On 02/04/2014 07:47 PM, Graeme Gill wrote: > Elle Stone wrote: > >> Why do some references give that second set of values? Is there a reason >> to prefer x=0.3127, y=0.3291 to the more commonly quoted x=0.3127, y=0.3290? > > I don't have a specific answer, but you will note that the x,y numbers are > rounded to 4 digits, which seems to be for practical calculation reasons, > as well as notions of excessive meaningless precision. Why the profile specifications give values to 4 digits is an interesting question. Rounding or not rounding the source xy white point values to 4 digits does affect the resulting profile.
> > Things that add uncertainty to D65 values: > > Very slight change in the CIE standard observer definition from 1931 to > current standards. > > Whether the D65 spectrum is from the Daylight equation or > the specific D65 distribution (they are not quite the same). > > How the CIE calculation dealt with the 5nm spacing of the > D65 spectrum - ie. whether it computed the integration at 5nm > (which is permitted by the CIE standard), or whether it up samples > to 1nm (which is encouraged by the standard). > > Whether the CIE x,y tables were used instead. > > Whether the Daylight x,y chromaticity coordinate equation was > used instead of the spectral calculation or table values. > > Round any one of these up and you may get a different number. > Different sources give different values for D65, calculated in different ways and presumably for different purposes. What I'm trying to ask is, of all the possible values for D65, which xy values are the right values to use when the purpose is making an sRGB profile? To give a parallel case, the AdobeRGB1998 specifications give the D65 values x=0.3127, y=0.3290. So the right D65 values to use when making an AdobeRGB1998-compatible profile are x=0.3127, y=0.3290. Perusing references available online, I came up with two possibilities: x=0.3127, y=0.3290 and x=0.3127, y=0.3291. http://www.w3.org/Graphics/Color/sRGB gives x=0.3127, y=0.3291 as correct for the "sRGB reference viewing environment". It gives x=0.3127, y=0.3290 as correct for the "Colorimetric definitions and digital encodings". The LCMS API says x=0.3127, y=0.3291. The built-in LCMS sRGB profile uses the CCT white point 6004K, which is x=0.312720535, y=0.329125536. When making an sRGB profile, these three sets of xy values produce three slightly different profiles. Granted, the profiles are very close. Granted in actual practice it's not likely that one would ever make an editing move where which profile was used would make a noticeable difference. Nonetheless, it seems to me that when making an ICC profile, the logical and correct thing to do is to use the source white point given in the profile's specification. So for sRGB, are the correct values x=0.3127, y=0.3290 (colorimetric)? or x=0.3127, y=0.3291 (viewing)? or even some other set of values? As an aside, why is there a difference between colorimetric and viewing D65? Elle ------------------------------------------------------------------------------ Managing the Performance of Cloud-Based Applications Take advantage of what the Cloud has to offer - Avoid Common Pitfalls. Read the Whitepaper. http://pubads.g.doubleclick.net/gampad/clk?id=121051231&iu=/4140/ostg.clktrk _______________________________________________ Lcms-user mailing list Lcms-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/lcms-user
