Glossary¶

Alphabetical listing.

• ACES : Academy Color Encoding System is a standard of exchange, of color management for digital imaging. It aims to simplify color management by maintaining color fidelity throughout the production pipeline. The standard is free and open source and many companies contribute to its development. It uses in particular the OCIO* software library.

• Alpha : a fourth channel is sometimes added to the pixels of a video to store transparency information in addition to color. The RGBA and YUVA channels are usually noted in this case, and in the case of YUVA a fourth value is sometimes added to the chroma subsampling acronym: 4:4:4:4, 4:2:2:4*, etc.

• Bit : The basic unit of both binary computer calculation and storage. A bit is either 0 or 1. 8 bits make up a byte, a sequence of eight 0s or 1s. Not to be confused with the Byte. Noted b (whereas Byte is noted B and byte is noted o).

• Byte : smallest unit of usable memory on a given system (pronounced /baɪt/); usually made up of 8 bits and (only in this case but not necessarily) equivalent to one byte

• Candela : Unit of the international system representing a luminous intensity, i.e. a quantity of light emitted by a light source in a given direction (whereas the lumen* represents this quantity of light in total, in all directions). Symbol : cd. Cf. Intensity.

• CIE : La Commission internationale de l’éclairage is an international organization dedicated to light, lighting, color and color spaces. It was founded in Berlin in 1913 and is currently based in Vienna, Austria. Note: the French acronym CIE is the one used internationally although in English it is the International Commission on Illumination.

• Codec : abbreviation for encoder-decoder. Software used to encode and decode a video or audio stream into a certain standard*. By the way, the codec is often confused with the standard or format, but it is indeed software and not a format. For example, x264 or nvenc are codecs allowing to encode a video in the standard h.264 (in format mp4 for example).

• (Colors) complementary : set of colors that, when mixed, cancel the perception of color, producing a neutral gray. Two complementary colors are diametrically opposed on the color wheel. In the chromaticity diagram CIE XYZ, the points that represent them are aligned on either side of the white point.

• Container : synonym for file type. Defines how multimedia streams (audio, video, subtitles, metadata…) should be saved together in a specific file. For example: Quicktime (*.mov), MP4 (*.mp4), Matroska (*.mkv) are containers (but h.264 is a standard, and x264 is a codec*). Some containers are specialized and impose a certain standard (e.g., a MP4 should always use the h.264 or h.265 standard), while others allow a large number of different standards (e.g., Quicktime allows PNG, Prores, RLE/Animation, MJPEG*, etc.).

• Contrast : difference in value between the most intense and the least intense point in an image.

• Channel (color) : AIn a pixel, the color is described by several values; each value is a channel of the pixel (or image). In RGB the three channels are red, green, and blue, in YUV, the luminance and two channels of chrominance.

• Transfer curve : mathematical function used to convert the intensities of a given color space to a linear scale (and vice versa). It is also called gamma even if the name gamma should rather be limited to only those functions that consist of a mathematical function of power, while transfer functions can take more varied and complex forms; they can however all have a relatively accurate approximation in a simple gamma. see also gamma and the chapter entitled Transfer curves, linear space and gamma.

• Proofing or Softproofing: Simulation of color conversion in a specific space which will be that of the final export (followed by a conversion towards the space of the screen for a correct display). This technique makes it possible to check that the successive conversions of the colors do not degrade the image too much and that the new colors are close to what one seeks, by checking what will see the final spectator. It is important to note that softproofing can remain far from reality if the space of exit is larger than that of the screen or too different (for example CMJN on a screen sRGB or Rec.2020 on a screen sRGB); it makes it possible however to preview what a user will see on a similar screen.

• Format : synonym for file type. See container. Not to be confused with standard and codec.

• Frequency : in the ondulatory representation of the light (and of all the range of electromagnetic waves), the frequency, measured in Hertz (Hz) is the number of undulations per second; it is the inverse of the wavelength* (F = 1 / λ with F for the frequency and λ for the wavelength): when the frequency increases, the wavelength decreases. In the visible part of the electromagnetic waves (the light), towards 10¹⁵Hz (1000 TeraHertz), each frequency corresponds to a precise color, a monochromatic light.

