Wolf Faust schrieb:
So I see a number of advantages in building a database and trying to
add the spectral data of scanner, ink and everything else that is
easy to get/use. It's not perfect but should get us very close.
Wolf,
I agree, a database is important. In order that a method as described in
http://www.ece.rochester.edu/~gsharma/papers/ntargcaljist00.pdf can be
used *without needing a spectrometer*, a database of media
characteristics (for various printer/settings/ink/paper combinations)
and scanner light source spectra is actually mandatory - only scanners
in the database can be used, and only media types in the database can be
scanned with this or a similar a method.
If the media model parameters (e.g. spectral eigenvectors) for the
desired medium are not available in the database, they would need to be
measured and computed, requiring a spectrometer.
Graeme: I agree, spectral characterization of a scanner most likely
needs more than three dyes. I was surprised about the IEC 61966-8
standard for spectral characterization of scanners. This standard
uses only a target simlar to the IT 8.7 target (same layout... just
more highly saturated colors in order to help the calculation).
If the goal is only the reconstruction of the sensor sensitivities, with
methods as described e.g. in http://tinyurl.com/9mvk2 or
http://tinyurl.com/bzyly, then I guess it is not so important that the
calibration target covers different metamers for the same colors.
On the other hand, for the calibration of a multi spectral (more than 3
channels) capturing system, which is expected to return the same XYZ
values for any captured metameric pair, I also have doubts, that an IT8
target will be sufficient, although it does not need to work for
arbitrary spectra, but can be limited to the union of all possible
inkjet spectra for various printers/inks/papers/..., and IT8 target
spectra. If the union of all inkjet and IT8 spectra is decomposed into a
reasonable number of principal components (say 9 or 12), such that all
these spectra can be described reasonably accurate as linear
combinations of these eigenvectors, then I also suspect, that the IT8
spectra will utilize most, but likely not all of these eigenvectors.
Btw, does 61966-8 actually specify the recommends maths how to
reconstruct the spectral sensitivities using this target, or is it
rather unspecific regarding the details? Is the method similar to the
two ones I referred above?
Regards,
Gerhard
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