04-02-2010, 07:50 PM
Hue is one of the main properties of a color, defined technically (in the CIECAM02 model), as “the degree to which a stimulus can be described as similar to or different from stimuli that are described as red, green, blue, and yellow,”[1] (the unique hues). The other main correlates of color appearance are colorfulness, chroma, saturation, lightness, and brightness.
Usually, colors with the same hue are distinguished with adjectives referring to their lightness and/or chroma, such as with "light blue", "pastel blue", "vivid blue". Exceptions include brown, which is a dark orange,[2] and pink, a light red with reduced chroma.
In painting color theory, a hue refers to a pure color—one without tint or shade (added white or black pigment, respectively).[3]. A hue is an element of the color wheel. Hues are first processed in the brain in areas in the extended V4 called globs.[4][5]
Computing hue
In opponent color spaces in which two of the axes are perceptually orthogonal to lightness, such as CIE L*a*b* (CIELAB) and CIE L*u*v* (CIELUV), 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:[6]
h_{ab} = \mathrm{atan2}(b^*, a^*)\;
while, analogously, in CIELUV:[6]
h_{uv} = \mathrm{atan2}(v^*, u^*) = \mathrm{atan2}(v', u')\;
Where, atan2 is a two-argument inverse tangent.
[edit] Computing hue from RGB
Preucil[7] describes a color hexagon, similar to a trilinear plot described by Evans, Hanson, and Brewer,[8] which may be used to compute hue from RGB. To place red at 0°, green at 120°, and blue at 240°, one may solve:
\tan h_{Preucil\ hexagon} = \frac{\sqrt{3}\cdot (G - B)}{2\cdot R - G - B}
He also used a polar plot, which he termed a color circle.[7] Using R, G, and B, rather than the R, G, and B densities Preucil used, one may compute hue angle using the following scheme: determine which of the six possible orderings of R, G, and B prevail, then apply the appropriate formula; see table below.
Usually, colors with the same hue are distinguished with adjectives referring to their lightness and/or chroma, such as with "light blue", "pastel blue", "vivid blue". Exceptions include brown, which is a dark orange,[2] and pink, a light red with reduced chroma.
In painting color theory, a hue refers to a pure color—one without tint or shade (added white or black pigment, respectively).[3]. A hue is an element of the color wheel. Hues are first processed in the brain in areas in the extended V4 called globs.[4][5]
Computing hue
In opponent color spaces in which two of the axes are perceptually orthogonal to lightness, such as CIE L*a*b* (CIELAB) and CIE L*u*v* (CIELUV), 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:[6]
h_{ab} = \mathrm{atan2}(b^*, a^*)\;
while, analogously, in CIELUV:[6]
h_{uv} = \mathrm{atan2}(v^*, u^*) = \mathrm{atan2}(v', u')\;
Where, atan2 is a two-argument inverse tangent.
[edit] Computing hue from RGB
Preucil[7] describes a color hexagon, similar to a trilinear plot described by Evans, Hanson, and Brewer,[8] which may be used to compute hue from RGB. To place red at 0°, green at 120°, and blue at 240°, one may solve:
\tan h_{Preucil\ hexagon} = \frac{\sqrt{3}\cdot (G - B)}{2\cdot R - G - B}
He also used a polar plot, which he termed a color circle.[7] Using R, G, and B, rather than the R, G, and B densities Preucil used, one may compute hue angle using the following scheme: determine which of the six possible orderings of R, G, and B prevail, then apply the appropriate formula; see table below.