Marti schrieb:
Just create a transform RGB profile -> XYZ using abs.
colorimetric, then feed (255, 0, 0), (0, 255, 0) and (0, 0, 255)
For example:
F:\lcms>icctrans -t3 -i "sRGB Color Space Profile.icm" -o*XYZ
little cms ColorSpace conversion calculator - v1.9
Enter values, 'q' to quit
R (0..255)? 255
G (0..255)? 0
B (0..255)? 0
X=41.2415 Y=21.2616 Z=1.9318
Hi Marti,
you just triggered me to bring up an issue I already wanted to discuss.
Obviously, the results depend on the sRGB profile being used. With the
old sRGB profile from 1998, one gets the result you mentioned above.
However, with the new sRGB profiles from July 2004, which can be
downloaded from the ICC web site www.color.org, the result is different:
$ icctrans -t3 -i sRGB_IEC61966-2-1_noBPC.icc -o \*XYZ
little cms ColorSpace conversion calculator - v1.8
Enter values, 'q' to quit
R (0..255)? 255
G (0..255)? 0
B (0..255)? 0
X=43.0878 Y=22.3145 Z=2.5116
And with Argyll CMS, the new sRGB profile gives me the same results for
relative colorimetric and for ICC-absolute intent:
$ icclu -ia sRGB_IEC61966-2-1_noBPC.icc
1 0 0
1.000000 0.000000 0.000000 [RGB] -> MatrixFwd -> 0.442668 0.232205
0.024054 [XYZ]
$ icclu -ir sRGB_IEC61966-2-1_noBPC.icc
1 0 0
1.000000 0.000000 0.000000 [RGB] -> MatrixFwd -> 0.442667 0.232205
0.024053 [XYZ]
According to the header, the new sRGB profile is a V 2.0.0 profile,
however, its media white point tag records D50 (not D65, but D65 adapted
to D50), and the profile also does contain a "chad" tag.
In the past, my understanding of ICC-absolute intent was the unchanged
reproducion of the source XYZ color on the destination device (without
performing any CAT), only the luminance may possibly differ.
In the mean time, I'm no longer sure. There seems to exist a seconds
interpretation as well, which interprets ICC-absolute intent as an
*illuminant relative* transformation (i.e. CAT from source illuminant to
destination illuminant), with the consequence, that relative
colorimetric and iCC-absolute intent give the same result, if source and
destination profiles are monitor profiles, since media WP equal to
illuminant is assumed for monitor profiles.
Basiaclly all three transformations make sense
1. Media relative, i.e. source media WP is mapped
to dst media WP (-> "relative colorimetric")
2. CAT from source illuminant to destination illuminant,
discounting only the illuminant, but not the paper color
3. Source XYZ colors map unchanged to dst XYZ colors,
(i.e. sRGB image is reproduced very blue-ish on a D50 monitor)
but there are only two colorimetric intents defined (relative
colorimetric and ICC-absolute), thus only either (2) or (3), but not
both, can be the "correct" interpretation of ICC-absolute intent.
So I'm a bit confused, and I'm wondering, what's now really the correct
interpretation of ICC-aboslute intent?
Thanks,
Gerhard
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