I've the author of DCamProf an open-source camera profiling tool that can do DCP and ICC profiles. I use LCMS2 for CAT02 and various other color management stuff.
Recently a question came up regarding how calibration in light different from D50 should be handled. Let's use StdA in this example. Currently DCamProf does like this: as it knows the spectra of the illuminants and the test patches (and observer CMF) it can regenerate XYZ reference values from scratch using spectral integration. This means that regardless if you calibrate for StdA or D50 the XYZ reference values will be calculated for D50, as the PCS is in D50. This means that the software makes profiles that try to make colors look like they were shot under D50. In other words an artwork shot under StdA and processed with an StdA profile will (ideally) produce the exact same result as if shot under D50 and processed with a D50 profile. Then someone told me that I should instead in the StdA case make XYZ reference values for StdA and then convert to D50 (like the PCS require) via CAT instead of regenerating from spectra. The reason would be that the CAT takes into account the minor appearance differences that is between D50 and StdA viewing condition so those will be preserved. I know there are such appearance differences in reality (it becomes obvious in more extreme light temperatures) but prior to this my humble understanding was that a CAT, including CAT02 (with average viewing condition in/out), did not intend to model those appearance differences but instead just tries to make a result as equal as possible to "relighting" (ie recalculate our D50 values from spectra in our case), and at the same time make some mathematical tradeoffs to make the transform reversible. That is a CAT result is just an approximation of the relighting result and any differences from actual relighting is not due to it tries to model slight appearance differences, but just simply limits in how precise the CAT can be. That is if my understanding is right it's always better to regenerate from spectra if you have spectral information and only use CAT if you can't. So who is right? /Anders ------------------------------------------------------------------------------ _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
