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

[Elle Stone <ellestone ninedegreesbelow com>]

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