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51-431: SuperPaint had the ability to capture images from standard video input or combine them with preexisting digital data. SuperPaint was also the first program to use now-ubiquitous features in common computer graphics programs such as changing hue , saturation and value of graphical data, choosing from a preset color palette, custom polygons and lines, virtual paintbrushes and pencils, and auto-filling of images. SuperPaint

102-607: 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

153-671: A stimulus can be described as similar to or different from stimuli that are described as red , orange , yellow , green , blue , violet ," within certain theories of color vision . Hue can typically be represented quantitatively by a single number, often corresponding to an angular position around a central or neutral point or axis on a color space coordinate diagram (such as a chromaticity diagram ) or color wheel , or by its dominant wavelength or by that of its complementary color . The other color appearance parameters are colorfulness , saturation (also known as intensity or chroma), lightness , and brightness . Usually, colors with

204-435: A color hexagon, similar to a trilinear plot described by Evans, Hanson, and Brewer, which may be used to compute hue from RGB . To place red at 0°, green at 120°, and blue at 240°, Equivalently, one may solve Preucil used a polar plot, which he termed a color circle. Using R, G, and B, one may compute hue angle using the following scheme: determine which of the six possible orderings of R, G, and B prevail, then apply

255-450: A color system with a hue was explored as early as 1830 with Philipp Otto Runge 's color sphere. The Munsell color system from the 1930s was a great step forward, as it was realized that perceptual uniformity means the color space can no longer be a sphere. As a convention, the hue for red is set to 0° for most color spaces with a hue. In opponent color spaces in which two of the axes are perceptually orthogonal to lightness, such as

306-403: 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

357-407: 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

408-439: 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

459-509: 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

510-432: 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

561-545: 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

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612-507: 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

663-644: Is Δ H a b ∗ {\displaystyle \Delta H_{ab}^{*}} in CIELAB and Δ H u v ∗ {\displaystyle \Delta H_{uv}^{*}} in CIELUV. There exists some correspondence, more or less precise, between hue values and color terms (names). One approach in color science is to use traditional color terms but try to give them more precise definitions. See spectral color#Spectral color terms for names of highly saturated colors with

714-449: 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 is a linear prediction of the human perception of that light. This distinction

765-528: 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

816-610: 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")

867-430: 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,

918-568: 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

969-468: Is quantified. The first is the simple difference between the two hue angles. The symbol for this expression of hue difference is Δ h a b {\displaystyle \Delta h_{ab}} in CIELAB and Δ h u v {\displaystyle \Delta h_{uv}} in CIELUV. The other is computed as the residual total color difference after Lightness and Chroma differences have been accounted for; its symbol

1020-467: 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

1071-423: 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

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1122-580: Is usually based on a 6-piece piecewise mapping, treating the HSV cone as a hexacone , or the HSL double cone as a double hexacone. The formulae used are those in the table above. One might notice that the HSL/HSV hue "circle" do not appear to all be of the same brightness. This is a known issue of this RGB-based derivation of hue. Manufacturers of pigments use the word hue, for example, "cadmium yellow (hue)" to indicate that

1173-641: 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

1224-463: 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

1275-525: 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

1326-484: The CIE 1976 ( L *, a *, b *) ( CIELAB ) and 1976 ( L *, u *, v *) ( CIELUV ) color spaces, hue may be computed together with chroma by converting these coordinates from rectangular form to polar form. Hue is the angular component of the polar representation, while chroma is the radial component. Specifically, in CIELAB while, analogously, in CIELUV where, atan2 is a two-argument inverse tangent. Preucil describes

1377-467: The Preucil circle agree with the hue angle computed for the Preucil hexagon at integer multiples of 30° (red, yellow, green, cyan, blue, magenta, and the colors midway between contiguous pairs) and differ by approximately 1.2° at odd integer multiples of 15° (based on the circle formula), the maximal divergence between the two. The process of converting an RGB color into an HSL color space or HSV color space

