Re: [Gimp-developer] darktable linear Rec709 doesn't match GIMP linear sRGB

[johannes hanika <hanatos gmail com>]

On Thu, Apr 28, 2016 at 10:18 AM, Elle Stone
<ellestone ninedegreesbelow com> wrote:
> On 04/27/2016 05:11 PM, Tobias Ellinghaus wrote:
> > Am Mittwoch, 27. April 2016, 10:55:14 schrieb Elle Stone:
> > > The darktable plug-in linear Rec 709 chromaticities don't match the GIMP
> > > built-in sRGB chromaticities.
> >
> >
> > Thanks for bringing that up, I changed the sRGB profile a while ago but
> > somehow
> > forgot the others. I pushed a change to create profiles that match your
> > sample
> > set. Could you please have a look at them [0]?
> >
> > [0] https://houz.org/tmp/darktable_profiles_2.0.4.tgz
>
> Using iccToXml to check the colorants and chad, the colorants and chad are
> perfect for all four profiles.
>
> The darktable sRGB profile is a "V2 according to V4" profile with a point
> TRC. So this profile clips out of gamut channel values in floating point
> images.
>
> GIMP's internal sRGB profile is a V4 profile with a parametric TRC, so
> GIMP's sRGB profile doesn't clip out of gamut channel values in floating
> point images.
>
> I don't know what the actual rationale is for darktable to make a V2 version
> of sRGB. But at this point in time a V2 profile is the right choice for
> output to the web, for maximum compatibility with the various browsers (even
> Firefox doesn't read V4 profiles unless the user changes the gfx settings in
> about.config).

yeah, the rationale was exactly the compatibility issue. fwiw, the
tonecurve code in darktable will not clip any values, but extrapolate
the tonecurves using a fitted gamma extension (that holds for all
curves, some are clipped at 0, others are extended both ways).

also, darktable will happily output negative floating point values
(out of gamut ones) and store them in pfm (making the exact choice of
output colour space somewhat irrelevant). i would need to check the
exr code about this.

it's probably a good idea to double check what kind of data makes it
into gimp in all those cases (clipped tonecurves, out of gamut
negative values).

-jo