Re: Calculating from AbsCol to RelCol from L*a*b*?
Re: Calculating from AbsCol to RelCol from L*a*b*?
- Subject: Re: Calculating from AbsCol to RelCol from L*a*b*?
- From: Marco Ugolini <email@hidden>
- Date: Mon, 20 Apr 2009 12:01:28 -0700
- Thread-topic: Calculating from AbsCol to RelCol from L*a*b*?
In a message dated 4/20/09 12:32 AM, Klaus Karcher wrote:
> Terence Wyse schrieb:
>> Hello all,
>>
>> Anybody here know where I can find the formulae for calculating relative
>> colorimetry from absolute L*a*b* where the L*a*b* for paper white is
>> known? I know that it's not as simple as subtracting paper white L*a*b*
>> but I've searched and can't find the math used to come up with relative
>> colorimetric values.
>
> see ICC spec Annex D.6 <http://color.org/ICC1v42_2006-05.pdf>:
>
> The ICC profile format does not include an explicit transform for the
> ICC-absolute colorimetric intent. On creating a profile only the
> XYZmediawhite-relative values are stored in a profile, not XYZunder D50.
> When using a profile, after obtaining the media-relative colorimetric
> transform of the profile, the PCS-side XYZ tristimulus values for the
> ICC-absolute colorimetric intent are calculated, if needed, from the
> media-relative colorimetric transform through a simple scaling operation:
> Xunder D50 = Xunder D50 mediawhite / X D50 * X mediawhite-relative
> Yunder D50 = Yunder D50 mediawhite / YD50 * Ymediawhite-relative
> Zunder D50 = Zunder D50 mediawhite / Z D50 * Z mediawhite-relative
In other words, one must convert from the measured Lab values to XYZ, then
from XYZ back to Lab -- but this time using the XYZ coordinates of the
measured reflective media white for the transform from XYZ to Lab, instead
of D50 white point coordinates.
Marco
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