AW: calculate Lab from RGB ...
AW: calculate Lab from RGB ...
- Subject: AW: calculate Lab from RGB ...
- From: Samer Mady <email@hidden>
- Date: Tue, 27 Nov 2001 18:20:39 +0100
>
How does Photoshop calculate Lab from RGB space? ... and vice versa? I'm
>
looking for a formula similar to what Charles Poynton describes:
>
http://www.inforamp.net/~poynton/notes/colour_and_gamma/ColorFAQ.html#RTFTo
C
>
18 ... but it is unclear as to how all color space parameters (gamma, Wp,
>
RGB tristimulus) fit into the matrix.
they do not fit into the matrix. this calculation is not comlete.
in order to calculate CIELab from RGB. you have to do the following steps:
* transform the CIELab values to CIEXYZ values using the standard
forumla
* transform the CIEXYZ for D50 to CIEXYZ with the white point of the
RGB space ( for an example the white point of sRGB is D65 ) using one the
following chromatic adaptation transforms: von Kries transformation,
Linearized Bradford/CIECAM97 or the "Wrong von Kries transformation"
* transform the XYZ to RGB by using a matrix just like Poynton
describes. this matrix only includes information about the color of the
phosphors of the RGB.
* and only after that you can take the Gamma in consideration by
calculating the R to the power of the inverse value of Gamma ; the same for
G and B
Ciao
Samer Mady
Color Management Developer
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