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

[Elle Stone <l elle stone gmail com>]

On 9/22/13, Michael Henning <drawoc darkrefraction com> wrote:

> > > Isn't the second (adapted) transform going to give us a D50 Y instead
> > > of a D65 Y?
> >
> > Yes, it will, and that's precisely what you want if you want to
> > correctly calculate an sRGB image's Luminance values from its RGB
> > values.
>
> The main usage for this transform (D65 RGB -> D65 Y, with the
> unadapted values) is to convert color images to grayscale. It seems to
> me that having a D65 grayscale image is correct, especially because
> when converting back to RGB D65 we simply copy the Y value into each
> of the components.

All sRGB images are encoded using D50-adapted primaries. To convert to
black and white using Luminance you need to use the "Y" values from
the actual sRGB profile that's actually used in actual sRGB images.
You shouldn't use the unadapted D65 Y values that are in the
babl/gegl/gimp code.

I can send you a test image and some profiles if you'd like to
demonstrate for yourself that the D50-adapted Y values are the right Y
values to use for converting an sRGB image to black and white.


> > As an aside, you can't assume that any of today's LCD monitors are
> > calibrated to match sRGB or even to have a D65 white point
> Nobody is claiming that our conversions are
> exact for an profileless, uncalibrated monitor.

I wasn't talking about a profileless, uncalibrated monitor. Profiling
and calibrating a monitor are completely independent actions.
Personally, I profile my monitor, but I don't calibrate it other than
setting how bright it is, because profiling an LCD monitor in its
uncalibrated, "native" state gives a larger color gamut and smoother
grayscale.


> Every monitor is different. D65 with the sRGB primaries is the same as
> ITU‐R BT.709‐5 (HDTV), which I think is a reasonable approximation for
> certain calculations. Nobody is claiming that our conversions are
> exact for an profileless, uncalibrated monitor.

I'm actually claiming that as far as converting an sRGB image to a
technically correct, luminance-based black and white image goes, your
unadapted D65 Y values are not the right values regardless of whether
a person is using a calibrated and/or profiled monitor or not. They
are the wrong values if it's a CRT or an LCD or a TV set from the
1950s or a super hi-def state-of-the-art TV from today. The technology
used to *display* the resulting image has nothing at all to do with
the right values for converting that image to black and white in a ICC
profile-based, color-managed workflow. The proper Y values for
converting an sRGB image to black and white using Luminance are the
D50-adapted Y values from the D50-adapted sRGB profile that's actually
embedded in the sRGB image.




-- 
Elle Stone
http://ninedegreesbelow.com

Just because it's a standard, doesn't mean it's right.