I am not doing any scaling at all. I am leaving all of the math up to Little CMS, which is my purpose for using it.
I know from experience that a 1.5 delta E is subtle. The difference in RGB values (displayed RGB versus lcms RGB calculated from measured Lab) that I am seeing are 6+ delta E off, which is in no way subtle. I have to assume that either I am doing something wrong, that Little CMS is seriously flawed, or that I am completely misunderstanding what a color management system does. For instance, if I display a swatch on my LCD with an RGB value of 85, 85, 85 and measure it with a spectrophotometer, I get an Lab value of 42, 0, -0.6. If I ask lcms, given my monitor's recent profile and the default Lab profile, to transform this Lab value to RGB, I expect to get 85, 85, 85, or something close to it. I don't. I get 96, 98, 99, which is off by 13%, 15%, and 16%. If it is something that I am doing wrong, I would be grateful to find out what it is so I can fix it, because having this much error in the results is unacceptable. Thanks for your reply, Mike. Kai-Uwe Behrmann wrote: > Am 08.12.07, 18:36 -0600 schrieb Michael A. Litscher: > > >> The short version: >> >> The issue I am having is that if I display an RGB value on the screen >> and measure it's Lab value, I should be able to feed that Lab value into >> LCMS, along with the monitor's profile, and get the original RGB value, >> or something very close to it visually. However, the RGB value I get >> back from LCMS is off, significantly, and lighter in almost every case. >> > > Does you proper scaling for luminance (lumi tag ~CIE*L 116)? > > >> The input profile I'm using is the built-in Lab4 profile. The output >> profile I'm using is created from the monitor's profile file. The >> transform I am doing is from Lab double to RGB 8, Absolute Colorimetrix >> Intent, with a No Precalc flag. >> > > Seems ok. > The profile looks internally ok. ICC Examin reports a DE of ~1.3. > > The spreadsheet shows not too much, as it has only results not the math. > > > regards > Kai-Uwe Behrmann > -- > developing for colour management > www.behrmann.name + www.oyranos.org > > ------------------------------------------------------------------------- SF.Net email is sponsored by: Check out the new SourceForge.net Marketplace. It's the best place to buy or sell services for just about anything Open Source. http://sourceforge.net/services/buy/index.php _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
