Misplaced Pages

#849150

109-478: Color names are not standardized by Xlib or the X11 protocol. The list does not show continuity either in selected color values or in color names , and some color triplets have multiple names. Despite this, graphic designers and others got used to them, making it practically impossible to introduce a different list. In earlier releases of X11 (prior to the introduction of Xcms ), server implementors were encouraged to modify

218-607: A perceptually uniform lightness scale. A question of interest is the relationship between the Munsell value scale and the relative luminance . Aware of the Weber–Fechner law , Albert Munsell remarked "Should we use a logarithmic curve or curve of squares?" Neither option turned out to be quite correct; scientists eventually converged on a roughly cube-root curve, consistent with the Stevens's power law for brightness perception, reflecting

327-429: A "generalized LHS model". The HSL and HSV model-builders took an RGB cube – with constituent amounts of red, green, and blue light in a color denoted R , G , B ∈ [0, 1] – and tilted it on its corner, so that black rested at the origin with white directly above it along the vertical axis, then measured the hue of the colors in the cube by their angle around that axis, starting with red at 0°. Then they came up with

436-485: A certain color has such variants seems random. If " color 1" is not the same as " color " , the base color is usually darker. That means its brightness in HSB color notation is less than 100%; about 30 of the base colors are fully bright. The four variants (1...4) have rounded brightness values of 100%, 93%, 80% and 55%, respectively. Their hue and saturation are usually the same except for rounding. In some cases they differ from

545-476: A characterization of brightness/value/lightness, and defined saturation to range from 0 along the axis to 1 at the most colorful point for each pair of other parameters. In each of our models, we calculate both hue and what this article will call chroma , after Joblove and Greenberg (1978), in the same way – that is, the hue of a color has the same numerical values in all of these models, as does its chroma. If we take our tilted RGB cube, and project it onto

654-466: A color precisely requires reporting not only HSL or HSV values, but also the characteristics of the RGB space they are based on, including the gamma correction in use. If we take an image and extract the hue, saturation, and lightness or value components, and then compare these to the components of the same name as defined by color scientists, we can quickly see the difference, perceptually. For example, examine

763-403: A cone response curve which approximates a power curve for a much larger range of stimuli, so hue and saturation are better preserved. All these proposals, as well as others relating to chromaticity, resulted in a new colour appearance model, CIECAM02. In this model, the formula for lightness remains the same: But all the quantities that feed into this formula change in some way. The parameter c

872-407: A cube root, an approximation that is found in much of the technical literature. However, the linear segment near black is significant, and so the 116 and 16 coefficients. The best-fit pure power function has an exponent of about 0.42, far from ⁠ 1 / 3 ⁠ . An approximately 18% grey card , having an exact reflectance of ( ⁠ 33 / 58 ⁠ ) , has a lightness value of 50. It

981-487: A discontinuity at 360°, it is difficult to use in statistical computations or quantitative comparisons: analysis requires the use of circular statistics . Furthermore, hue is defined piecewise, in 60° chunks, where the relationship of lightness, value, and chroma to R , G , and B depends on the hue chunk in question. This definition introduces discontinuities, corners which can plainly be seen in horizontal slices of HSL or HSV. Charles Poynton, digital video expert, lists

1090-439: A given color space , we will see that they may differ substantially from a different color space or model. For example, examine the following images of a fire breather ( fig. 1 ). The original is in the sRGB color space. CIELAB L ∗ {\displaystyle L^{*}} is a perceptually uniform lightness prediction that is derived from luminance Y {\displaystyle Y} , but discards

1199-599: A hue/saturation plane relative to either HSL or HSV space. Video editors also use these models. For example, both Avid and Final Cut Pro include color tools based on HSL or a similar geometry for use adjusting the color in video. With the Avid tool, users pick a vector by clicking a point within the hue/saturation circle to shift all the colors at some lightness level (shadows, mid-tones, highlights) by that vector. Since version 4.0, Adobe Photoshop's "Luminosity", "Hue", "Saturation", and "Color" blend modes composite layers using

SECTION 10

#1732870142850

1308-463: A lightness dimension, does not attempt to "fill" a cylinder by its definition of saturation. Instead of presenting color choice or modification interfaces to end users, the goal of HSI is to facilitate separation of shapes in an image. Saturation is therefore defined in line with the psychometric definition: chroma relative to lightness ( fig. 15 ). See the Use in image analysis section of this article. Using

1417-592: A luma/chroma/hue color geometry. These have been copied widely, but several imitators use the HSL (e.g. PhotoImpact , Paint Shop Pro ) or HSV geometries instead. HSL, HSV, HSI, or related models are often used in computer vision and image analysis for feature detection or image segmentation . The applications of such tools include object detection, for instance in robot vision ; object recognition , for instance of faces , text , or license plates ; content-based image retrieval ; and analysis of medical images . For

1526-509: A rough estimate for the useful range of nerve impulses per second, and which has a fairly large intermediate range where it roughly follows a square root curve. The brightness according to CIECAM97s is then: The factor 1.24 / c is a surround factor that reflects that scenes appear brighter in dark surrounding conditions. Suggestions for a more comprehensive model, CIECAM97C, were also formulated, to take into account several effects at extremely dark or bright conditions, coloured lighting, as well as

