Re: U.S. Web Coated (SWOP) v2 always produces blues that
Re: U.S. Web Coated (SWOP) v2 always produces blues that
- Subject: Re: U.S. Web Coated (SWOP) v2 always produces blues that
- From: Harold Boll <email@hidden>
- Date: Wed, 4 Jul 2007 22:56:14 -0400
> In Colorsync-users Digest, Vol 4, Issue 242, Msg. 1
> Rich Apollo writes:
> Okay we're talking about RGB, but... (and where are you setting Lab
> values in decimal?)
>not quite what I was talking about
You did a lot of work so I'd like to respond. Your generated a lot of data
but I didn't quite understand the point you're making. However, the first
thing I noted was the difference between the sRGB blue value you reported
in PhotoShop versus the one I reported. So I decided to look into that
disparity.
L a b
30 69 -110 sRGB IEC61966-2.1 (yours via PhotoShop)
32.3 62.5 -107.4 Mine via Matlab (results in floating
point)
It turns out there is more than one type of sRGB profile out there.
sRGB_IEC61966-2-1_noBPC.icc
Copyright: 'Copyright Hewlett Packard, 2004'
Description: sRGB IEC61966-2-1 noBPC
sRGB_IEC61966-2-1_withBPC.icc
Copyright Hewlett Packard, 2004
Description: sRGB IEC61966-2-1 withBPC
sRGB Color Space Profile.icm (PhotoShop CS3)
Copyright: 'Copyright (c) 1998 Hewlett-Packard Company'
Description: sRGB IEC61966-2.1
(Note: "BPC" stands for Black Point Compensation)
I used the 'noBPC' to obtain the sRGB blue value I initially reported.
It seems that 'withBPC' is identical to the 'PS' profile. The difference to
'noBPC' is as follow:
RGB = [0 0 0]
L* a* b*
0.0042 -0.0042 10.91 noBPC
0 0 0 withBPC
The above 3 profiles yield the following Lab values for RGB = [001] inputs:
(note: RGB range below is from 0-1 whereas in PS it is 0-255)
RGB = [0 0 1]
L* a* b*
32.3 62.5 -107.4 withBPC
29.6 68.3 -112.0 noBPC
29.6 68.3 -112.0 photoShop
I looked at sRGB blue in photoShop and got following values. (It differs
slightly from ones you reported)
L* a* b*
30 68 -112
The above PhotoShop values match the Matlab evaluations after rounding to
nearest integer value.
Using this 'new' sRGB blue value in the analysis I did in a previous email
yields the same result:
i) Cyan > Magenta
ii) hue angles of both mapped colors essentially identical.
If you send me the ECI rgb profile, I'll run those numbers.
Harold
TOSHIBA AMERICA BUSINESS SOLUTIONS 2 Musick, Irvine, CA 92618
Harold Boll | Color Scientist | 1.781.856.5174 |
email@hidden
" The sunlights differ, but there is only one darkness. "
Ursula K. Leguin
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