• Gamma : Mathematical function used as a transfer curve and for colorimetric retouching, consisting of a power; the gamma value is that of the power (or the inverse of the power for inverse retouching): x1/2.4 for a gamma 2.4, for example. see also transfer curve and the chapter titled Transfer Curves, Linear Space, and Gamma.

• Gamut : extent of colors that can be represented by a color space, represented by a subsurface in the chromaticity diagram CIE XYZ, in triangle when the space uses three primary. The area then represents the gamut; we also speak of the “width” of the gamut: the more colors the space contains, the more its gamut is “wide”.

• (Colors) Homochromes : cf. Metamers

• Illuminant : coordinates (in the CIE XYZ) of the point representing conventionally the white in a given color space. It can also be referred to by a standardized name, such as D65 or D60, or by a blackbody temperature, in Kelvin, 6500 K for example. See also White Temperature.

• Intensity : quantity of light received by a surface (reflecting or a sensor) or emitted by a light source, measured in number of photons received by a given surface in a given time (for example in photons per second per square centimeter). In the case of a light emitter (candle, lamp, screen), we speak in terms of a cone (a solid angle) of emission and not a receiving surface and the unit is the candela* noted cd¹.

• Linear : graphic representation of a proportional function, a mathematical function known as affine, represented by a straight line. The term linear is used to speak about color spaces whose correspondences of values with the physical intensity are proportional and thus represented by a straight line on a graph.

• Wavelength : in the ondulatory representation of the light (and of all the range of electromagnetic waves), the wavelength, measured in nanometers (nm) is the “size” of the undulations ; it is the reverse of the frequency* (λ = 1 / F with F for the frequency and λ for the wavelength) : when the frequency increases, the wavelength decreases. In the visible part of the electromagnetic waves (the light thus), in 400nm and 700nm approximately, each wavelength corresponds to a precise color, a *monochromatic* light.

• Luminance : representation of the physical intensity of light (or color) corresponding to a color on a linear scale. Different from luminosity or luma.

• Luminosity or Luma : representation of the intensity of the light (or the color), but on a scale adapted to the human perception, using a gamma (cf Transfer curve), contrary to the luminance.

• LUT : from Lookup Table. A LUT allows in computer science (especially in its early days) to replace complex mathematical functions (and especially time-consuming to realize) by tables of values; instead of calculating correspondences and conversions, one finds the value in the table. They are still used a lot in colorimetry: they allow to convert colors between different color spaces without worrying about the corresponding mathematical formula (and are thus an easy way to make a conversion between spaces in a software which does not manage them natively). They can cause a (small) loss of information if they are not precise enough (do not contain enough values). They are also used as calibration or colorimetry presets and can allow to easily replicate an effect, even in a software that would not allow it with its native color tools (as long as it can apply a LUT anyway). See the section entitled The LUTs for more details.

• (Colors) Metamers : two (or more) colors are said metamers when they are of identical appearance (the eye and the brain do not make the difference) although they are composed in reality of a different mixture of monochromatic rays*.

• Lumen : Unit of the international system representing luminous flux, symbol: lm. Measured by the total amount of light emitted by a light source in a given time.

• (Light) Monochromatic : pure light, composed of rays of a single precise frequency, without mixing frequencies*, like the light generated by a laser. The color of such a light is part of the spectrum of visible electro-magnetic waves, of the rainbow, going from blue to pure red (passing by cyan, green, yellow, orange…).

• Lux : Unit of the international system representing illumination (the amount of light received by a surface, which can be described as surface brightness), symbol: lx. 1 lux = 1 lm/m², 1 lux is equivalent to one lumen* per square meter. For example, a typical office lighting environment is between 300 and 500 lux, a cloudless sunset or sunrise around 400 lux, an overcast day at 1000 lux, a sunny day between 10,000 and 25,000 lux, with areas in full sunlight ranging from 32,000 to 100,000 lux.

• Black : The black is the color resulting from the absence of light. It is thus the color of weakest luminosity (null) and its saturation* cannot be defined.

