# So what is the 'Dehaze' actually doing?



## Hoggy

(Can't believe nobody else has asked yet...)
It almost seems to act as a shadows contrast.

But unlike with the other types of contrast, the 'standard basic-contrast' and mid-tone "clarity" contrast, which both seem to stretch/pull-apart the histogram values -- the '+' dehaze (via presets) version seems to compress/pull-in the histogram values toward the shadows.  And the '-' dehaze values seem to stretch them away from the shadows..  Just the opposite of the other contrast types.  ....  Strange.

So does anyone know what is actually happening with it?  How does it work?


And in a possibly related question, would being able to use the adjustment-brush's new black/white settings be able to help haze-wise in any way?
   (Unrelated -- I'm having trouble in understanding exactly what or how the new black/white adjustment brush settings could help with.  Would they be useful in any way?  I don't get it.)


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## Ian.B

Hoggy said:


> (Can't believe nobody else has asked yet...)
> It almost seems to act as a shadows contrast.
> 
> But unlike with the other types of contrast, the 'standard basic-contrast' and mid-tone "clarity" contrast, which both seem to stretch/pull-apart the histogram values -- the '+' dehaze (via presets) version seems to compress/pull-in the histogram values toward the shadows.  And the '-' dehaze values seem to stretch them away from the shadows..  Just the opposite of the other contrast types.  ....  Strange.
> 
> So does anyone know what is actually happening with it?  How does it work?
> 
> 
> And in a possibly related question, would being able to use the adjustment-brush's new black/white settings be able to help haze-wise in any way?
> (Unrelated -- I'm having trouble in understanding exactly *what or how the new black/white adjustment brush settings could help with.*  Would they be useful in any way?  I don't get it.)



I could only guess it's more than the old basic desaturate tool. I don't have CC/6 but it would be good to be able edit the b/w part when doing part B+W part colour editing. Naturally I cannot help with the Dehaze tool; but I have done similar with aerial photos by adding clarity,contrast and yellow [mainly], so again only guessing the dehaze tool would be something similar although  more refined and easy repeatable


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## Johan Elzenga

The Blacks and the Whites sliders have nothing to do with B&W photography. They are sliders that adjust the far ends of the histogram, to set the black point and the white point. Having that in a brush means you can change this locally, which can be useful at times.

Dehaze adjusts the contrast, but in a kind of 'intelligent' way so that haze in the background gets removed, without increasing the contrast in clear areas in the foreground.


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## davidedric

There have been some pointers from Adobe.  Sorry don't have the reference right now, but it analyses the image to understand what the haze is, then algorithmically attempts to derive what the scene would look like without it.   Incidentally, it uses the whole images to do this, so it is highly unlikely ever to appear in local adjustments  (according to Jeff Schewe).   There are various threads on Luminous Landscapes and CPR.

Dave


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## Johan Elzenga

davidedric said:


> There have been some pointers from Adobe.  Sorry don't have the reference right now, but it analyses the image to understand what the haze is, then algorithmically attempts to derive what the scene would look like without it.   Incidentally, it uses the whole images to do this, so it is highly unlikely ever to appear in local adjustments  (according to Jeff Schewe).   There are various threads on Luminous Landscapes and CPR.



You can still apply it locally, but then you have to use Photoshop. In Photoshop you can use ACR (and so Dehaze) as a smart filter, which means you'll have a mask to apply that filter only locally.


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## Conrad Chavez

Hoggy said:


> the '+' dehaze (via presets) version seems to compress/pull-in the histogram values toward the shadows. And the '-' dehaze values seem to stretch them away from the shadows.. Just the opposite of the other contrast types...Strange...So does anyone know what is actually happening with it?  How does it work?


Julianne Kost wrote up an Adobe explanation of Dehaze in a blog post:
Dehaze in Camera Raw 9.1 for Photoshop CC and Lightroom CC



> The Dehaze technology is based on a physical model of how light is transmitted, and it tries to estimate light that is lost due to absorption and scattering through the atmosphere.



Looking at the histogram alone won't tell you how Dehaze works, because it isn't simply redistributing the histogram like a Contrast or Tone Curve adjustment does. Dehaze is more like Shadows, Highlights, and Clarity in that the changes it makes depend on what tones are near other tones in an image (it is somewhat content-aware). But Dehaze goes beyond that in using its physical model of haze to try and recognize 3D distances in the image and making larger corrections to what it thinks is more distant haze. So different image content creates different results, like the other three corrections I grouped it with. Compare that to Contrast or Tone Curve where the type of image doesn't matter; the same curve will always affect the image numbers and histogram in the same way.

The reason you see tones move toward the shadows is probably a consequence of trying to reverse the effects of haze. Hazy areas are usually lighter than the rest of the image, so dehazing it will involve darkening it. Along with restoring its contrast and color balance.


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## Hoggy

Ah, ok..  It's bit less hazy now.

That kind of falls in line with my original thinking that it was pulling more information in where info wasn't fully/directly there..  And why I often have to raise the blacks, shadows, or exposure to try and compensate.  Although I'm finding it can be a delicate balance, because raising those will also tend to bring some of the haze back in.

It may not be a miracle 'slider', but it's still amazing how the engineers at Adobe can think all these things up and have them work pretty darn well much of the time.


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## Johan Elzenga

Hoggy said:


> Ah, ok..  It's bit less hazy now.
> 
> That kind of falls in line with my original thinking that it was pulling more information in where info wasn't fully/directly there..  And why I often have to raise the blacks, shadows, or exposure to try and compensate.  Although I'm finding it can be a delicate balance, because raising those will also tend to bring some of the haze back in.
> 
> It may not be a miracle 'slider', but it's still amazing how the engineers at Adobe can think all these things up and have them work pretty darn well much of the time.



Not everything in Lightroom is proprietary Adobe technology, far from it. They buy a lot of stuff too. I think dehaze was also not developed by Adobe: http://www-scf.usc.edu/~qiyu/Papers/ivcnz2011.pdf


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