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A colorspace defined by CIE in 1976 to achieve (or at least approximate) perceptual uniformity.

CIE XYZ space and RGB space are linear transformations of each other, which among other things means that an RGB color cube projected into XYZ will still be cube-ish, but scaled, sheared and rotated.

However, RGB is not perceptually uniform: the effect of changes in intensity is nonlinear. Neither, then, is XYZ or CIE (x, y). L*u*v* is a nonlinear transform of CIE (x, y) (chrominance) space so that equal distances in the color solid will corespond to approximately equally different colors.

L*u*v* is defined in terms of xyY (or plain XYZ), in two steps. First, (x, y) or XYZ space is distorted into quantities u' and v' so that changes in u' v' space are equally visible everywhere:

u' = 2x/(6y - x + 1.5) = 4X/(X+15Y+3Z)
v' = 4.5y/(6y - x + 1.5) = 9Y/(X+15Y+3Z)

Secondly L*u*v* is defined in terms of Yu'v':

L* = 116(Y/Yn)1/3 - 16
u* = 13L*(u' - u'n)
v* = 13L*(v' - v'n)

Where Yn, u'n, and v'n are the u', v', and Y values of the color you use as reference white. Note that the quantity L* is CIE lightness, the perceptually corrected version of the luminance Y.

This second transform moves the color solid so that the greys all fall on the L* axis (u* = v* = 0). Also, the factor L* in the definitions of u* and v* means that the solid shrinks to a cone, so that all colors with zero lighness (i.e, black) fall on a single point, namely the origin.

The final shape of L*u*v* is quite similar to the HSV space: the "value" of the color is the lightness L*. The horizontal direction, or angle in the u*v* plane, from the L* axis determines "what" color we get - the "hue". And the distance from the L* axis determines the "saturation" - at the axis itself we have greyscale, and at the edge of the solid are the pure colors. The difference is that L*u*v*, unlike HSV, is based on experiments to achive perceptual uniformity. This makes it suitable for user interfaces: many programs that use HSV in their "color selection" dialogs could profit from moving to L*u*v*.

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