Hello Graeme,
thank you very much for this detailed explanation. This will certainly help
understanding what I see with the profiles.
In my naïve view of the input device "gamut" I thought that the device
dependent colour space of my scanner is a cube with the R,G,B-values along
the axis and this cube is completely filled by the scanner, because its
R,G,B values can take all the values from 0 to 1. Then the ICC-profile tells
me how to transform each R,G,B-triple into Lab-values. So the projection of
the cube into the Lab-space gives me a volume that represents the "gamut" of
my scanner. Of course, since I can adjust the exposure time, this volume
represents the scanner "gamut" only for the exposure time used for
calculating the ICC-profile. I hope this is a correct view?
Best wishes
Hermann-Josef
--- Begin Message ---Hermann-Josef Röser wrote:
- From: "Graeme Gill" <graeme@xxxxxxxxxxxxx>
- To: <argyllcms@xxxxxxxxxxxxx>
- Date: Sat, 20 Feb 2021 01:51:02 +0100
Hi,
Another observation in this direction is the following. I have IT8-targetsfrom 2 vendors.
One seems to be somewhat underexposed with brightest patches having L =85, the other has
in the brightest patches L-values of about 93. For the same AG setting,the profile
corresponding to the slightly underexposed target extends not as far inthe L-direction as
the better exposed one. I guess the two observations are related to eachother?
a "feature" of the ICC profile format is that it stores the basic profiling
information
in a Relative Colorimetric form. This means that the data is normalised to
the white
point. For output profiles this is not of great significance. For input
profiles
it has an impact in that it may not model values above the relative white
point.
- I'll come back to this point later.
Of even greater impact on making input profiles is the question of gamut.
For an
output device you can explore the whole of the possible color gamut by
exploring the whole of the possible device value gamut. General input
devices such as
cameras do not have a gamut in the same sense, since you can adjust exposure
to
ensure that the sensors are never saturated, and they will produce a device
output
value for any possible input spectrum. A properly setup scanner should not
have
a gamut limit either, putting aside unusual materials such as fluorescence
or
specular reflective type materials. (The input devce equivalent of a gamut
limit is really a multi-dimensional plot of spectra vs. colorimetric
accuracy.
Not easy to represent, and you need to know the sensor spectral
sensitivities to
compute it.)
But a test chart used to characterise an input device does have a limited
gamut. So from a purely information point of view, it's not as accurate to
extrapolated outside the gamut of the test chart as it is to interpolate
within
measured data points.
Back to white point - the default ICC representation is white point
relative,
and the test chart doesn't test anything above its white point, and the
default ICC data representation is white point relative, so by default
the test chart white comes out at L*a*b* 100, 0, 0.
But viewed in an absolute sense, the L* gamut varies with the test chart
white.
The implication is that the input profile is only really accurate when
applied to the same media and process as used to make the test chart.
You can make an Absolute Colorimetric input profile using ArgyllCMS by
using the -ua option. This attempts to make the white point 100%
reflection rather than the chart white. But it has to use extrapolation
to do this, and this can make it inaccurate outside the test chart gamut.
Cheers,
Graeme Gill.
--- End Message ---