1635-432: A sample will appear darker on a light background than on a dark background. See contrast effect for more information on the topic. When n = ⁠ 1 / 5 ⁠ , cz = 1, representing the assumption that most scenes have an average relative luminance of ⁠ 1 / 5 ⁠ compared to bright white, and that therefore a sample in such a surround should be perceived at its proper lightness. The quantity A models

1744-716: A saturated yellow and saturated blue may be designated as the same 'lightness' but have wide differences in perceived lightness. These flaws make the systems difficult to use to control the look of a color scheme in a systematic manner. If much tweaking is required to achieve the desired effect, the system offers little benefit over grappling with raw specifications in RGB or CMY. If these problems make HSL and HSV problematic for choosing colors or color schemes, they make them much worse for image adjustment. HSL and HSV, as Brewer mentioned, confound perceptual color-making attributes, so that changing any dimension results in non-uniform changes to all three perceptual dimensions, and distorts all of

1853-500: A significantly lighter tone than plain "Gray"   , because "Dark Gray" was descended from X11 – for it did not exist in HTML nor CSS level 1 – while "Gray" was descended from HTML. Even in the current draft for CSS 4.0, dark gray continues to be a lighter shade than gray. Some browsers such as Netscape Navigator insisted on an "a" in any "Gr a y" except for "Light Gr e y". Recent X releases (since 2014, xorg-rgb version 1.0.6) also support

1962-403: A single color, they ignore much of the complexity of color appearance. Essentially, they trade off perceptual relevance for computation speed, from a time in computing history (high-end 1970s graphics workstations, or mid-1990s consumer desktops) when more sophisticated models would have been too computationally expensive. HSL and HSV are simple transformations of RGB which preserve symmetries in

2071-545: A study on the Munsell neutral value scale, considering several proposals relating the relative luminance to the Munsell value, and suggest: Sidney Newhall, Dorothy Nickerson , and Deane Judd prepare a report for the Optical Society of America (OSA) on Munsell renotation. They suggest a quintic parabola (relating the reflectance in terms of the value): Using Table II of the OSA report, Parry Moon and Domina Spencer express

2180-441: A transformation, hue is precisely the angle around the origin and chroma the distance from the origin: the angle and magnitude of the vector pointing to a color. Sometimes for image analysis applications, this hexagon-to-circle transformation is skipped, and hue and chroma (we'll denote these H 2 and C 2 ) are defined by the usual cartesian-to-polar coordinate transformations ( fig. 11 ). The easiest way to derive those

2289-507: A tristimulus value of Y = 100 . Since the reflectance of magnesium oxide (MgO) relative to the perfect reflecting diffuser is 97.5%, V = 10 corresponds to Y = ⁠ 100 / 97.5 ⁠ % ≈ 102.6 if MgO is used as the reference. Irwin Priest , Kasson Gibson , and Harry McNicholas provide a basic estimate of the Munsell value (with Y running from 0 to 1 in this case): Alexander Munsell, Louise Sloan , and Isaac Godlove launch

SECTION 20

#1732870142850

2398-691: Is a linear prediction of the human perception of that light. This distinction is meaningful because human vision's lightness perception is non-linear relative to light. Doubling the quantity of light does not result in a doubling in perceived lightness, only a modest increase. The symbol for perceptual lightness is usually either J {\displaystyle J} as used in CIECAM02 or L ∗ {\displaystyle L^{*}} as used in CIELAB and CIELUV . L ∗ {\displaystyle L^{*}} ("Lstar")

2507-424: Is also often called HSB ( B for brightness ). A third model, common in computer vision applications, is HSI , for hue , saturation , and intensity . However, while typically consistent, these definitions are not standardized, and any of these abbreviations might be used for any of these three or several other related cylindrical models. (For technical definitions of these terms, see below .) In each cylinder,

2616-528: Is called " mid grey " because its lightness is midway between black and white. As early as in 1967 a hyperbolic relationship between light intensity and cone cell responses was discovered in fish, in line with the Michaelis–Menten kinetics model of biochemical reactions. In the 70s the same relationship was found in a number of other vertebrates and in 1982, using microelectrodes to measure cone responses in living rhesus macaques, Valeton and Van Norren found

2725-485: Is controlled by three sliders ranging from 0–255 , one controlling the intensity of each of the red, green, and blue primaries. If we begin with a relatively colorful orange   , with sRGB values R = 217 , G = 118 , B = 33 , and want to reduce its colorfulness by half to a less saturated orange   , we would need to drag the sliders to decrease R by 31, increase G by 24, and increase B by 59, as pictured below. [REDACTED] Beginning in

2834-664: Is held to have the same saturation as the green primary   , even though the former color has almost no chroma or saturation by the conventional psychometric definitions. Such perversities led Cynthia Brewer, expert in color scheme choices for maps and information displays, to tell the American Statistical Association : Computer science offers a few poorer cousins to these perceptual spaces that may also turn up in your software interface, such as HSV and HLS. They are easy mathematical transformations of RGB, and they seem to be perceptual systems because they make use of

2943-548: Is no particular reason to strictly mimic human color response. John Kender's 1976 master's thesis proposed the HSI model. Ohta et al. (1980) instead used a model made up of dimensions similar to those we have called I , α , and β . In recent years, such models have continued to see wide use, as their performance compares favorably with more complex models, and their computational simplicity remains compelling. While HSL, HSV, and related spaces serve well enough to, for instance, choose

