Editing HSL and HSV (section)

==Basic principle==
{{multiple image
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| image1 = HSL color solid cylinder saturation gray.png
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| caption1 = Fig. 2a. HSL cylinder.
| image2 = HSV color solid cylinder saturation gray.png
| width2 = 197
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| caption2 = Fig. 2b. HSV cylinder.
| footer = 
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HSL and HSV are both cylindrical geometries ({{nobr|fig. 2}}), with hue, their angular dimension, starting at the [[red]] [[primary color|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 color|additive]] primary and [[secondary color]]s – red, [[yellow]], green, [[cyan]], blue and [[magenta]] – and linear mixtures between adjacent pairs of them, sometimes called ''pure colors'', are arranged around 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 ''[[tints and shades|shades]]'' – leaves saturation unchanged. In HSL, saturation is also unchanged by ''[[tints and shades|tinting]]'' with white, and only mixtures with both black and white – called ''tones'' – have saturation less than 1. In HSV, tinting alone reduces saturation.

{{multiple image
| align             = right
| image1            = HSL color solid dblcone chroma gray.png
| width1            = 197
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| caption1          = 
| image2            = HSV color solid cone chroma gray.png
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| caption2          = 
| footer            = Fig. 3a–b. If we plot hue and (a) HSL lightness or (b) HSV value against chroma ([[range (statistics)|range]] of RGB values) rather than saturation (chroma over maximum chroma for that slice), the resulting solid is a [[bicone]] or [[cone]], respectively, not a cylinder. Such diagrams often claim to represent HSL or HSV directly, with the chroma dimension confusingly labeled "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, {{colorsample2|#005456}} from the bottom right in the sliced HSL cylinder or {{colorsample2|#d4ffff}} from the top right) – conflict with the intuitive notion of color purity, often a [[cone|conic]] or [[bicone|biconic]] solid is drawn instead ({{nobr|fig. 3}}), with what this article calls ''[[Colorfulness|chroma]]'' as its radial dimension (equal to the [[range (statistics)|range]] of the RGB values), instead of saturation (where the saturation is equal to the chroma over the maximum chroma in that slice of the (bi)cone). Confusingly, such diagrams usually label this radial dimension "saturation", blurring or erasing the distinction between saturation and chroma.{{refn|group=upper-alpha |In the [[#Joblove|Joblove and Greenberg (1978)]] paper first introducing HSL, they called HSL lightness "intensity", called HSL saturation "relative chroma", called HSV saturation "saturation" and called HSV value "value". They carefully and unambiguously described and compared three models: hue/chroma/intensity, hue/relative chroma/intensity, and hue/value/saturation. Unfortunately, later authors were less fastidious, and current usage of these terms is inconsistent and often misleading.}} [[#Formal derivation|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 the "hexcone model" while HSL is often called the "bi-hexcone model" ([[#Color-making attributes|{{nobr|fig. 8}}]]).{{refn|group=upper-alpha |The name ''hexcone'' for hexagonal pyramid was coined in [[#Smith|Smith (1978)]], and stuck.}}
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