How-to: visually assess colour regions (spatial distribution) in photos

There have been posts (not too frequently, though), when some wanted to check the colours of the image, ignoring lightness. Some LUTs have been posted, but those are limited to brightness = 100%, so are not really compatible with the scene referred workflow.
However, darktable’s development version has a new module, composite, which makes this possible:


  1. create a grey image (or use this):
  2. Import it into darktable
  3. Open it in the darkroom
  4. Enable the composite module.
  5. Drag the same grey image from the filmstrip to the designated area of the composite module:
  6. Save this as a preset:


  1. open any photo, and enable the composite module with the preset created above. Since composite is normally before exposure, it’ll probably be very bright:

    (I do not know why there is a brighter area at the bottom – is that a bug or a feature of the module?)
  2. Adjust the scale parameter so it covers the image completely. Here, I just yanked it all the way up:
  3. Set the blending mode to uniform (circle icon), and blending mode to lightness:

    (Again, the preview showed some weird artefacts when I made the darktable window smaller for this screenshot, so I don’t bombard the forum with 4K screenshots. Bug?)
  4. Drag to the desired place in the pipeline (some place above exposure, usually). Here, I just dragged it to the end:

I hope some will find this useful.


Here is a screenshot showing how filmic desaturates highlights. Top: filmic off; bottom: filmic on, highlights saturation mix: -50%.

Interesting - seems similar to the GIMP’s ‘Decompose’ which can actually show a distribution histogram where the 0-255 level is equivalent to HSL Hue degree value:

Equally, Saturation distribution:

The cool thing is that any of the H,S or L components can be edited and then re-composed.

And the histogram can be messed with to get actual percentile number for any range of interest.

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Yep (I’m aware of those tools in Gimp, have used them), but:

  • in HSL/HSV hue 0° = 360°, which can be problematic when shown on a histogram (related colours may appear at distant ends of the histogram);
  • this shows hue and saturation (= colour) in a single view, simply ignoring brightness (so you don’t have to rely on numbers);
  • the aim was to do it inside darktable;
  • this is not intended an editing tool, just an assessment / visualisation.

I don’t use colour harmonies and such, but maybe people who analyse their images more will find it useful. If not, it wasn’t much effort to do this, so no big loss. The idea just occurred to me and I tested if it works.

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Agreed that, if interest were in a range of reds, two separate selections would be needed in the histogram. Then add the percentiles so obtained

So alternative methods are not welcome?

I find histograms very useful for assessing/visualizing color distribution.

That is not what I meant. I’m sorry for the harsh tone.

Yes, they are useful for statistical purposes. But just like a brightness / lightness histogram is different from the lightness channel in a Gimp decomposition (or from a B&W conversion), the hue / saturation histograms are different from the colour-only view. For example, spatial arrangement is not reported.

I’m not saying it’s a ‘great invention’ (I did not come up with the idea of such a view, in the first place), and all other methods are bad/wrong/obsolete. Use it if you find value in it, ignore it if you don’t.

I guess I was misled by the word “distribution” in the title, sorry.

Thanks for pointing out the confusing wording. I’ve updated the title. Please let me know if I only made it worse. :slight_smile:

Looks good to me, Sir.

Another play in the GIMP, this time sans histogram. Lightness filled with gray; Saturation increased; Hue left as-is; re-composed:


@kofa Thanks for this. How does it compare with using the Colorize module with the following settings?



Oh, I was not aware one could do that. :slight_smile:

This appears to be a rather good way of establishing a raster mask over a specific but diverse color.
Colorize + Preset

Not sure I follow. You basically get this view if you hold the C key on the hue channel when creating a mask so I’m not sure of the advantage of this for masking…

I don’t think you’ll find it in the manual. It was a while ago now, but if I remember rightly, I figured it out after watching a YT video by Joanna Kustra about colour harmonies. She does something similar in Photoshop using a 50% grey layer, so I converted the process to the Colorize module. I think there was a thread about it on here a couple of years ago, and we also managed to come up with something similar using the Tone Curve module:


There are probably differences with all of the various techniques, but they can all be used to visualize hues.

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The colorize solution:

hz channel of a parametric mask (pressing ‘c’ while hovering over the slider – hue only, saturation not reflected in the output):

The composite module:

And the original image:


Wasn’t happy with my last effort. This time, left the GIMP ‘precision’ at 32-bit floating point, Linear and this time decomposed to HSV, instead of HSL, with V filled with medium-light gray and then recomposed.


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