Screen captures mentioned in text below.

Le 19-01-26 à 15 h 03, Normand Fortier a écrit :
So far, my understanding is this.

DT works in LAB color space. Most modules work in that space ("The local color pickers run in the color space of the individual module, which is usually L"; see also http://www.darktable.org/usermanual/en/color_management.html). The main histogram and the global color picker, at least, display rgb values. One would think that those values are converted from LAB values, but instead, those modules obtain RGB values after converting to the monitor color space.

If I am correct, it means DT modules do not all work on the same image: most work in LAB color space but some work on the image after conversion to the (RGB) monitor color space. See for example the two files appended: hist_mon.png shows the main histogram and the tone curve with my monitor profile selected for display, while hist_srgb.png shows the same information with srgb profile selected for display: the histogram in the tone curve looks identical, while the main histogram visibly differs. Note that the tone curve histogram is set to display rgb.

To me, this inconsistent behaviour is undesirable: I would expect all modules to work on the same underlying image.

The latter is how at least some other programs work.

- Lightroom:
"Lightroom uses a wide gamut RGB space similar to ProPhoto RGB to do all the image calculations, and the histogram and RGB percentage readouts are based on this native Lightroom RGB space."
http://www.adobepress.com/articles/article.asp?p=1930486

- RawTherapee:
Uses ProPhoto as default working profile (https://rawpedia.rawtherapee.com/Color_Management#Working_Profile). The UI indicates "If enabled, the working profile is used for rendering the main histogram and the Navigator panel, otherwise the gamma-corrected output profile is used". If I open my original png image with patches, the display of pixel rgb values (in the Navigator module) correspond to values written into the patch.

My impression is that it would be possible for modules displaying rgb values numerically or graphically to obtain such values through a conversion of the LAB values at the appropriate step of the pipeline -- this is what the global color picker does when it offers to display LAB and RGB values of a given area.

Can anyone provide feedback as to whether the above is correct?

If so, I could write a bug report, although I am not sure of the title or what should be requested. Note that there was a discussion around a request for rgb curves, that could be relevant:
https://redmine.darktable.org/issues/9559
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