On 9/21/13, Michael Henning <drawoc@xxxxxxxxxxxxxxxxxx> wrote: > Both babl's RGB format and Y format are currently defined with a white > point of D65. Because of this, I believe the code's current luminance > values are correct. > The whitepoint of an ICC profile might have a D65 white point without a chad tag, if it's a V2 profile. Or it might have a D50 white point with a chad (chromatic adaptation) tag if it's a V4 profile. Either way, the XYZ values are the adapted values, NOT the unadapted values. In other words "D65" as the white point for the V2 profiles doesn't mean "use the unadapted primaries", which means the unadapted "Y" values are wrong. So are the unadapted X and Z values. > Out of curiosity, how did you determine the Y values from the code in > gegl/operations/external/lcms-from-profile.c ? I put in a couple additional lines of code in the gegl file telling lcms to save the profile to disk. Then I recompiled gegl, retrieved the profile, and used iccToXml to examine the primaries. It's a bog standard sRGB V4 lcms-generated profile. If you would like to look at it, it's attached to this email. > If you're somehow > dumping the icc profile and analyzing it, then it would make sense > that those are relative to D50 because icc mandates D50. > That's kinda the point. If a person opens an sRGB image file with Gimp, that image's profile has the adapted primaries, not the unadapted primaries. > In other words, I think that the whitepoints are all currently > correct, because the constants in code are for converting between two > color spaces with D65, and the icc profile is relative to D50. > But that means the code is assuming a very odd color space, not the same sRGB as is used by the vast majority of all the sRGB images ever created. This includes all images to which Gimp assigns the built-in sRGB profile created by lcms. V2 or V4, lcms1 or lcms2, the sRGB primaries are the adapted primaries, which means the adapted R,G, and B "Y" values. So how can it be right that the babl/gegl/gimp code uses unadapted primaries for image processing, while operating on images that are created using the adapted primaries? If you assign an sRGB profile with unadapted primaries to an sRGB image that was created with a proper sRGB image, that image will have a false blue color cast. And if you use the unadapted Y values for Luminance desaturating in a linear gamma color space, the resulting image will be slightly wrong. > (As a side note, I believe some of the grayscale conversions are > currently very broken for other reasons. They operate in a gamma > space, but use constants that are meant for linear color spaces. It's true that Luminance conversions to black and white in a nonlinear color space makes much more wrong results than merely using the unadapted Y values. But the Y values for the Luminance desaturate code, for example, are still wrong. They should be the adapted values. Elle _______________________________________________ gimp-developer-list mailing list List address: gimp-developer-list@xxxxxxxxx List membership: https://mail.gnome.org/mailman/listinfo/gimp-developer-list
