Thread: [Lcms-user] Re: Printer-profiling via scanner
An ICC-based CMM for color management
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From: Wolf F. <mai...@wf...> - 2005-06-23 20:47:07
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Just found out that I am still subscribed to this ML but I simply do not receive anything (incl. my email password reminder). I don't have the slightest idea why... Anyway, to answer Jan-Peter: > Wolf, if some people would be interested on such thinks, would you like > to distribute also such charts additional to your photographic produced > charts ? Why not. I simply do not see how this can be done cost effective as you would need to produce very small amounts of targets on a very large number of different setups. After all we need a cost effective solution for these customers (otherwise we better buy a spectrometer). But there are a number of things we should not forget: there are many cases where costs or technical reasons do not allow producing scanner targets using the wanted materials. And even simple cases like yours can turn out tricky: you are talking of a 4-7 ink colors printer that produces shades by halftoning. So in reality, you still only have 4-7 inks and the scanner does simply see every dot with all it's problems. So halftoning might matter not only on the print, but also on the scan. Another problem is, as Graeme noted, that we are talking about converting >3 dimensional output space back into a three dimensional Lab value. As a result, the separation original used for printing does matter as the scanner for instance can't easily tell a true black ink from a mixed one. 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. 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). There is even free source and binary code available from http://www.ite.rwth- aachen.de/Inhalt/Forschung/AixSpecScan/scan_cal.html implementing the standard. So, there is a standard and tools to get experience. I bought the standard some time ago and even added spectral data for my targets in hope some applications turns up using the standard so I could produce them. But so far, I haven't even found some info on the practical experience of using the standard nor had I time to do my own tests. As you know, there are actualy a number of articles available from SPIE and others handling spectral characterization of scanners. Anybody out there with experience about spectral characterization of scanners and the accuracy achived willing to share his info? Otherwise I am sure to find some time to at least provide the necessary targets and data for tests. With kind regards Wolf Faust -- Wolf Faust Tel: +49-69-5486556 mailto:wf...@wf... Fax: +49-69-95409598 http://www.coloraid.de Mobile: +49-179-6924769 |
From: Graeme G. <gr...@ar...> - 2005-06-24 06:47:12
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Wolf Faust wrote: > 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). For it to work, I would expect it to be constructed using a wide selection of colorants, not combinations of 3. The IEC 61966-8 is meant to be such a chart. The main problem with this approach seems to be that the problem is highly under constrained, and some sort of normalization is needed to produce feasible looking spectral sensitivities. Two recent articles were presented at CIC12; "Spectral Characterization using Continuous functions" by Tomasz J. Cholewo and Steven F. Weed. The preceding article "Emissive chart for image calibration" by Jeffrey M. DiCarlo, Glen Eric Montgomery and Steven W. Trovinger was also avery interesting, different approach. Some comments at the conference were that the IEC chart seemed to be difficult to get hold of (your link provided a contact though.) Graeme Gill. |
From: Gerhard F. <nos...@gm...> - 2005-06-24 00:58:07
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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 |