3052-447: Is no systematic variation apparent. Several sets, however, feature a Dark variant with 55% brightness and some have their Medium at about 80%. "Light Goldenrod Yellow" and "Dark Olive Green" are special, because there are no corresponding color entries without Dark and Light prefixes. Several groups of colors share the same lightness or brightness and saturation. These nuances differ only by hue. Several groups of colors share

3161-430: Is not to be confused with L {\displaystyle L} as used for luminance. In some color ordering systems such as Munsell , Lightness is referenced as value . Chiaroscuro and Tenebrism both take advantage of dramatic contrasts of value to heighten drama in art. Artists may also employ shading , subtle manipulation of value. In some colorspaces or color systems such as Munsell, HCL , and CIELAB,

3270-568: Is now continuously variable as discussed above and z = 1.48 + √n. Although this is higher than z in CIECAM97s, the total effective power factor is very similar because the effective power factor of the achromatic response is much lower: As before, this formula assumes bright conditions. Apart from 1220, which results from an arbitrarily assumed cone response constant, the various constants in CIECAM02 were fitted to experimental data sets. The expression for

3379-467: Is roughly similar, but differs at high chroma, deviating most from an achromatic signal such as linear luminance Y {\displaystyle Y} or non-linear lightness L ∗ {\displaystyle L^{*}} . HSL L {\displaystyle L} and HSV V {\displaystyle V} are neither perceptually uniform, nor uniform as to luminance. The Munsell value has long been used as

X11 color names - Misplaced Pages Continue

3488-542: Is shown. The latter type of GUI exhibits great variety, because of the choice of cylinders, hexagonal prisms, or cones/bicones that the models suggest (see the diagram near the top of the page ). Several color choosers from the 1990s are shown to the right, most of which have remained nearly unchanged in the intervening time: today, nearly every computer color chooser uses HSL or HSV, at least as an option. Some more sophisticated variants are designed for choosing whole sets of colors, basing their suggestions of compatible colors on

3597-592: Is simply called "Lime", as no such name existed before. It aliases to "Green", i.e. "Lime" = "Green" = "X11 Green" ≠ "Web Green". The following chart presents the standardized X11 color names from the X.org source code. The list of names accepted by browsers following W3C standards slightly differs as explained above. The table does not show numbered gray and brightness variants as described below. Actual rgb.txt files and other color databases or palettes may differ since they are freely editable by vendors and users. The table shows component values in several notations of

3706-399: Is simply the maximum of the other two components. This chroma is M in the particular case of a color with a zero component, and M − m in general. The hue is the proportion of the distance around the edge of the hexagon which passes through the projected point, originally measured on the range [0, 1] but now typically measured in degrees [0°, 360°) . For points which project onto

3815-411: Is specifically reserved for the 128 triplet (50% gray)   . However, in X11, "gray" was assigned to the 190 triplet (74.5%)   , which is close to W3C "Silver" at 192 (75.3%)   , and had "Light Gray" at 211 (83%)   and "Dark Gray" at 169 (66%)   counterparts. As a result, the combined CSS 3.0 color list that prevails on the web today produces "Dark Gray"   as

3924-423: Is tangent to the formula above at the point at which the linear extension takes effect. First, the transition point is determined to be ⁠ Y / Y n ⁠ = ( ⁠ 6 / 29 ⁠ ) ≈ 0.008856 , then the slope of ( ⁠ 29 / 3 ⁠ ) ≈ 903.3 is computed. This gives the two-part function: The lightness is then: At first glance, you might approximate the lightness function by

4033-579: Is via a pair of cartesian chromaticity coordinates which we'll call α and β : (The atan2 function, a "two-argument arctangent", computes the angle from a cartesian coordinate pair.) Notice that these two definitions of hue ( H and H 2 ) nearly coincide, with a maximum difference between them for any color of about 1.12° – which occurs at twelve particular hues, for instance H = 13.38° , H 2 = 12.26° – and with H = H 2 for every multiple of 30°. The two definitions of chroma ( C and C 2 ) differ more substantially: they are equal at

4142-641: The X {\displaystyle X} and Z {\displaystyle Z} , of the CIE XYZ color space . Notice this appears similar in perceived lightness to the original color image. Luma ( Y ′ ) {\displaystyle (Y^{\prime })} is a gamma-encoded luminance component of some video encoding systems such as ( Y ′ I Q ) {\displaystyle (Y^{\prime }IQ)} and ( Y ′ U V ) {\displaystyle (Y^{\prime }UV)} . It

4251-463: The CIE 1964 color space . CIELAB uses the following formula: where Y n is the CIE XYZ Y tristimulus value of the reference white point (the subscript n suggests "normalized") and is subject to the restriction ⁠ Y / Y n ⁠ > 0.01 . Pauli removes this restriction by computing a linear extrapolation which maps ⁠ Y / Y n ⁠ = 0 to L * = 0 and

4360-525: The Helmholtz–Kohlrausch effect , where highly chromatic samples appear lighter and brighter in comparison to a neutral grey. To model the latter effect, in CIECAM97C the formula for J is adjusted as follows: where C is the chroma and h the hue angle Q is then calculated from J HK instead of from J. This formula has the effect of pulling up the lightness and brightness of coloured samples. The larger

