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

2016-04-28 Thread Elle Stone 

On 04/28/2016 06:00 AM, Tobias Ellinghaus wrote:

Am Mittwoch, 27. April 2016, 18:18:38 schrieb Elle Stone:



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.


Yes, what would you suggest? Using a different encoding for the TRC?


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.


So is darktable's internal sRGB profile (the one you can select in "input color
profile"). We just export with a V2 to make it easy to actually look at the
image. And I don't expect people to export float images in sRGB.



I don't have any suggestions. You pinpoint the problem exactly:

* A V2 version of sRGB is better for images that are uploaded to the 
web, which I think is what you mean by "make it easy to actually look at 
the image". But if you mean something else, please clarify.


* Exporting floating point images that aren't encoded linearly seems 
like an odd thing to do, and especially so if the image contains out of 
gamut channel values.


The only solution to the problem that I can think of is user education:

1. For output of a finished "ready to display" image directly from 
darktable, export the image using darktable's V2 sRGB profile.


2. For exporting to disk for further editing at floating point 
precision, export a floating point linear gamma image. And deal with any 
out of gamut (and especially negative) channel values before using 
editing operations that involve multiplying by a non-gray color, and 
before trying to edit using nonlinearized RGB. This is a bit difficult 
to keep track of in GIMP as some editing operations do work by default 
on RGB encoded using the sRGB TRC, even if the image was opened in GIMP 
as a linear gamma image.


People wanting to export images for printing on wider gamut photographic 
printers of course aren't covered by these two options. They probably 
would want to export using a wider gamut color space.


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

2016-04-28 Thread Tobias Ellinghaus 
Am Mittwoch, 27. April 2016, 18:18:38 schrieb Elle Stone:
> 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.

Good, thanks for checking.

> 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.

Yes, what would you suggest? Using a different encoding for the TRC?

> 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.

So is darktable's internal sRGB profile (the one you can select in "input color 
profile"). We just export with a V2 to make it easy to actually look at the 
image. And I don't expect people to export float images in sRGB.

> 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).

That.

> Best,
> Elle

Tobias

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

2016-04-28 Thread johannes hanika 
On Thu, Apr 28, 2016 at 10:18 AM, Elle Stone
 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
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Re: [Gimp-developer] darktable linear Rec709 doesn't match GIMP linear sRGB

2016-04-27 Thread Tobias Ellinghaus 
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]?

[...]

> Best,
> Elle

Tobias

[0] https://houz.org/tmp/darktable_profiles_2.0.4.tgz


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