Re: [Gimp-developer] Questions about the histogram


first of all thank you, Elle, for your - as usual - detailed answer, especially with your health restrictions!

Being an amateur photographer myself I know about the purpose of the histogram for photographers and use it 
all the time as one step among others. I cannot tell much about the needs of the other mentioned user groups, 
such as artists creating art for web and print.

However, I have not seen buttons like these in any other image editing software, neither commercial nor 
free/libre (not even in RT with its technology filled GUI). I wished the developers themselves could tell 
about their intentions. At least I found in commit e09e563 from 07/21/2018:
- change all "linear" properties to "trc", in all config objects like
  for levels and curves, in the histogram, in the widgets. This results
  in some GUI that now has three choices instead of two.
  TODO: we might want to reduce that back to two later.
Ah so, it's in an unfinished state.
To get some input from other relevant software I did some checks:
Lightroom 8.3.1: Article [1] describes how Lightroom handles this in its Develop module: it works with a 
profile of ProPhotoRGB gamut and whitepoint. Internally it computes with a linear TRC, but the histogram in 
the Develop module uses a gamma ~2.21 sRGB TRC (they named it MelissaRGB profile). This sounds similar to 
GIMPs behaviour, but with a greater color space. (I don't want to revive the old discussions here). I ran a 
test by converting images to ProPhotoRGB and clicking the "perceptual" button which shows the histogram using 
the sRGB TRC. It's visually almost equal to LR's histogram in the Develop module and Library module.
Photoshop CC: Histogram view and histogram in Camera Raw Filter look close to the perceptual view.
Darktable 2.6.0: No match at all. It shows almost nothing where the other histograms have massive peaks.
Darktable 2.7.0+1612~g9e212dc9d-dirty: Shows nothing at all.
Krita 4.2.5: Looks like the perceptual view.
RawTherapee 5.6.1: Looks like the perceptual view.
DxO Optics 11: Looks like the perceptual view.
As it seems, the perceptual view is common. Elles lines read as perceptual uniformity has benefits or is 
required - why?

I think one crucial step to finish the work while avoiding further confusion about mysterious buttons is to 
do some user research among the aforementioned user groups and then do decide whether and how these buttons 
should stay in the GUI. Perhaps different histogram views as outlined in [2] could be a solution.

Sad, that the promising space invasion project has stalled. I hope it will not end like the OpenCL efforts.

I checked out Elle's CCE versions of babl, GEGL and GIMP for reference.
To build GEGL with the current ffmpeg library three commits have to be cherry-picked from stock GEGL's master 
Commit 28185ee7 by Franz Brausse, 2018-05-06 13:38,
Commit 02f90ebb by Franz Brausse, 2018-05-06 13:40,
ommit d1a6f855 by Øyvind Kolås, 2018-05-06 15:09.

I hope my lines help somebody to get forward in this field.


[1] R. Griffith, The Truth about Lightroom Colour Management,, retrieved on 08/11/2019.
[2] A. Prokoudine, Work in progress on histogram redesign,,
retrieved on 08/11/2019.

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