4469-588: The Helmholtz–Kohlrausch effect . Though the CIELAB space and relatives do not account for this effect on lightness, it may be implied in the Munsell color model. Light levels may also affect perceived chromaticity, as with the Purkinje effect . [REDACTED] Media related to Lightness at Wikimedia Commons HSL and HSV HSL and HSV are the two most common cylindrical-coordinate representations of points in an RGB color model . The two representations rearrange

X11 color names - Misplaced Pages Continue

4578-697: The RGB color space, with conversions to HSL and HSV assuming sRGB color space. * Prior to standardization as a web color, Gainsboro was included as one of the X11 color names. It was, however, absent from the original 1987 version of the list, but present in Paul Raveling's version which added, amongst other things, "[l]ight and off-white colors, copied from several Sinclair Paints color samples". The complete rgb.txt defines 101 shades from 'Gray0' (black) up to 'Gray100' (white) in addition to 'Gray' and its variants listed above. The shades are apparently defined by

4687-440: The interval [0, 1] , except those for H and H 2 , which are in the interval [0°, 360°) . The original purpose of HSL and HSV and similar models, and their most common current application, is in color selection tools . At their simplest, some such color pickers provide three sliders, one for each attribute. Most, however, show a two-dimensional slice through the model, along with a slider controlling which particular slice

4796-561: The red primary at 0°, passing through the green primary at 120° and the blue primary at 240°, and then wrapping back to red at 360°. In each geometry, the central vertical axis comprises the neutral , achromatic , or gray colors ranging, from top to bottom, white at lightness 1 (value 1) to black at lightness 0 (value 0). In both geometries, the additive primary and secondary colors – red, yellow , green, cyan , blue and magenta – and linear mixtures between adjacent pairs of them, sometimes called pure colors , are arranged around

4905-518: The spectral distribution of light entering the eye, while lightness and chroma are measured relative to some white point, and are thus often used for descriptions of surface colors, remaining roughly constant even as brightness and colorfulness change with different illumination . Saturation can be defined as either the ratio of colorfulness to brightness, or that of chroma to lightness. HSL, HSV, and related models can be derived via geometric strategies, or can be thought of as specific instances of

5014-413: The value of HSV and the saturation of HSL are particular offenders. In HSV, the blue primary   and white   are held to have the same value, even though perceptually the blue primary has somewhere around 10% of the luminance of white (the exact fraction depends on the particular RGB primaries in use). In HSL, a mix of 100% red, 100% green, 90% blue – that is, a very light yellow   –

5123-424: The "chromaticity plane " perpendicular to the neutral axis, our projection takes the shape of a hexagon, with red, yellow, green, cyan, blue, and magenta at its corners ( fig. 9 ). Hue is roughly the angle of the vector to a point in the projection, with red at 0°, while chroma is roughly the distance of the point from the origin. More precisely, both hue and chroma in this model are defined with respect to

5232-403: The "hexcone model" while HSL is often called the "bi-hexcone model" ( fig. 8 ). Most televisions, computer displays, and projectors produce colors by combining red, green, and blue light in varying intensities – the so-called RGB additive primary colors . The resulting mixtures in RGB color space can reproduce a wide variety of colors (called a gamut ); however, the relationship between

5341-405: The (bi)cone). Confusingly, such diagrams usually label this radial dimension "saturation", blurring or erasing the distinction between saturation and chroma. As described below , computing chroma is a helpful step in the derivation of each model. Because such an intermediate model – with dimensions hue, chroma, and HSV value or HSL lightness – takes the shape of a cone or bicone, HSV is often called

5450-413: The 1950s, color television broadcasts used a compatible color system whereby " luminance " and " chrominance " signals were encoded separately, so that existing unmodified black-and-white televisions could still receive color broadcasts and show a monochrome image. In an attempt to accommodate more traditional and intuitive color mixing models, computer graphics pioneers at PARC and NYIT introduced

5559-634: The CSS Color module include a similar function gray() . They are still coded without 'Grey' alternatives, but with no space before the digit. For 78 colors (not counting grays), rgb.txt offers four variants " color 1", " color 2", " color 3", and " color 4", with " color 1" sometimes corresponding to " color ", so e.g. "Snow1" is the same as "Snow". Unlike base colors, e.g. cadet blue and CadetBlue , these are only coded without spaces, e.g. CadetBlue3 . These variations are neither supported by popular browsers nor adopted by W3C standards. Whether or not

SECTION 50

#1732870142850

5668-597: The Computer Graphics Standards Committee recommended it in their annual status report ( fig. 7 ). These models were useful not only because they were more intuitive than raw RGB values, but also because the conversions to and from RGB were extremely fast to compute: they could run in real time on the hardware of the 1970s. Consequently, these models and similar ones have become ubiquitous throughout image editing and graphics software since then. Some of their uses are described below . The dimensions of

5777-492: The HSL and HSV geometries – simple transformations of the not-perceptually-based RGB model – are not directly related to the photometric color-making attributes of the same names, as defined by scientists such as the CIE or ASTM . Nonetheless, it is worth reviewing those definitions before leaping into the derivation of our models. For the definitions of color-making attributes which follow, see: Brightness and colorfulness are absolute measures, which usually describe

