Hi, Every time I peek over current ICC v4 specification - appendix D always makes me scratch my head with referece to this particular method (simple scaling of D50 illuminated media white -> D50 illuminant). On LCMS mailing list I found a few past discussions touching this subject (though I think [some of] them were related specifically to v2).
But in color.org's faq (the one in pdf - http://www.color.org/faqs.pdf) this particular issue is sensibly(?) explained (with reference to v4 spec, my questions after the quote): ------------- begin question as in pdf: "For reflection prints there is a distinction between three whites: 1) The illuminant 2) The media illuminated by the illuminant 3) The PCS Relative Colorimetric White (D50) The 'wtpt' white point tag defines 2) under the standard D50 illuminant. If the measurement illuminant wasn't actually D50, then it is made as if it was using a chromatic transform, and this is recorded in the 'chad' tag. ie. the 'chad' tag defines how 1) is transformed to D50. For reflective media this is all clear. The transformation of 2) under D50 into 3) is done using a "wrong Von-Kries", which I think is a mistake, but at least it is clear what's going on." and the answer: There are actually four whites. The 4th is -- the media illuminated by D50. Transformation of 2) into 4 is handled by the chromatic adaptation matrix which should be built into the profile transforms and then recorded in the chromatic adaptation 'chad' tag. The final step - conversion from illuminant [D50] relative values to media-relative values [which are the PCS side values of the relative colorimetric RI] is handled by a simple scaling - see equation D3 in Annex D. This may look like "wrong Von-Kries" but should not actually do any chromatic adaptation. It should just shift the colors so that the max white possible is assigned to LAB=100,0,0. This scaling is the difference between ICC media-relative colorimetric [after scaling result of eqn D3] and ICC absolute [result of eqn D2 if computing from measurement data and result of eqns 1, 2, 3 in section 6.3.2 when computing from existing previously scaled media-relative colorimetric values]. There is and should be no chromatic adaptation relationship between ICC media relative colorimetric and ICC absolute colorimetric - it should just be a scaling relationship so the data is either scaled to the media white == max PCS value or scaled to the illuminant white == max PCS value. Viewed in this way perhaps it clarifies why a monitor media white point == illuminant white point in V4. -------------- end Now my questions: 1) Is that conceptually correct approach ? I mean, if we have media white point chromatically adapted to PCS D50 (using e.g. chad data) can we simply scale that m.w.p. to D50 PCS (as per: 'This may look like "wrong Von-Kries" but should not actually do any chromatic adaptation.') ? It always felt kind of "off" to me. But perhaps it's ok after chromatic adaptation. In that faq, it's been actually stressed multiple times to not confuse chromatic adaptation with later scaling which is always done under the same conditions. 2) In case of monitor profiles (and following the faq above - m.w.p. == illuminant, which in turns would imply that relative and absolute colorimetric are de-facto identical as scaling would be a no-op) - what would be the problem with monitor V4 icc ? In one of the past discussions ...: http://sourceforge.net/p/lcms/mailman/message/31985055/ ... it was mentioned that this kind of profile - with chad tag and D50 as white point is not suitable for soft proofing. Why wouldn't it be ? As far as I understand this, we do absolute colorimetric from printer profile (itow what we try to preview) -> PCS and then PCS -> monitor profile. So as long as we're adapted to monitor's white then all should be fine; if we're adpated to other conditions (say D50 + fullscreen preview), then not doing reverse adaptation back to the monitor would also give expected results (or doing different one depending on adaptation conditions, e.g. to D55). We lose direct information about monitor's whitepoint - but we have the chad matrix now so we can always recover it and everything else as needed (though it was not possible in earlier standards due to lack of 'chad' tag). This kind of reasoning was also covered in another point of that faq: -------- begin question as in pdf: "If the user asks for absolute rendering, then a (correctly working application) will reproduce an absolute rendering. If you send it D50, then it produces D50, and against the users adaptation to the presumably D65'ish white point of the monitor, it will look yellow. It's supposed to. If you measure the color though, it should be as close to D50 as it can get, just like you asked for when specifying absolute rendering." answer: This is mixing absolute [illuminant relative] with chromatic adaptation. Asking for absolute colorimetric does NOT mean "do not chromatically adapt to my display" -- it does mean show the colors of the file [e.g., prepared for print viewing in D50] relative to the white of a perfect diffuser viewed under the same illuminant [e.g., D50]. So the white of the file should look less white than a perfect diffuser in the same illuminant if the paper is not a perfect diffuser. However, in either case if the actual monitor is not set up as a D50 monitor then the monitor profile should chromatically adapt the colors to the display illuminant -- i.e., should NOT show a difference due to the difference between D50 and D65 if the monitor is calibrated to D65. Using ICC absolute colorimetric you should get a paper white simulation that is more gray than yellow, as it should be. -------- end Well, hope my questions at least make sense. ------------------------------------------------------------------------------ Download BIRT iHub F-Type - The Free Enterprise-Grade BIRT Server from Actuate! Instantly Supercharge Your Business Reports and Dashboards with Interactivity, Sharing, Native Excel Exports, App Integration & more Get technology previously reserved for billion-dollar corporations, FREE http://pubads.g.doubleclick.net/gampad/clk?id=157005751&iu=/4140/ostg.clktrk _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
