darktable Exposure: clipping threshold

The Exposure module has a parameter “clipping threshold”, which as documented in the dt manual:

clipping threshold

darktable can calculate correct black level and exposure values for your image based on the content of a rectangular area. The adjustment slider lets you define what percentage of bright values are to be clipped out in the calculation. Pressing the icon starts the calculation and lets you draw a rectangular area of your choice using your mouse. This feature is only available in “manual” mode.

You can indeed draw different rectangles on the image to affect the calculation, and use the clipping threshold slider to alter the result. What you cannot do is turn it off, unless you reset the Exposure module to its defaults or throw away the results of the Exposure module by clicking the previous item in your history stack. You can click the color picker, and it toggles between highlighted and non-highlighted states, but it has not effect on the result…once it’s on it’s on.

Is it the intended behavior of the tool that once it’s on it’s irrevocably on?


What would be your expected behaviour? If you change your mind and don’t want it, isn’t that where you would just reset the tool?

To be clear, by reset the tool, do you mean reset the clipping threshold tool, or the Exposure module?

Perhaps a concrete example would be useful. When I first access the Exposure module for a given photo, I see this (1):
Screenshot from 2020-09-14 23-50-41
Then I adjust the exposure up significantly, since the shot was underexposed (2):
Screenshot from 2020-09-14 23-51-24
and then I click the color-picker for clipping threshold (without altering the rectangle that covers almost the entire image), resulting in this (3):
Screenshot from 2020-09-14 23-52-12
Finally, if I click the color-picker again, which causes the color-picker to dim, the image remains unchanged.

Clearly, the act of activating the clipping threshold tool can cause a large difference to the exposure value. So what I’m getting at is this: is it intended that if I click the color-picker a second time to toggle it off as described above, I stay at (3) or go back to (2)? Either answer is OK. I’m not asking for a design change, just clarity.

I see what you mean. It obviously is not acting as an on/off toggle to the previous state. Once you click the eye dropper, the exposure changes “irreversibly” (although it can be reversed using ctrl+z).

I would imagine this is by design because it’s not writing your previous manual adjustment to history. I also think this is standard behaviour for all color pickers within modules. At least, from memory, it’s the same behaviour with color balance, filmic, negadoctor, etc. Once you click the eye dropper, the image changes and toggling it off does not take you back to the previous state.

Maybe the simplest thing to do is:

  • change the word “toggle” in the tool tip to something else; and
  • disable the on->off transition for the color pickers that cannot be turned off within the module.

Well, you use a color picker to set one or more values. Alternatively, you use sliders slider to set these values.
Why would you want the values to be reset when you have finished with the colour picker?
That’s about the same as asking for a slider to revert to the previous value when it loses input focus.

In other words, a color picker is a tool to set parameter values within a module, exactly like the sliders set parameter values. The main difference is that the sliders also display the current values for the parameter they represent.

That’s impossible with a picker, so it has its current value translated to the relevant module parameters, which are then used to update the sliders. The picker doesn’t hold any values in itself, so switching it on and switching it off have no effect, the action occurs when you pick a point or zone

And as @europlatus said, you can revert the changes from the picker through xtrl+z. Or just use the sliders (they are not magically blocked at the values the picker decided)

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I agree that would be ridiculous. But, of course, that’s not at all what I said.

Ctrl+z does work. Clicking the color picker again to toggle it, as implied by the wording of its tool tip (“percentage of bright values clipped out, toggle color picker to activate”), does not. At the end of the day, it’s a simple matter of the tool tip needing to be reworded.