5886-438: The HSL and HSV models scale the chroma so that it always fits into the range [0, 1] for every combination of hue and lightness or value, calling the new attribute saturation in both cases (fig. 14). To calculate either, simply divide the chroma by the maximum chroma for that value or lightness. The HSI model commonly used for computer vision, which takes H 2 as a hue dimension and the component average I ("intensity") as

5995-455: The HSL or HSV relationships between them. Most web applications needing color selection also base their tools on HSL or HSV, and pre-packaged open source color choosers exist for most major web front-end frameworks . The CSS 3 specification allows web authors to specify colors for their pages directly with HSL coordinates. HSL and HSV are sometimes used to define gradients for data visualization , as in maps or medical images. For example,

6104-911: The HSV model for computer display technology in the mid-1970s, formally described by Alvy Ray Smith in the August 1978 issue of Computer Graphics . In the same issue, Joblove and Greenberg described the HSL model – whose dimensions they labeled hue , relative chroma , and intensity – and compared it to HSV ( fig. 1 ). Their model was based more upon how colors are organized and conceptualized in human vision in terms of other color-making attributes, such as hue, lightness, and chroma; as well as upon traditional color mixing methods – e.g., in painting – that involve mixing brightly colored pigments with black or white to achieve lighter, darker, or less colorful colors. The following year, 1979, at SIGGRAPH , Tektronix introduced graphics terminals using HSL for color designation, and

6213-594: The RGB cube unrelated to human perception, such that its R , G , and B corners are equidistant from the neutral axis, and equally spaced around it. If we plot the RGB gamut in a more perceptually-uniform space, such as CIELAB (see below ), it becomes immediately clear that the red, green, and blue primaries do not have the same lightness or chroma, or evenly spaced hues. Furthermore, different RGB displays use different primaries, and so have different gamuts. Because HSL and HSV are defined purely with reference to some RGB space, they are not absolute color spaces : to specify

6322-415: The RGB gamut (the gray parts of the slices in figure 14). The creators of these models considered this a problem for some uses. For example, in a color selection interface with two of the dimensions in a rectangle and the third on a slider, half of that rectangle is made of unused space. Now imagine we have a slider for lightness: the user's intent when adjusting this slider is potentially ambiguous: how should

6431-417: The RGB values in the reference color database to account for gamma correction. As of X.Org Release 7.4 rgb.txt is no longer included in the roll up release, and the list is built directly into the server. The optional module xorg/app/rgb contains the stand-alone rgb.txt file. The list first shipped with X10 release 3 (X10R3) on 7 June 1986, having been checked into RCS by Jim Gettys in 1985. The same list

6540-494: The W3C definitions. In X11, the original definitions have been preserved (so "Dark Gray" remains a darker shade of "Gray"), but for every conflicting name pair, "Web" and additional "X11" prefixes have been added to ease disambiguation after the merger. The "X11" prefix is an alias for the non-prefixed version, i.e. "X11 Gray" = "Gray" ≠ "Web Gray". The W3C also defined a color that is equal to X11's "Green", but called it "Lime". In X11, this

6649-453: The X11 colors as the basis for the web colors list, as both were originally X applications. The W3C specifications SVG and CSS level 3 module Color eventually adopted the X11 list with some changes. The present W3C list is a superset of the 16 " VGA colors" defined in HTML 3.2 and CSS level 1. One notable difference between X11 and W3C is the case of "Gray" and its variants. In HTML, "Gray"

SECTION 60

#1732870142850

6758-463: The achromatic cone response; it is colour dependent but for a grey sample under bright conditions it works out as: Here Y is the relative luminance compared to white on a scale of 0 to 1 and L A is the average luminance of the adapting visual field as a whole, measured in cd/m . The achromatic response follows a kind of S-curve , ranging from 1 to 123, numbers which follow from the way the cone responses are averaged and which are ultimately based on

6867-414: The angle around the central vertical axis corresponds to " hue ", the distance from the axis corresponds to " saturation ", and the distance along the axis corresponds to " lightness ", "value" or " brightness ". Note that while "hue" in HSL and HSV refers to the same attribute, their definitions of "saturation" differ dramatically. Because HSL and HSV are simple transformations of device-dependent RGB models,

6976-506: The base color, though, which may indicate that these variants were specified with alternate definitions of the bases in mind, i.e. their values were adapted to a certain monitor which was commonly done by vendors until the 1990s. The fixed brightness settings correspond closely to these formulae to determine the RGB values: Examples: Some color names appear to be brightness or saturation modifications of others because they bear prefixes such as Dark, Light, Medium, Pale or Deep , but there

7085-408: The bottom right in the sliced HSL cylinder or   from the top right) – conflict with the intuitive notion of color purity, often a conic or biconic solid is drawn instead ( fig. 3 ), with what this article calls chroma as its radial dimension (equal to the range of the RGB values), instead of saturation (where the saturation is equal to the chroma over the maximum chroma in that slice of

7194-507: The brightness has also changed considerably: Note that contrary to the suggestion from CIECAM97C, CIECAM02 contains no provision for the Helmholtz–Kohlrausch effect. This subjective perception of luminance in a non-linear fashion is one thing that makes gamma compression of images worthwhile. Beside this phenomenon there are other effects involving perception of lightness. Chromaticity can affect perceived lightness as described by

