This glossary contains definitions important for the GIMP development. These include functional definitions i.e. from computer graphics and color theory, photography, terms from BABL and GEGL etc.
Table of Contents
- 'A (uppercase)' -
- The alpha channel in a color model, for instance in RGBA.
- [babl] the alpha value in a color model. If combined with alpha-premultiplied color channels: the alpha value each color channel R, G, B was multiplied with to achieve Ra, Ga, Ba. Needed to get from RaGaBa back to RGB . The same applies to R'aG'aB'aA, YaA, Y'aA.
- 'a (lowercase)' -
- 'alpha' - opacity. An alpha value of 0 means 'transparent', an alpha value of 100 means 'fully opaque', values in between mean 'partly opaque'.
- 'B (uppercase)' -
- Blue color channel in the XYZ and RGB color models.
- brightness component of the HSB color model.
- In photography: symbol for brightness: obsolete, but may be found on older light meters. 
- Bulb (in photography): Used for long exposures timed by the photographer. Setting for shutter at which shutter opens when shutter button is pressed and stays open as long as shutter button is kept down. 
- 'b (lowercase)' - the axis in the CIELAB color model which denotes the blue-yellow component of the color. A low numerical value on this axis denotes blue and a high value yellow.
- 'babl' - a dynamic, any to any, pixel format translation library.
See also: babl website
- 'brightness' - attribute of a visual perception according to which an area or color appears to emit, or reflect, more or less light. This correlates directly with the amplitude of the lightwave.   Translation glossary
- 'C (uppercase)' -
- 'CIE' - Commission Internationale de l´Eclairage (International Commission on Illumination). Independent, non-profit organization for recommendations on photometry and colorimetry. Its aim is to exchange information on all matters relating to the science and art of light and lighting, colour and vision, photobiology and image technology worldwide. It is recognized by ISO as an international standardization body. The CIE has defined several "color spaces" that describe the range of visible colors in unambiguous numerical terms.  
- 'CIELAB' - a device independent color model defined by CIE in 1976. It describes colors as points on the three axes L, a, b. It is also said to be the reference color model in Adobe Photoshop (but not GIMP), e.g. internal computations from one color space to another are done there via CIELAB.
See also CIE's formal definition.
- 'CIEL*a*b*' - CIELAB
- 'CIE 1976 L*a*b*' - CIELAB
- 'Clang' - the C compiler of the LLVM project. Clang is able to cross-compile code, i.e. build GIMP for Windows on a Linux machine .
- 'CMY' - Cyan, Magenta, Yellow. The three "subtractive" primary colors used as the basis of all dye or pigment-based printing systems and color photography. A full range of colors including neutral grays and blacks can be produced with good cyan, yellow and magenta colorants (e.g. color film dyes) but typical printing inks are less than perfect. 
- 'CMYK' - Cyan, Magenta, Yellow, Key color (usually Black). The four colors commonly used in process color printing. Black is added to CMY to enhance the density of dark areas and solve gray balance problems encountered when trying to make neutral grays with CMY alone. 
- 'compiler' - a program to translate code in a programming language into executable machine code, i.e. gcc or Clang. In GIMP there's no recommended default C compiler. Instead the platform default will be used automatically.
- 'color model' - a mathematical model to describe colors and handle them formally, such as points in a coordinate system. This is the more theoretically abstract background of color computation. Practically this term is often used synonymously to color space.
Examples are CIELAB , RGB and XYZ .
- 'color space' - a geometric representation of color in space, usually of 3 dimensions . While color models are the theoretical background color spaces describe the set of visible or reproducable colors . Practically this term is often used synonymously to color model. To get a visual notion of various color spaces and their relationships see Bruce Lindblooms 3D Gamut Viewer demo.
Examples are AdobeRGB, sRGB and Wide Gamut RGB.
To assign a numerical vector to a real color you need both a color model and a color space. As an example the numbers (0,1,0) are given. In XYZ color model with sRGB color space they represent a pure green. In XYZ color model with AdobeRGB color space you get a pure, but more saturated green. In HSB color model+sRGB color space these numbers represent a pure black.
- 'color temperature' - formally the temperature of an ideal black body radiator whose radiation has the same chromaticity as that of a given stimulus.  Practically it's a synonym for white point . Not to be confused with the artistic classification into warm colors (i.e. yellow, orange) and cold colors (i.e. blue).
Unit: K (Kelvin).
- 'chroma' - Colorfulness of an area judged as a proportion of the brightness of a similarly illuminated area that appears white or highly transmitting.  
See also saturation.
- 'chromaticity' - The color properties of a sample judged independently of luminance, i.e. in terms of hue and saturation only. 
- 'D50' - a CIE standard illuminant with the spectral power distribution of daylight (which has a color temperature of 5003 Kelvin). It is chosen for the special needs of the paper printing industry and thus the preferred reference white for evaluating prints on paper. Compared to D65 it's a bit more yellowish.
- 'D65' - a CIE standard illuminant with the spectral power distribution of noon mid-latitude daylight (which has a color temperature of 6504 Kelvin). It's the white balance standard used for sRGB color space and to calibrate display screens. Compared to D50 it's a bit more bluish.
