CIELAB numbers are not fully specified, if you don't specify the reference white
point as well, which was used for the XYZ -> CIELAB conversion. That like
saying the length is 5, w/o specifying whether you mean 5 inches, or 5 feet, or 5
meter.
Usually, D50 is assumed if the reference white point is not specified, but for
numbers from an unknown origin you can never be sure how they are supposed to
be interpreted, if it is not explicitly specified.
I guess that the CIELAB values in your reference file were calculated for a
different reference white than colprof's CIELAB numbers (the latter are IMO
D50-based).
Regards,
Gerhard
Am 09.06.20 um 10:30 schrieb Hermann-Josef Röser:
Hello Graeme and Gerhard,
thanks a lot for your replies. Unfortunately I do not really understand this
(yet).
Here are the values for patch A1 in the reference file:
SAMPLE_ID XYZ_X XYZ_Y XYZ_Z LAB_L LAB_A LAB_B
LAB_C LAB_H D_RED D_GREEN D_BLUE D_VIS STDEV_X
STDEV_Y STDEV_Z MEAN_DE STDEV_DE
A1 2.622 1.970 0.758 15.331 10.511 2.674 8.89
15.75 1.47 1.88 1.93 1.70 0.06 0.05
And here is the corresponding line from the ti3-file:
SAMPLE_ID XYZ_X XYZ_Y XYZ_Z RGB_R RGB_G RGB_B STDEV_R STDEV_G STDEV_B
A01 2.622000 1.970000 0.758000 28.91538 19.94871 19.18301 0.720745 0.465900
0.899324
So the XYZ-data are identical. RGB_R ... are the measured values from the
target scan. This I understand.
And here is what profcheck tells me:
[1.146434] A01: 0.28915380 0.19948710 0.19183010 -> 14.802257 14.722387
13.324269 should be 15.329014 15.318528 12.124771
, but not mangled otherwiseWhat is the meaning of the number in [] at the beginning? What are the three
numbers following after the A01: ?
The values following "->" are quite different from the Lab-values in the
reference table. So is this due to the rendering intent?
My naive understanding of the ICC-profile was, that it is "just" the
transformation of measured scanner RGB-values to the XYZ/Lab values supplied
by the reference table. So for any measure triple (R,G,B) interpolation via
the profile gives me the absolute colour in (X,Y,Z) or (L,a,b). So still I
do not see, why I need a rendering intent here :-( : Once I have the
absolute colours I then can render into a colour space of my choice (e.g.
sRGB) using one of the rendering intents.
I would be very happy for a reference so I can read and learn about all
these connections. What I have found up to now (e.g. the white papers on the
color.org pages and several books from the Wiley series) was not of much
help regarding an understanding of these technical details.
Many thanks again and best wishes
Hermann-Josef
