Hi,
The procedure is quite simple, and does not involve any magic constant but
D50.
- First convert from Lab to XYZ
- Then scale it from D50, which is the illuminant the ICC profile is in, to
the real monitor white point, which is stored in 'lumi' tag. You can do
that by applying:
Xout = Xin * (X_lumi/X_D50)
Yout = Yin * (Y_lumi/Y_D50)
Zout = Zin * (Z_lumi/Z_D50)
- Then for RGB=(0,0,0) you have the XYZ of veiling glare, that is, what is
going out of your monitor with no RGB stimuli. You have to subtract this
value to all patches. Obviously RGB=(0,0,0) will then result in XYZ=(0,0,0)
- Finally convert this XYZ to Lab
There are some white papers in the ICC site explaining part of this process.
Please double-check the excel file, as I may be done some errors. I'm not
very used to excel after all...
Hope this helps
All the best
Marti
Original Message:
-----------------
From: Michael A. Litscher [EMAIL PROTECTED]
Date: Wed, 19 Dec 2007 10:53:06 -0600
To: lcms-user@lists.sourceforge.net
Subject: [Lcms-user] [Fwd: [Fwd: RE: Lab to RGB not yielding
expectedvalues]]
Marti,
Thank you again for replying, and please excuse my ignorance on this
subject. I have spent many years coming to understand the math and
constants used in the reflective end of this business, and I've spent
the last year attempting to understand the same for the emissive end.
I have been looking closely at the changes you made to the spreadsheet.
I have searched the ICC web page (color.org <http://color.org>)
specifically and the web in general, but I have been unable to determine
the source of the math, the constants, and the procedure that you used
for correcting for veiling glare and scaling the luminance.
The closest I've come is a reference to standard ISO 22028-1:2004, found
here:
http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnum
ber=37161
<http://www.iso.org/iso/iso_catalogue/catalogue_tc/catalogue_detail.htm?csnu
mber=37161>
It was referenced here:
http://www.color.org/rgbchardata.xalter
At the suggestion of someone else, I've even grep'd the lcms and argyll
source code for these constants, hoping to learn what they were, which
equations they are used in, and what published standards I may find them
in, but I came up empty. Except for the D50 XYZ white point, which is
oddly very close but not exactly the same as that found in the ASTM E308
standard. Perusing the source code of lcms and argyll would, I am sure,
be very educational, but I doubt I would recognize the needle I would be
looking for in these two rather large haystacks of source code.
I am not opposed to purchasing standards, but having done so in the past
based on their description, I have wasted considerable money purchasing
standards which did not contain the answers I was looking for. So, if I
may ask for your help once more, in which published standard would I
find the constants and the math that you used in that spreadsheet for
removing veiling glare and scaling the luminance?
Also, I will re-profile my monitor and create a LUT-based profile.
Thank you for that advice.
>
>
> Hi Mike,
>
>
>
> I've taken my time to check what you are reporting. After comparing
> results with Photoshop, both gives same RGB values, so the problem, if
> any, is not on lcms but in the profile. At that point, I wondered why
> you got such big differences and here are some hints.
>
>
>
> * First and most important: you should not trust on RGB differences.
> They are meaningless. Use Lab and DeltaE instead.
>
> * ICC states that some "pre cooking" should be applied to measured
> data, specially on emissive devices:
>
> - You need to remove veiling glare (flare)
>
> - You need to scale the luminance (across 'lumi' tag).
>
>
>
> I've done those on the excel file you provided and the results, again
> in RGB contone, are:
>
> http://www.littlecms.com/test%20chart%20simplified2.xls
> <http://www.littlecms.com/test%20chart%20simplified2.xls>
>
>
>
> MIN
>
>
>
> -5
>
>
>
> -1
>
>
>
> 0
>
> MAX
>
>
>
> 5
>
>
>
> 13
>
>
>
> 16
>
> AVG
>
>
>
> 0.161616
>
>
>
> 3.69697
>
>
>
> 3.959596
>
>
>
> If you compare those with your initial results, there is some difference:
>
>
>
> MIN
>
>
>
> -4
>
>
>
> -1
>
>
>
> 0
>
> MAX
>
>
>
> 21
>
>
>
> 22
>
>
>
> 28
>
> AVG
>
>
>
> 8.080808
>
>
>
> 11.83838
>
>
>
> 15.77778
>
>
>
>
>
> But again, that is meaningless. Two contone values may differ 10 or
> more units and be just on 0.1 dE colorimetrically.
>
>
>
> Finally, this is a Matrix-shaper profile, which may model a CRT quite
> closely, but would probably fail on LCD. I think a LUT-based profile
> would be more adequate, anyway that is up to the profile manufacturer.
>
>
>
> Hope this helps
>
> All the best
>
> Marti Maria
>
> The littleCMS project
>
> www.littlecms.com <http://www.littlecms.com>
>
>
>
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