7303-499: The chroma, the stronger the effect; for very saturated colours C can be close to 100 or even higher. The absolute sine term has a sharp V-shaped valley with a zero at yellow and a broad plateau in the deep blues. The achromatic response in CIECAM97s is a weighted addition of cone responses minus 2.05. Since the total noise term adds up to 3.05, this means that A and consequentially J and Q aren't zero for absolute black. To fix this, Li, Luo & Hunt suggested subtracting 3.05 instead, so

7412-402: The color dimensions used. Because the R , G , and B components of an object's color in a digital image are all correlated with the amount of light hitting the object, and therefore with each other, image descriptions in terms of those components make object discrimination difficult. Descriptions in terms of hue/lightness/chroma or hue/lightness/saturation are often more relevant. Starting in

7521-418: The color relationships in the image. For instance, rotating the hue of a pure dark blue   toward green   will also reduce its perceived chroma, and increase its perceived lightness (the latter is grayer and lighter), but the same hue rotation will have the opposite impact on lightness and chroma of a lighter bluish-green –   to   (the latter is more colorful and slightly darker). In

7630-443: The constituent amounts of red, green, and blue light and the resulting color is unintuitive, especially for inexperienced users, and for users familiar with subtractive color mixing of paints or traditional artists' models based on tints and shades ( fig. 4 ). Furthermore, neither additive nor subtractive color models define color relationships the same way the human eye does. For example, imagine we have an RGB display whose color

7739-420: The corners of our hexagon, but at points halfway between two corners, such as H = H 2 = 30° , we have C = 1 , but C 2 = 3 4 ≈ 0.866 , {\textstyle C_{2}={\sqrt {\frac {3}{4}}}\approx 0.866,} a difference of about 13.4%. While the definition of hue is relatively uncontroversial – it roughly satisfies the criterion that colors of

7848-403: The displayed luminance is relative to the hue and chroma for a given lightness value, in other words the selected lightness value does not predict the actual displayed luminance nor the perception thereof. Both systems use coordinate triples, where many triples can map onto the same color. In HSV, all triples with value 0 are pure black. If the hue and saturation are held constant, then increasing

7957-504: The example below ( fig. 21 ), the image (a) is the original photograph of a green turtle . In the image (b), we have rotated the hue ( H ) of each color by −30° , while keeping HSV value and saturation or HSL lightness and saturation constant. In the image right (c), we make the same rotation to the HSL/HSV hue of each color, but then we force the CIELAB lightness ( L *, a decent approximation of perceived lightness) to remain constant. Notice how

8066-421: The fact that lightness is proportional to the number of nerve impulses per nerve fiber per unit time. The remainder of this section is a chronology of lightness models, leading to CIECAM02 . Note. – Munsell's V runs from 0 to 10, while Y typically runs from 0 to 100 (often interpreted as a percentage). Typically, the relative luminance is normalized so that the "reference white" (say, magnesium oxide ) has

8175-461: The following images of a fire breather ( fig. 13 ). The original is in the sRGB colorspace. CIELAB L * is a CIE-defined achromatic lightness quantity (dependent solely on the perceptually achromatic luminance Y , but not the mixed-chromatic components X or Z , of the CIEXYZ colorspace from which the sRGB colorspace itself is derived), and it is plain that this appears similar in perceptual lightness to

8284-589: The following relationship: where V is the measured potential, I the light intensity and σ a constant. In 1986 Seim and Valberg realised that this relationship might aid in the construction of a more uniform colour space. This inspired advances in colour modelling and when the International Commission on Illumination held a symposium in 1996, objectives for a new standard colour model were formulated and in 1997 CIECAM97s (International Commission on Illumination, colour appearance model, 1997, simple version)

8393-506: The formula Gray N  := round( N % × 255) resulting in e.g. 'Gray96'   , which happens to be the same as 'White Smoke'. Similarly 'Dim Gray' is the same as 'Gray41'   . On the other hand, 'Gray'   lies between 'Gray74'   and 'Gray75'   ; 'Dark Gray'   is not the same as 'Gray66'   ; and 'Light Gray'   is not the same as 'Gray83'   . These shades are not included in W3C specifications, although drafts for level 4 of

8502-446: The four formulations yields a lightness equal to the value of R , G , or B . For a graphical comparison, see fig. 13 below . When encoding colors in a hue/lightness/chroma or hue/value/chroma model (using the definitions from the previous two sections), not all combinations of lightness (or value) and chroma are meaningful: that is, half of the colors denotable using H ∈ [0°, 360°) , C ∈ [0, 1] , and V ∈ [0, 1] fall outside

8611-572: The functional color names other , unknown and transparent alongside seven basic colors from ISO 10175 ( DPA ) and ISO 10180 ( SPDL ), and JTAPI. This standard has four variants for each non-monochromatic color: clear (50% transparent), dark , light and the default. Wherever possible, the values are the same as in the W3C adaptation of the X11 list, except for Turquoise which is   instead of   . Missing variant values have been added systematically. Buff and Mustard are completely new color names. Light Black and Gray correspond to

8720-464: The geometry of RGB in an attempt to be more intuitive and perceptually relevant than the cartesian (cube) representation. Developed in the 1970s for computer graphics applications, HSL and HSV are used today in color pickers , in image editing software, and less commonly in image analysis and computer vision . HSL stands for hue , saturation , and lightness , and is often also called HLS . HSV stands for hue , saturation , and value , and

