Glossary¶

Alphabetical order.

• ACES: the Academy Color Encoding System is a standard for exchange and color management in 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. In this case, the channels are usually noted RGBA and YUVA, 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: basic unit of both binary computer calculation and storage. A bit is either 0 or 1. 8 bits make up an octet, a sequence of eight 0s or 1s. Not to be confused with the Byte. Noted b (whereas Byte is noted B and octet is noted o).

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

• Byte: smallest unit of usable memory on a given system; usually made up of 8 bits and (only in this case but not necessarily) equivalent to one octet. Symbol: B.

• 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.

• (Color) Channel: In 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.

• CIE: the 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’s the International Commission on Illumination.

• Codec: abbreviation for coder-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’s indeed a software and not a format. For example, x264 or nvenc are codecs allowing to encode a video in the h.264 standard (in the mp4 format for example).

• Color Purity: ratio between the luminance of the monochromatic component of a light and the luminance of the total light.

• Complementary colors: set of colors that, when mixed, cancel the perception of hue, producing a neutral gray. Two complementary colors are diametrically opposed on the color wheel. In the CIE XYZ chromaticity diagram, 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., an 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.

• Dynamic range (of an image): difference between the highest and lowest light intensities, in terms of light (therefore in an infinite range), and not in numerical values (in this case it’s rather called contrast).

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

• Exposure: total quantity of light received by the sensor (film or digital sensor) during shooting. An overexposed image has a lack of detail in highlights. The latter appear uniformly white, or “burned”. An underexposed image has a lack of detail in the shadows, which appear black.

• Floating point (float): in computing, using a decimal number, the number of digits before and after the decimal point can vary (the decimal point isfloating). When using this type of number to represent colors, it makes it possible to record intensities over a range from 0.0 to an almost infinite value (and therefore store data representing reality), and where, by convention, the value 1.0 represents the maximum intensity of color display devices (screen, projector, etc.). In other words, when converting to integers, necessary for display, all values above 1.0 are ignored.

• 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 electromagnetic spectrum), the frequency, measured in Hertz (Hz) is the number of undulations per second; it’s the inverse of the wavelength* (F = 1 / λ with F being the frequency and λ being the wavelength): when the frequency increases, the wavelength decreases. In the visible part of the electromagnetic spectrum (the light), around 10¹⁵Hz (1000 TeraHertz), each frequency corresponds to a precise color, a monochromatic light.

• Gamma: mathematical function used as a transfer curve and for color corrections, consisting of a power; the gamma value is that of the power (or the inverse of the power for inverse conversion): for example, x1/2.4 is a 2.4 gamma. See also transfer curve and the chapter entitled Transfer Curves, Linear Space, and Gamma.

• Gamut: extent of colors that can be represented by a color space, represented by a subsurface in the CIE XYZ chromaticity diagram, a 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 “wider” its gamut is.

• Homochromes: cf. Metamers.

• Illuminant: coordinates (in CIE XYZ) of the point conventionally representing 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.

• Integer (int): in computing, an integer value (strictly positive if it’s unsigned, or relative if it’s signed) is capped. When using this type of number to represent colors, the maximum intensity is given by the maximum possible value, which depends on the number of bits used to record it. With 8 bits it’s 256, with 16 bits it is 65536.

• 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* symbol: 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.

• 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.

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

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

• LUT: Lookup Table. In computer science (especially in its early days), a LUT allows to replace complex mathematical functions (and especially time-consuming methods) by tables of values; instead of computing correspondences and conversions, one finds the value in the table. They’re still used a lot in color management: 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 doesn’t manage them natively). They can cause a (small) loss of information if they’re not precise enough (if they don’t contain enough values). They’re also used as calibration or color 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 LUTs for more details.

• 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.

• 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 actually composed of a different blend of monochromatic rays*. cf. Metamerism on Wikipedia

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

• OCIO: OpenColorIO is a software library (a “brick”/developer tool usable in other software/applications) dedicated to color management. It’s 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… or via plugins and add-ons or LUTs*). This inclusion in different applications allows to share a single color management configuration throughout the production pipeline and ensure identical color reproduction at all stages.

• Octet: 8 bits. Noted o.

• 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’s interdependent with OCIO*.

• OpenColorIO: cf. 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 back 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 actually). The photon can be seen as the particular counterpart of the light ray which would be the ondulatory counterpart. It’s in reality both a particle and a 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 (γ) per square centimeter per second γ/cm²/s). We also measure the triggering threshold (the lower level) and saturation (the upper level) of photo-sensitive sensors (such as retinal cells) in photons per second.

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

• Proofing or Soft-proofing: 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 the final spectator will see. It’s important to note that soft-proofing can remain far from reality if the output space is larger than that of the screen or too different (for example CMYK on an sRGB or Rec.2020 screen); however, it makes it possible to preview what a user will see on a similar screen.

• Purity: cf. Saturation

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

• Saturation: the most saturated colors possible are the monochromatic colors to which we add the purples (blends of red and blue). As soon as the colors are a blend of several monochromatic* lights (except blends 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. This is also the light purity. cf. Colorimetric purity and Excitation purity.

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

• (White) Temperature: 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’s a way to precisely define 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, an orange candle flame around 1850 K, etc. See chapter Back to whites: temperature

• 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. Tints are usually located on a circle with the primary colors equally spaced, with the primary red at 0°, the primary green at 120°, the primary blue at 240°, and the blends at intermediate angles, in proportion to the two primaries that compose them.

• Transfer curve: mathematical function used to convert the intensities of a given color space to a linear scale (and vice versa). It’s also called gamma even if the name gamma should rather be limited to only those functions that consist of a mathematical power function, 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.

• Violet: see Purple.

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

• White Point: cf. Illuminant

• YCbCr or YCC: digital version of YUV.

• YPbPr: alternate name of YUV.

• YUV: color coding system in three channels: one of luminance or luminosity and two of chrominance. Several notations bring details on the system, YUV is the most used:

  • 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.