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why use 0.008856 in Lab color space?

조회 수: 4 (최근 30일)
Yoon ThiriZaw
Yoon ThiriZaw 2019년 5월 26일
In Lab color space equations, why use 0.008856 as thresholds if we transform RGB to Lab? Lab color space equation are the following:
The conversion formula of RGB image into Lab image is:
X = 0.4124*R + 0.3576*G + 0.1805*B
Y = 0.2126*R + 0.7152*G + 0.8220*B
Z = 0.0193*R + 0.1192*G + 0.9505*B
If X/X_n,Y/Y_n,Z/Z_nare all greater than 0.008856,then
L* = 116∛("Y/" Y_n )- 16
a* = 500 ( ∛("X/" X_n )- ∛("Y/" Y_n ))
b* = 200 (∛("Y/" Y_n )- ∛("Z/" Z_n ))
If X/X_n,Y/Y_n,Z/Z_nare all equal to or less than 0.008856,then
L* = 903.3(Y/Y_n,)
a* = 500 (f(X/X_n,) – f(Y/Y_n))
b* = 200 (f(Y/Y_n) – f(Z/Z_n))
Why let these thresholds 0.008856?
If less than 0.008856, what we get?
And, if greater than 0.008856, what we get?
Please!!!

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