• Standard (video and audio) : defines the way in which the audio and video data are encoded, the standard used, and therefore with which codec these data can be decoded.

• OCIO : OpenColorIO is a software library (a “brick”/developer tool usable in other software/applications) dedicated to color management. It is free and open source, and has become standard with its inclusion and use in a large number of applications (natively on Maya, Arnold, Krita, Blender, Nuke… ; via plugins and add-ons on After Effects… ; via LUTs*/Colorimetric profiles on Photoshop and others). This inclusion in different applications allows to share a single configuration of color management throughout the production pipeline and ensure identical color reproduction at all stages.

• Byte : 8 bits. Noté B.

• OpenColorIO : cf. OCIO

• OIIO : OpenImageIO mainly defines the standard for the openEXR image data storage format, and is a free and open source software library to handle this open format (and others), chosen as the default format by ACES*; it is interdependent with OCIO*.

• OpenEXR : Open and extremely versatile file format dedicated to images, and used in most production pipelines. Cf. OIIO.

• OpenImageIO : cf. OIIO

• Photon : Although the word dates from 1926, it was in 1905 that Albert Einstein theorized the photon as the quantum of light: the smallest indivisible quantity of light (of all electromagnetic waves in reality). The photon can be seen as the particular counterpart of the radius of light which would be the ondulatory counterpart. It is in reality both particle and wave. The intensity of light (and thus of colors as we perceive them) can be expressed in number of photons received by a given surface in a given time (thus for example in photons per square centimeter per second γ/cm²/s). We also measure the threshold of triggering (the lower level) and saturation (the upper level) of photo-sensitive sensors (such as retinal cells) in photons per second.

• White Point : cf. Illuminant

• Purple : The purples are the colors resulting from the mixture (only) of the two extremes of the spectrum of the visible monochromatic* lights: red and blue, in proportions which can vary. They are considered as saturated* colors.

• (Colors) Primaries : Colors chosen in a colorimetric space as basic colors for the representation, defining the gamut. Their mixture must make it possible to obtain white (in an additive system). In general, in digital imaging, a red, green and blue tint.

• Purity : cf. Saturation

• Colorimetric Purity : Ratio between the luminance of the monochromatic component of a light and the luminance of the total light.

• Purity of excitement : ratio between the amount of white and saturated color (monochromatic) in a light.

• Saturation : The most saturated colors possible are the monochromatic colors to which we add the purples (mixtures of red and blue). As soon as the colors are a mixture of several monochromatic* lights (except mixture of the blue and red extremes), the saturation decreases until the color becomes gray or white. The saturated colors go from black to the colors of the rainbow. We also speak of purity of light. cf. Colorimetric purity and Excitation purity.

• Tint : Color of the dominant wavelength of a light (of the pure equivalent of a given light), and thus the color of the pure or saturated equivalent of the light. We usually locate the colors on a circle by considering that the primary colors are equally spaced, with the primary red at 0 °, the primary green at 120 °, the primary blue at 240 °, and the mixtures, at intermediate angles, in proportion to the two primaries that compose them.

• Temperature of whites : Comparison of a “white” light to the color of a black body (a body that only emits light, and whose color is a direct result of its temperature, such as the sun, an ember, a flame, molten metal …). It is a way to define precisely the hue of a white, which is then expressed in temperature, using the Kelvin as unit. The yellow-orange of sunlight is around 5800 K, the blue of an electric flash at 9000 K, the orange of a candle around 1850 K, etc. See the chapter Back to whites: temperature

• Violet : see Purplee.

• YCbCr or YCC : digital version of the YUV..

• YPbPr : alternate name of YUV.

• YUV : System of coding of the colors in three channels: one of luminance or luminosity and two of chrominance. Several notations bring details on the system, although one uses very majority the term YUV:

  • When a prime is added to the Y, it is specified as luminosity (with a gamma) and not luminance (linear).
  • The correct terms in analog are: YUV or YPbPr with the luminance and Y’UV or Y’PbPr with the luminosity.
  • The correct terms in digital are: YCbCr or YCC with the luminance and Y’CbCr or Y’CC with the luminosity.