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

[Michael Henning <drawoc darkrefraction com>]

I hope you don't mind; I reordered parts of your email when responding to it.

> > 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.

I haven't really looked at the other cases of the values you pointed
out yet, but none of those are widely used code paths. They should
probably be examined individually.

> D50 has two uses in an ICC profile: the profile illuminant and also
> the profile white point for profiles with D50 white points.

I don't think we should be accounting for the illuminant when
converting to grayscale.

> 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

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.

> So is there some
> other place in the babl/gegl/gimp code where there is an actual
> hard-coded matrix transform between two D65 color spaces? If so, which
> two D65 color spaces?

The YCbCr transform in babl might be wrong (assumes D65, in a case
where we don't want D65), but you already pointed that out, and I
wasn't really talking about that conversion.

> Well, what I really want babl/gegl/gimp to do is stop using sRGB as
> the "one space to rule them all" and instead, if there has to be a
> single internal color space, make it the D50 Identity color space ...

I think the main reason for choosing an sRGB-like space is because of
convenience. Many images are in sRGB, and it's nice to be able to
store them in 8-bits/channel without severe quality loss. People using
other color spaces are more likely to use high bitdepths, where the
profile choice is less critical for image quality.