8829-652: The hexagonal shape of the projection. The chroma is the proportion of the distance from the origin to the edge of the hexagon. In the lower part of the adjacent diagram, this is the ratio of lengths OP / OP ′ , or alternatively the ratio of the radii of the two hexagons. This ratio is the difference between the largest and smallest values among R , G , or B in a color. To make our definitions easier to write, we'll define these maximum, minimum, and chroma component values as M , m , and C , respectively. To understand why chroma can be written as M − m , notice that any neutral color, with R = G = B , projects onto

8938-432: The hue-shifted middle version without such a correction dramatically changes the perceived lightness relationships between colors in the image. In particular, the turtle's shell is much darker and has less contrast, and the background water is much lighter. Image (d) uses CIELAB to hue shift; the difference from (c) demonstrates the errors in hue and saturation. Because hue is a circular quantity, represented numerically with

9047-501: The hue–lightness/value–saturation terminology. But take a close look; don't be fooled. Perceptual color dimensions are poorly scaled by the color specifications that are provided in these and some other systems. For example, saturation and lightness are confounded, so a saturation scale may also contain a wide range of lightnesses (for example, it may progress from white to green which is a combination of both lightness and saturation). Likewise, hue and lightness are confounded so, for example,

9156-402: The late 1970s, transformations like HSV or HSI were used as a compromise between effectiveness for segmentation and computational complexity. They can be thought of as similar in approach and intent to the neural processing used by human color vision, without agreeing in particulars: if the goal is object detection, roughly separating hue, lightness, and chroma or saturation is effective, but there

9265-514: The late-1980s, but various more complicated color tools have also been implemented. For instance, the Unix image viewer and color editor xv allowed six user-definable hue ( H ) ranges to be rotated and resized, included a dial -like control for saturation ( S HSV ), and a curves -like interface for controlling value ( V ) – see fig. 17. The image editor Picture Window Pro includes a "color correction" tool which affords complex remapping of points in

9374-532: The lightness (value) achromatically constrains the maximum and minimum limits, and operates independently of the hue and chroma . For example Munsell value 0 is pure black, and value 10 is pure white. Colors with a discernible hue must therefore have values in between these extremes. In a subtractive color model (e.g. paint, dye, or ink) lightness changes to a color through various tints, shades, or tones can be achieved by adding white, black, or grey respectively. This also reduces saturation . In HSL and HSV ,

9483-407: The lightness value is set at a given number. While HSL, HSV, and similar spaces serve well enough to choose or adjust a single color, they are not perceptually uniform. They trade off accuracy for computational simplicity, as they were created in an era where computer technology was restricted in performance. If we take an image and extract the hue, saturation, and lightness or value components for

9592-402: The most part, computer vision algorithms used on color images are straightforward extensions to algorithms designed for grayscale images, for instance k-means or fuzzy clustering of pixel colors, or canny edge detection . At the simplest, each color component is separately passed through the same algorithm. It is important, therefore, that the features of interest can be distinguished in

9701-402: The origin and so has 0 chroma. Thus if we add or subtract the same amount from all three of R , G , and B , we move vertically within our tilted cube, and do not change the projection. Therefore, any two colors of ( R , G , B ) and ( R − m , G − m , B − m ) project on the same point, and have the same chroma. The chroma of a color with one of its components equal to zero ( m = 0)

9810-416: The origin in the chromaticity plane (i.e., grays), hue is undefined. Mathematically, this definition of hue is written piecewise : Sometimes, neutral colors (i.e. with C = 0 ) are assigned a hue of 0° for convenience of representation. These definitions amount to a geometric warping of hexagons into circles: each side of the hexagon is mapped linearly onto a 60° arc of the circle ( fig. 10 ). After such

9919-453: The original color image. Luma is roughly similar, but differs somewhat at high chroma, where it deviates most from depending solely on the true achromatic luminance ( Y , or equivalently L *) and is influenced by the colorimetric chromaticity ( x,y , or equivalently, a*,b* of CIELAB). HSL L and HSV V , by contrast, diverge substantially from perceptual lightness. Though none of the dimensions in these spaces match their perceptual analogs,

10028-401: The other of them is often more convenient than RGB, but both are also criticized for not adequately separating color-making attributes, or for their lack of perceptual uniformity. Other more computationally intensive models, such as CIELAB or CIECAM02 are said to better achieve these goals. HSL and HSV are both cylindrical geometries ( fig. 2 ), with hue, their angular dimension, starting at

10137-661: The outside edge of the cylinder with saturation 1. These saturated colors have lightness 0.5 in HSL, while in HSV they have value 1. Mixing these pure colors with black – producing so-called shades – leaves saturation unchanged. In HSL, saturation is also unchanged by tinting with white, and only mixtures with both black and white – called tones – have saturation less than 1. In HSV, tinting alone reduces saturation. Because these definitions of saturation – in which very dark (in both models) or very light (in HSL) near-neutral colors are considered fully saturated (for instance,   from

10246-465: The physical colors they define depend on the colors of the red, green, and blue primaries of the device or of the particular RGB space, and on the gamma correction used to represent the amounts of those primaries. Each unique RGB device therefore has unique HSL and HSV spaces to accompany it, and numerical HSL or HSV values describe a different color for each basis RGB space. Both of these representations are used widely in computer graphics, and one or

