I've recorded a screencast of the issue here.

I have a test image that I made so I could see how various modules in Darktable effected the histogram:

enter image description here

I import this image in darkable and switch on shadows and highlights, this gives me a result like the below:

enter image description here

(not much has changed, but the bottom stripes are slightly darker)

If I then increase the exposure, everything works as I would expect (as in, the image gets lighter), until I move over about +1EV, which is when the background of the image starts to darken. By +2EV, the image looks like this:

enter image description here

If I switch off the shadows and highlights module - the image returns to what I would expect (more interestingly, if I switch the shadows and hightlights to bilateral filter as suggested in another thread, it works as expected as well).

My question is:

  • Why does increasing the exposure of this image make it darker?

...and I wouldn't mind some context around

  • Is it possible to switch on 'shadows and highlights' in a neutal state so that it doesn't change the image (like switching on crop or exposure)
  • Is there a repository anywhere of more professionally generated 'see what the histogram looks like' images?

Edit - I upgraded to 3.0.2 this morning and tested again - same result.

  • 1
    Very interesting. I don't understand what's going on, but I did notice the following playing around with your test image: this has something to do with the Gaussian filter. If you switch to "soften with: bilateral filter" the effect disappears, at least in darktable 2.4.2. – Kahovius Jun 16 '20 at 12:18

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