Thanks for the reply, @JenSCDC, but I’m afraid you’ve lost me. How does one “subtract with an offset of 127”?
I’m no expert in this stuff, so I could do with an “Idiot’s Guide”. 
Thanks again. 
Sorry, should have been more verbose.
Do “apply image”, mode = subtract, and there will be an entry for offset.
Ok, let’s see if I’ve got this:-
Apply the dehazed image to the non-dehazed image with blend mode = subtract, and offset = 127
Apply the resulting image to the non-dehazed image using blend mode = Linear Dodge (Add)?
Is this correct? If so the results don’t look like I’d expected. I’m probably doing something wrong, so any advice would be appreciated. Thanks.
Sorry, I simply don’t have the time to demonstrate but what I will say is that Linear Dodge is definitely wrong because it doesn’t subtract or add all of the pixels in the same way. What you need is straightforward subtraction and addition.
1 Normal - Dehazed = Difference
2 Normal + Difference = Normal with more haze
Not sure if step 1 should be Dehazed - Normal instead. It depends on how PS orders its layers.
A long time ago, I liked adding some “haze” by simply applying a gaussian blur with large radius to a duplicate layer, and then play with blend mode and transparency.
@porchard could you post an image we could play with?
Thanks @sguyader - that seems to get fairly close to what I was looking for. I tried this method some time ago, but I couldn’t have been using a large enough radius, because the results weren’t as good. The larger radius seems to to do a decent job. I still think that a negative dehaze would be a good feature to have in RT, though!
Thanks to you, and to all the others who replied. 
Hello @porchard
If you are really interested in having this feature considered for inclusion in some future RawTherapee versions it might be helpful to add it as an “enhancement” in the Github web-page:
In short, you need to create a new “issue” in this web-page and add your request. Needless to say, you need to have a personal account before adding this proposal on Github.
Only having this request on this forum might be less effective for your proposal in the long run…
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I decided to verify my instructions above. I didn’t use GIMP since you don’t use it, only G’MIC for a strict subtraction and addition. Raw file was processed using RT with neutral profile and some lens corrections and raw tab tweaks.
No dehaze
More haze = ( no dehaze - dehazed ) + no haze
More haze = ( no dehaze - dehazed_luminance ) + no haze
Hint You can control the strength of the haze by multiplying the difference with a factor; i.e., controlling opacity or adding new layers to stack on top.
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I’m in front of PS right now. Here are the steps to “reverse dehaze”.
-
Take the dehazed image. “Apply image” with original image, mode = subtract, offset = 127. Let’s call the result “diff” because it’s the offset difference between the two images.
-
Take the original (or copy thereof). Do Apply Image, using diff as the source, mode = subtract, offset = 127, opacity as desired.
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Many thanks again @JenSCDC. I’ve just tried this again, and it’s working well.
I really appreciate the time and trouble to which you - and the other contributors - have gone, to help with my request. Thanks, all!
so …,might we see an implementation of this in RT sometime 
?
Don’t know if you are aware of it, but the local adjustment dehaze does allow negative values. I’ve been using it for a while now.
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thanks for the heads up; i didn’t know. I was however hoping to be able to do it overall
even its named “local adjustments”, you can apply it overall. There are three types of spots, one of them is called “full image”
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I must admit I was very glad when local adjustments first appeared in RT …then proceeded to never actually use them so my experience with them is as is obvious quite minimal. Thanks for the feedback, I will check it out
kudos to the RT team. I cannot begin to understand the beauty of code you guys write; but I do appreciate that it exists. thank you!
Morning, @stefan.chirila,
Download the image of the particularly noisy girl and work through this wonderful tutorial:
https://rawpedia.rawtherapee.com/Local_Adjustments#Using_the_Denoise_module
Have fun!
Claes in Lund, Sweden
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thank you sir! will do
Thank you for these positive reviews. However, improvements to Local adjustments are always possible, for example it should be possible to associate ‘black Ev’ with Dehaze (even if you can do it by opening LA Tone equalizer before).
I remain convinced (of course you can say that I have a biased opinion) that LA with full-image is better than the “main” versions (of course when possible). Using scope=100, from my point of view must be a very marginal use. What needs to be improved, but very difficult for me, is the GUI to facilitate access to these features.
jacques
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