some of the colors in the original are lost in the process. [WKPD-GAMUT]

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 photographs, 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“.

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.

The name came about from the naming conventions which were in practice at MIT, where Stallman worked at the time. For programs which were similar to other programs, recursive acronyms were chosen as names. Since the new system was to be based on the widespread operating system, Unix, Stallman looked for that kind of name and came up with GNU, which stands for „GNU is not Unix“. In order to avoid confusion, the name should be pronounced with the „G“, not like „new“. There were several reasons for making GNU Unix-compatible. For one thing, Stallman was convinced that most companies would refuse a completely new operating system, if the programs they used wouldn't run on it. In addition, the architecture of Unix made quick, easy and distributed development possible, since Unix consists of many small programs that can be developed independently of each other, for the most part. Also, many parts of a Unix system were freely available to anyone and could therefore be directly integrated into GNU, for example, the typesetting system, TeX, or the X Window System. The missing parts were newly written from the ground up.

GIMP (GNU Image Manipulation Program) is an official GNU application [WKPD-GNU].

Grayscale is a mode for encoding the colors of an image which contains only black, white and shades of gray.

When you create a new image, you can choose to create it in Grayscale mode (which you can colorize later, by changing it to RGB mode). You can also change an existing image to grayscale by using the Grayscale, Desaturate, Decompose, Channel Mixer, although not all formats will accept these changes. Although you can create images in Grayscale mode and convert images to it, it is not a color model, in the true sense of the word.

As explained in RGB mode, 24-bit GIMP images can have up to 256 levels of gray. If you change from Grayscale to RGB mode, your image will have an RGB structure with three color channels, but of course, it will still be gray.