Re: [Gimp-developer] Questions about the histogram

On 08/12/2019 10:07 AM, Elle Stone wrote:
On approximately 08/11/2019, Steve Pricks wrote:
(text below copied from Nabble)
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).

On 08/11/2019 04:34 PM, Elle Stone wrote:
I'll try to continue responding to your post tomorrow or hopefully
later today.

Continuing on where I left off,

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.

Lightroom is somewhat similar to GIMP, in that some (all?) Lightroom operations are done "under the hood" in linear RGB. But apparently Lightroom "lies" to you about the TRC encoding, and never let's you see the linearly encoded histogram, I'm guessing because their intended audience is used to using perceptual histograms. I think Lightroom is pretty funny given that for years and years PhotoShop users in various forums (eg Luminous Landscape) ridiculed anyone editing in linear RGB color spaces.

But seeing the actual linear histograms is sometimes helpful and necessary, for example if you want a physically meaningful "ev/stop-based" presentation at the information captured by your digital camera. Take a look at software like the old Rawnalyze ( - closed source Windows software, runs under WINE - an example screenshot is in Section A of this article:

Also, if you have access to Mac, or don't mind reading through the documentation, take a look at RPP's histograms (,

Also consider dealing with HDR images. I don't mean tone-mapped images, I mean actual linearly encoded HDR images, which today's camera raw files many times actually already are HDR images after an appropriate middle gray point is set.

GIMP's floating point processing already can be used for HDR images, and histograms for such images make no sense at all if presented (or manipulated) using perceptually uniform RGB.

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?

For Krita and PhotoShop, and also my "CCE" version of GIMP, after opening an image in any RGB color space, the histogram always and only reflects the color space's actual TRC.

So for these softwares, if the embedded ICC profile has a linear TRC (gamma=1.0), then you get a linear histogram. And if the ICC profile has the sRGB TRC, you get the sRGB-encoded histogram (almost perceptually uniform). And if the ICC profile has the gamma=1.8 TRC, well, you get the point.

As an exercise to understanding how GIMP histogram buttons work:

Open an image in the regular sRGB color space in my CCE version of GIMP or in Krita or PhotoShop. Look at the histogram - it will match GIMP's perceptual histogram. Then convert the image to one of the linear gamma versions of sRGB included in my github ICC profile collection (I know you already have the link to these profiles), and watch the histogram change to match GIMP's linear histogram.

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.

The aforementioned user groups aren't using GIMP, and so what works for the aforementioned user groups is only tangentially relevant to what will work for GIMP users.

GIMP histograms need improvement in two very different areas:

1. Ability to zoom in on a portion of the histogram to more precisely view and set points for manipulating, especially important for shadow tones. For a possible direction on solving this problem see these links which show histogram interfaces with the option to zoom in: - Alexandre, as an aside and if you are reading this, it looks like the old PI did allow Curves manipulation of LCh hue and Chroma - look under "real time preview" at the bottom of the page.

The above links are documentation for a version of PixInsight that's no longer available. But the current version of PI runs on Windows and Linux and can be downloaded in trial mode - I think you need to request a key - if anyone wants to take a closer look at the Histogram (really Levels) and Curves interfaces.

2. How to deal with RGB channel values that exceed 1.0f when working with HDR images. For which the appropriate places to look for existing implementations of the histogram would be softwares that already allow to work with HDR images, such as HDR Luminance. Sliding windows seems to be the usual way to deal with this sort of data. But maybe a logorithmic scale could also be used. Or perhaps "slide the data" using stop-based exposure adjustments to bring various portions of the data into view.

Also take a close look at how RawDigger handles histograms:

On 08/12/2019 06:18 PM, Elle Stone wrote:
I used PhotoShop extensively in the past, and I quite much prefer using GIMP, having switched entirely over from PhotoShop to GIMP five or six years ago, long before that horrible CC version was released.

Sorry, that wasn't written very clearly. The last version of PS I used before switching to GIMP was released long before CC came out. But CC itself came out quite a few years ago.

Best regards,

Color management and free/libre photography

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