Re: [Gimp-developer] Adapted and unadapted sRGB luminance values

[Michael Henning <drawoc darkrefraction com>]

Those conversion factors are attempting to get a D65 Y from D65 RGB.

So, we currently do a conversion like this, using unadapted values:
D65 Y = 0.212656 * D65 Red + 0.715158 * D65 Green + 0.072186 * D65 Blue

And you want us to do this, using the adapted values:
Y = 0.222491 * D65 Red + 0.716888 * D65 Green + 0.060621 * D65 Blue

Isn't the second (adapted) transform going to give us a D50 Y instead
of a D65 Y?

On Sat, Sep 21, 2013 at 8:21 PM, Elle Stone <l elle stone gmail com> wrote:
> On 9/21/13, Michael Henning <drawoc darkrefraction com> 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