10355-422: The popular GIS program ArcGIS historically applied customizable HSV-based gradients to numerical geographical data. Image editing software also commonly includes tools for adjusting colors with reference to HSL or HSV coordinates, or to coordinates in a model based on the "intensity" or luma defined above . In particular, tools with a pair of "hue" and "saturation" sliders are commonplace, dating to at least

10464-410: The reflectance to the power of 0.352: Realizing this is quite close to the cube root , they simplify it to: Glasser et al. define the lightness as ten times the Munsell value (so that the lightness ranges from 0 to 100): Günter Wyszecki simplifies this to: This formula approximates the Munsell value function for 1% < Y < 98% (it is not applicable for Y < 1% ) and is used for

10573-423: The same color. Color value Lightness is a visual perception of the luminance ( L ) {\displaystyle (L)} of an object. It is often judged relative to a similarly lit object. In colorimetry and color appearance models , lightness is a prediction of how an illuminated color will appear to a standard observer. While luminance is a linear measurement of light, lightness

10682-489: The same hue and HSL saturation. Tints are lighter than a base color, shades are darker. Some pairs of colors share the same lightness and hue. These tones differ only by saturation. Tones are far less common in the X11 set than nuances, tints and shades. The Printer Working Group (PWG) of the IEEE publishes a standard, PWG 5101.1 , whose mandatory color names are based upon RFC 3805 , successor to RFC 1759 which imported

10791-506: The same name for these three different definitions of saturation leads to some confusion, as the three attributes describe substantially different color relationships; in HSV and HSI, the term roughly matches the psychometric definition, of a chroma of a color relative to its own lightness, but in HSL it does not come close. Even worse, the word saturation is also often used for one of the measurements we call chroma above ( C or C 2 ). All parameter values shown below are given as values in

10900-411: The same perceived hue should have the same numerical hue – the definition of a lightness or value dimension is less obvious: there are several possibilities depending on the purpose and goals of the representation. Here are four of the most common ( fig. 12 ; three of these are also shown in fig. 8 ): All four of these leave the neutral axis alone. That is, for colors with R = G = B , any of

11009-450: The scale starts at zero. Although CIECAM97s was a successful model to spur and direct colorimetric research, Fairchild felt that for practical applications some changes were necessary. Those relevant for lightness calculations were to, rather than use several discrete values for the surround factor c, allow for linear interpolation of c and thereby allowing the model to be used under intermediate surround conditions, and to simplify z to remove

11118-455: The shape of the cone response S-curve, when the luminance of a colour is reduced, even if its spectral composition remains the same, the different cone responses do not quite change at the same rate with respect to each other. It is plausible therefore that the perceived hue and saturation will change at low luminance levels. But CIECAM97s predicts much larger deviations than are generally thought likely and therefore Hunt, Li and Luo suggested using

11227-436: The software deal with out-of-gamut colors? Or conversely, If the user has selected as colorful as possible a dark purple   , and then shifts the lightness slider upward, what should be done: would the user prefer to see a lighter purple still as colorful as possible for the given hue and lightness   , or a lighter purple of exactly the same chroma as the original color   ? To solve problems such as these,

11336-431: The special case for large stimuli because he felt it was irrelevant for imaging applications. Based on experimental results, Hunt, Li, Juan and Luo proposed a number of improvements. Relevant for the topic at hand is that they suggested lowering z slightly. Li and Luo found that a colour space based on such a modified CIECAM97s using lightness as one of the coordinates was more perceptually uniform than CIELAB. Because of

11445-459: The time, so would have worked to that device. In some applications multipart names are written with spaces, in others joined together, often in camel case . They are usually matched insensitive of case and the X Server source code contains spaced aliases for most entries; this article uses spaces and uppercase initials except where variants with spaces are not specified in the actual code. The first versions of Mosaic and Netscape Navigator used

11554-567: The value in terms of the relative luminance: Jason Saunderson and B.I. Milner introduce a subtractive constant in the previous expression, for a better fit to the Munsell value. Later, Dorothea Jameson and Leo Hurvich claim that this corrects for simultaneous contrast effects . Ladd and Pinney of Eastman Kodak are interested in the Munsell value as a perceptually uniform lightness scale for use in television . After considering one logarithmic and five power-law functions (per Stevens' power law ), they relate value to reflectance by raising

11663-402: The value increases the luminance, such that a value of 1 is the lightest color with the given hue and saturation. HSL is similar, except that all triples with lightness 1 are pure white. In both models, all pure saturated colors indicate the same lightness or value, but this does not relate to the displayed luminance which is determined by the hue. I.e. yellow is higher luminance than blue, even if

11772-531: Was in X11R1 on 18 September 1987. Approximately the full list as is available today shipped with X11R4 on 29 January 1989, with substantial additions by Paul Ravelling (who added colors based on Sinclair Paints samples), John C. Thomas (who added colors based on a set of 72 Crayola crayons he had on hand) and Jim Fulton (who reconciled contributions to produce the X11R4 list). The project was running DEC VT240 terminals at

11881-429: Was standardised. CIECAM97s distinguishes between lightness, how light something appears compared to a similarly lit white object, and brightness, how much light appears to shine from something. According to CIECAM97s the lightness of a sample is: In this formula, for a small sample under bright conditions in a surrounding field with a relative luminance n compared to white, c has been chosen such that: This models that

#849150