> Objective: > I want to get a fair estimation of the Lab colours from the ICC profile, > when using relative colorimetric conversion.
Hi Auke, do you mean, you want to estimate those L*a*b* values, which you can _measure_ on the printed paper, if you perform a relative colorimetric transformation from e.g. Adobe RGB to printer RGB? This would corespond to a proofing transformation Adobe RGB --relative-colorimetric--> printer RGB --ICC-absolute--> *Lab Example: Q: Which CIELAB color can I expect to measure on the printed paper, if I convert AdobeRGB=[150,160,170] with relative colorimetric intent to printer RGB? A: $ icctrans -c0 -t1 -i AdobeRGB1998.icc -o *Lab -p printer.icc -m3 little cms ColorSpace conversion calculator - v1.8 Enter values, 'q' to quit R (0..255)? 150 G (0..255)? 160 B (0..255)? 170 L*=59.5236 a*=-3.4766 b*=-12.1094 Is this what you want? > c. I don't *wan*t to use absolute colorimetry as it changes the > RGB values too much. For instance white (Lab 100,0,0) is trnaformed > to 229,246,255. > If I print this, I don't get paper white anymore, yet my paper is > becoming even more blue :( That's pretty strange, IMO rather the opposite should happen. If PCS white (D50) is transformed with ICC-absolute intent to the printer color space, then I would rather expect that the printer puts down a bit of yellow ink to make the blue-ish paper more neutral. Or do you possibly talk about an ICC-absolute conversion from AdobeRGB (or sRGB) to the printer color space? This will indeed produce a _strong_ blue-cast, because AdobeRGB and sRGB have a D65 whitepoint. Please notice, Marti and I are just talking about about an ICC-absolute transform from the _profile connection space_ (PCS) to the printer color space, and _NOT_ about an ICC-asolute transform from a source RGB space (e.g. AdobeRGB) to printer RGB space! Btw, I'm still not sure: Do you use the generic profile supplied by the manufacturer, or did you build your own profile? I think you said, that you're using ProfileMaker? > Some thoughts: > The ICC profile *knows* about the actual colour <> RGB > relation (it is the major function of the profile). > Therefore, I would think, it should be able to specify, > approximately, using its 3D LUT, what the Lab values will > be when I print using a particular RGB combination. Right, that's exactly what the ICC-absolute intent is supposed to do. If you transform printer RGB values with ICC-absolute intent to CIELAB, then you are supposed to obtain the CIELAB values _as you can measure_ them on the printed paper. Example: $ icctrans -c0 -t3 -i printer.icc -o *Lab little cms ColorSpace conversion calculator - v1.8 Enter values, 'q' to quit R (0..255)? 255 G (0..255)? 255 B (0..255)? 255 L*=91.7417 a*=0.3047 b*=-6.5195 where the result [91.7, 0.3, -6.5] is approximately the measured paper color. Btw, all what I said, applies under the ancillary conditions that - you built the profile for D50 illuminant - you did not create the profile with FWA compensation - and that the measured L*a*b* values are D50-based L*a*b* values As Marti said, first try to compare the results of "icctrans -c0 -t3 -i printer.icc -o *Lab" for a couple of printer RGB values (e.g. 100 patches) with actual measurements. If you achieve an average error of <= 3dE for say 100 patches, using a custom profile, then IMO you can be pretty contented, for a consumer printer. Don't expect accurate results from a generic, manufacturer supplied profile (they often have a large error, nevertheless the perceptual intent can still give reasonably pleasing results). Also don't overestimate the capabilities of consumer inkjet printers. In his paper "To Develop a Universal Gamut Mapping Algorithm", Jan Morivic has also briefly evaluated the repeatability, spatial uniformity and temporal stability of a DeskJet 850C. My conclusion of his evaulation is, that depending on the used consumables, an average error of up to 3dE, and max. error of up to 10dE can be easily explained by the flaws of the printer. Another conclusion is, always grant 24-48 hours (!) of drying time after printing a test chart, before making measurements! Regards, Gerhard -- NEU +++ DSL Komplett von GMX +++ http://www.gmx.net/de/go/dsl GMX DSL-Netzanschluss + Tarif zum superg�nstigen Komplett-Preis! ------------------------------------------------------- SF email is sponsored by - The IT Product Guide Read honest & candid reviews on hundreds of IT Products from real users. Discover which products truly live up to the hype. Start reading now. http://productguide.itmanagersjournal.com/ _______________________________________________ Lcms-user mailing list [EMAIL PROTECTED] https://lists.sourceforge.net/lists/listinfo/lcms-user