- 'gcc' - a C compiler. Part of the free GNU Compiler Collection. On Linux it usually ships with your distribution. On Windows it's part of MinGW. On OS X XCode versions before 4.2 used GCC; then Apple moved to llvm-gcc and llvm. FreeBSD 10 deprecates GCC support in favor of Clang.  GCC is able to cross-compile, i.e. build GIMP for Windows on a Linux machine.
- 'GEGL' - Generic Graphics Library, a graph based image processing framework. It is GIMP's foundation for nondestructive image editing in high bit depths. GEGL provides the infrastructure to do demand based, cached, nondestructive image editing on larger than RAM buffers. Through babl it provides support for a wide range of color models and pixel storage formats for input and output. GEGL support in current GIMP versions is experimental and will be official part of GIMP in version 2.10.
See also: GEGL website, GEGL porting matrix
- 'GPU computing' - a means to utilize the computing power of graphic-cards processors for general purposes.
- 'gradation' - relationship of reproduced lightness values to original lightness values in an imaging process. It is usually expressed as a 'tone curve', which is the Curves tool in GIMP. 
- 'GSoC' - Google Summer of Code. Annual mentoring program hosted by Google Inc. to offer and award students to work on open source projects during summer. GIMP has been participating for many years.
See also: GSoC website, GIMP's GSoC participation
- 'illuminance' - measurement of the amount of light coming from a light source. Illuminance is measured in footcandles or Lux. 
- 'illuminant' -
- a mathematical description of a real or imaginary light source described by its spectral power distribution. This definition applies especially for standard illuminants like D50 and D65. For examples and calculations see CIE's selected colorimetric tables (xls). Illuminant A was designated to represent tungsten light and Illuminant D was designated to represent daylight. 
- [coll.] any kind of light falling on a body or scene .
- 'K (uppercase)' -
- 'L (uppercase)' - The axis in the CIELAB color model which denotes the lightness component of the color. A low numerical value on this axis denotes a dark color and a high value a light color.
- 'Lab' - CIELAB
- 'L*a*b*' - CIELAB
- 'LGM' - Libre Graphics Meeting. Annual conference of developers and users of graphics open source software. GIMP has been participating for many years. Website
- 'light, linear' - light as it is in nature and captured by camera sensors (and thus is encoded in RAW images).
- 'light, perceived' - light as it is processed by eye and brain. The difference between linear and perceived light is that human eyes react to the same change of (linear) light with different sensitivity in dark or light environments. perceived light is the basis of color (e.g. color is the result of light, processed by eyes and brain).
- 'lightness' - a number that corresponds to the human perception of light from a surface (e.g. the brightness of a color relative to the brightness of a similar illuminated white area)  .
- 'luma' - the weighted sum of R', G' and B' color component (=in perceived light). It is often confused with luminance which actually refers to linear light. Wikipedia article
- 'LLVM' - Low Level Virtual Machine. A collection of modular and reusable compiler and toolchain technologies. Despite its name, LLVM has little to do with traditional virtual machines . You can get if for Linux, Windows and FreeBSD 9 from the LLVM download page. On OS X LLVM is part of XCode 4.2 and later. LLVM's Clang is the default C compiler in FreeBSD 10.
- 'luminance' -
- 'luminance, relative' - luminance with values normalized to 1...100 (100=reference white), see Wikipedia.
- 'Mired' - former, obsolete unit of reciprocal color temperature .
- 'OpenCL' - open standard for parallel programming of heterogeneous systems, i.e. using additional processors or GPU computing. In GIMP used to improve performance in GEGL.
See also website, GEGL porting matrix.
- 'RaGaBaA'- [babl] a color model with the alpha premultiplied channels R,G,B and alpha. It operates in linear light.
- 'R'aG'aB'aA'- [babl] a color model with the alpha premultiplied channels R,G,B and alpha. It operates in perceived light.
- 'R'G'B'' - [babl] a color model with the channels R,G,B. It operates in perceived light with an sRGB TRC which is roughly perceptually uniform.
- 'RGBA' - generally and in babl a color model with the channels R,G,B and alpha. It operates in linear light.
- 'R'G'B'A' - [babl] a color model with the channels R,G,B and alpha. It operates in perceived light with an sRGB TRC which is roughly perceptually uniform.
- 'TRC'- Tonal Response Curve. A curve describing the signal response of the eye or an sensor to changes in linear light and as such the foundation for perceived light.
- 'Value' -
- Level of brightness of a pixel or color as defined in HSV color model. 
- Designation of lightness of color in the Munsell system of color nomenclature. 
- Tone or brightness of a color as assessed subjectively: high values give light colours, low value colors are dark in appearance.
- [photo] in the Zone System the step or zone which corresponds to a certain range of subject brightness. 
- [photo] Camera setting or range of settings based on scales of shutter time, lens aperture, exposure value, luminance and film speed. 
- 'white point' - the light-source's or sensor's interpretation of white.
See also CIE's definition, ColorWiki.
- 'XYZ' - the first color model, based on experimental observations on humans (the 'CIE standard observer'). It operates in linear light.
- 'Y (uppercase)' -
FreeBSD 10 To Use Clang Compiler, Deprecate GCC by Michael Larabel at Phoronix
Article on cross-compiling using Clang