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In the sRGB color space, the luminance values for the red, green, and blue primaries are specified as 0.21216, 0.7152, and 0.0722, respectively. The white point is defined to have luminance 1. In other words, the sRGB values <1,0,0>, <0,1,0>, <0,0,1>, and <1,1,1> map to xyY values <0.64, 0.33, 21.216>, <0.3, 0.6, 71.52>, <0.15, 0.06, 7.217>, and <0.31273, 0.32902, 100> (with Y scaled by 100 by convention).

How are the luminance values for the primaries determined? Are they purely a function of the xy primaries, or a combination of the primaries and the illuminant (e.g. D65)? If so, what is the relationship? More generally, how can I determine the luminance values for an arbitrary set of primaries?

MooseBoys
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1 Answers1

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Finding the RGB-to-XYZ matrix is determined by the chromaticities (xy values) of the red, green, and blue primaries and by the chromaticies of the white point. The white point, in turn, is determined, at least in part, by the light source and by the color matching functions in use (for example, the D65 illuminant and the CIE 1931 standard observer, respectively).

The conversion is explained in further detail on Bruce Lindbloom's Web site:

http://www.brucelindbloom.com/index.html?Eqn_RGB_XYZ_Matrix.html

After generating the matrix, the luminances (Y values) of the three primaries are given in the second row of that matrix (see the pregenerated matrices further down on that page). Note that the formula given there takes the xy form of the primaries and the XYZ form of the white point, which can be converted from xy form by [x/y, 1, (1-(y+x))/y].

Peter O.
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