Editing HSL and HSV (section)

==={{Anchor|Lightness|Value|Brightness|Intensity}} Lightness===

[[File:hsl-hsv chroma-lightness slices.svg|thumb|right|300px|Fig. 12a–d. Four different possible "lightness" dimensions, plotted against chroma, for a pair of complementary hues. Each plot is a vertical cross-section of its three-dimensional color solid.|alt=When we plot HSV value against chroma, the result, regardless of hue, is an upside-down isosceles triangle, with black at the bottom, and white at the top bracketed by the most chromatic colors of two complementary hues at the top right and left corners. When we plot HSL lightness against chroma, the result is a rhombus, again with black at the bottom and white at the top, but with the colorful complements at horizontal ends of the line halfway between them. When we plot the component average, sometimes called HSI intensity, against chroma, the result is a parallelogram whose shape changes depending on hue, as the most chromatic colors for each hue vary between one third and two thirds between black and white. Plotting luma against chroma yields a parallelogram of much more diverse shape: blue lies about 10 percent of the way from black to white, while its complement yellow lies 90 percent of the way there; by contrast, green is about 60 percent of the way from black to white while its complement magenta is 40 percent of the way there.]]

While the definition of ''hue'' is relatively uncontroversial – it roughly satisfies the criterion that colors of 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 ({{nobr|fig. 12}}; three of these are also shown in [[#Color-making attributes|{{nobr|fig. 8}}]]):

* The simplest definition is just the [[arithmetic mean]], i.e. average, of the three components, in the HSI model called ''intensity'' ({{nobr|fig. 12a}}). This is simply the projection of a point onto the neutral axis – the vertical height of a point in our tilted cube. The advantage is that, together with Euclidean-distance calculations of hue and chroma, this representation preserves distances and angles from the geometry of the RGB cube.<ref name=Hanbury2008/><ref>Rafael C. Gonzalez and Richard Eugene Woods (2008). ''Digital Image Processing'', 3rd ed. Upper Saddle River, NJ: Prentice Hall. {{ISBN|0-13-168728-X}}. [https://books.google.com/books?id=8uGOnjRGEzoC&pg=PA410&lpg=PA410 {{nobr|pp. 407–413}}].</ref>
*: <math>I = \operatorname{avg}(R, G, B) = \tfrac{1}{3}(R + G + B)</math>
* In the HSV "hexcone" model, ''value'' is defined as the largest component of a color, our ''M'' above ({{nobr|fig. 12b}}). This places all three primaries, and also all of the "secondary colors" – cyan, yellow, and magenta – into a plane with white, forming a [[hexagonal pyramid]] out of the RGB cube.<ref name=Smith/>
*: <math>V = \max(R, G, B) = M</math>
* In the HSL "bi-hexcone" model, ''lightness'' is defined as the average of the largest and smallest color components ({{nobr|fig. 12c}}), i.e. the [[mid-range]] of the RGB components. This definition also puts the primary and secondary colors into a plane, but a plane passing halfway between white and black. The resulting color solid is a double-cone similar to Ostwald's, [[#Motivation|shown above]].<ref name=Joblove/>
*: <math>L = \operatorname{mid}(R, G, B) = \tfrac{1}{2}(M + m)</math>
* A more perceptually relevant alternative is to use [[luma (video)|''luma'']], {{nobr|''{{prime|Y}}''}}, as a lightness dimension ({{nobr|fig. 12d}}). Luma is the [[weighted average]] of gamma-corrected ''R'', ''G'', and ''B'', based on their contribution to perceived lightness, long used as the monochromatic dimension in color television broadcast. For [[sRGB]], the [[Rec. 709]] primaries yield {{nobr|''{{prime|Y}}''<sub>709</sub>}}, digital [[NTSC]] uses {{nobr|''{{prime|Y}}''<sub>601</sub>}} according to [[Rec. 601]] and some other primaries are also in use which result in different coefficients.<ref>[[#Poynton|Poynton (1997)]]. [http://www.poynton.com/notes/colour_and_gamma/ColorFAQ.html#RTFToC9 "What weighting of red, green and blue corresponds to brightness?"]</ref>{{refn|group=upper-alpha |For a more specific discussion of the term ''luma'', see Charles [[#Poynton2008|Poynton (2008)]]. See also [[RGB color space#Specifications]]. Photoshop exclusively uses the NTSC coefficients for its "Luminosity" blend mode regardless of the RGB color space involved.<ref>Bruce Lindbloom (2001-09-25). http://lists.apple.com/archives/colorsync-users/2001/Sep/msg00488.html {{Webarchive|url=https://web.archive.org/web/20110707173255/http://lists.apple.com/archives/colorsync-users/2001/Sep/msg00488.html |date=2011-07-07 }} "Re: Luminosity channel...".</ref>}}
*: <math>Y'_\text{601} = 0.299\cdot R + 0.587\cdot G + 0.114\cdot B</math> (SDTV)<!--525 lines-->
*: <math>Y'_\text{240} = 0.212\cdot R + 0.701\cdot G + 0.087\cdot B</math> [[Adobe RGB color space|(Adobe)]]<!--also SMPTE 145-->
*: <math>Y'_\text{709} = 0.2126\cdot R + 0.7152\cdot G + 0.0722\cdot B</math> [[Rec. 709|(HDTV)]]
*: <math>Y'_\text{2020}  = 0.2627\cdot R + 0.6780\cdot G + 0.0593\cdot B</math> [[Rec. 2020|(UHDTV, HDR)]]<!--also Rec. 2100-->

All four of these leave the neutral axis alone. That is, for colors with {{nobr|1=''R'' = ''G'' = ''B''}}, any of the four formulations yields a lightness equal to the value of ''R'', ''G'', or ''B''.

For a graphical comparison, see [[#Disadvantages|fig. 13 below]].

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