1428-585: The SuperPaint configuration was an 8-bit video digitizer , and direct conversion to standard NTSC video. The system is now in the permanent collection of the Computer History Museum in Mountain View, California. Hue In color theory , hue is one of the main properties (called color appearance parameters ) of a color , defined technically in the CIECAM02 model as "the degree to which

1479-463: 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

1530-507: 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

1581-499: 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

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1632-410: The coordinates of the color in question, until it intersects the spectral locus . The wavelength at which the line intersects the spectrum locus is identified as the color's dominant wavelength if the point is on the same side of the white point as the spectral locus, and as the color's complementary wavelength if the point is on the opposite side. There are two main ways in which hue difference

1683-403: 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

1734-421: 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

1785-589: 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)

1836-475: The formula given in the table below. In each case the formula contains the fraction M − L H − L {\displaystyle {\frac {M-L}{H-L}}} , where H is the highest of R, G, and B; L is the lowest, and M is the mid one between the other two. This is referred to as the "Preucil hue error" and was used in the computation of mask strength in photomechanical color reproduction. Hue angles computed for

1887-487: The hue from ≈ 0° (red) up to ≈ 275° (violet), and line of purples#Table of highly-saturated purple colors for color terms of the remaining part of the color wheel. Alternative approach is to use a systematic notation. It can be a standard angle notation for certain color model such as HSL/HSV mentioned above, CIELUV , or CIECAM02 . Alphanumeric notations such as of Munsell color system , NCS , and Pantone Matching System are also used. Lightness Lightness

1938-532: 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 ,

1989-407: 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

2040-405: The original pigmentation ingredient, often toxic, has been replaced by safer (or cheaper) alternatives whilst retaining the hue of the original. Replacements are often used for chromium , cadmium and alizarin . Dominant wavelength (or sometimes equivalent wavelength) is a physical analog to the perceptual attribute hue. On a chromaticity diagram , a line is drawn from a white point through

2091-410: 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

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2142-452: The same hue are distinguished with adjectives referring to their lightness or colorfulness - for example: "light blue", " pastel blue", "vivid blue", and "cobalt blue". Exceptions include brown , which is a dark orange . In painting , a hue is a pure pigment —one without tint or shade (added white or black pigment, respectively). The human brain first processes hues in areas in the extended V4 called globs . The concept of

2193-450: 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

2244-455: 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

2295-431: 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

2346-421: The total framebuffer be implemented as a 307,200 byte shift register that shifted in synchronization with the television output signal. The primary drawback to this scheme was that memory was not random access . Rather, a given position could be accessed only when the desired scan-line and pixel time rolled around. This gave the system a maximum latency of 33 ms for writing to the framebuffer. Also included in

2397-567: 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

2448-402: 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

2499-429: Was a custom computer system built around a Data General Nova 800 minicomputer CPU and a hand-wired shift register framebuffer . This system had 311,040 bytes (303.75 KB ) of memory and was capable of storing 640 by 480 pixels of data with 8 bits of color depth . The memory was scattered across 16 circuit boards , each loaded with multiple 2- kilobit shift register chips. While workable, this design required that

2550-997: Was also one of the first graphics programs to use a graphical user interface and was one of the earliest to feature anti-aliasing . SuperPaint was used in the mid-1970s to make custom television graphics for KQED-TV in San Francisco, and later to make technical graphics and animations for the NASA Pioneer Venus project mission in 1978. Due to differences with management at PARC, Shoup left Xerox in 1979 to found graphics company Aurora Systems, while colleague Alvy Ray Smith went to work at New York Institute of Technology . In 1980, Smith and others joined Industrial Light & Magic , George Lucas 's movie special effects firm, and this group later founded Pixar . Shoup won an Emmy Award in 1983, and an Academy Scientific Engineering Award shared with Smith and Thomas Porter in 1998, for his development of SuperPaint. The SuperPaint system

2601-429: 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

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