What *exactly* is Clarity?

It has a sharpening function hence the ringing effect if pushed to hard.

It also crushes the blue end to much, pushing into the purple end of the spectrum.

thats one of the reasons I use it in LAB mode

actually I mean with *exactly* the concrete algorithm.

Over 10 years ago a company named Pixmantec created a nice piece of software known as RawShooter that was offered in “Essentials” and “Professional” versions. This product offered two “unique” tools – Clarity and Vibrance. A few years later, Adobe purchased Pixmantec and introduced Clarity and Vibrance into Adobe Camera Raw and Lightroom (however the values/numbers in Adobe’s implementation were not exactly the same or interchangeable with the now defunct RawShooter product).

In a way that I can programm it as a photoshop plugin or standalone image processor etc.

ThomasT, this is not known outside of the programmers – for good reason! This is known as Intellectual Property, I.P. or otherwise as a trade secret. You have stated that you would like to “recreate” this effect that Adobe purchased when acquiring the assets of Pixmantec.

As has been stated, the exact code is not known. What is known is the results of using the tool, which can be used to *try* to reverse engineer the results. Many have come “visually very close” using a recipe similar to this:

• Copy image to new layer
• Apply a density based mask, often known as a luminosity mask (however the density could be Lightness, channel mixed etc)
• Curve the luminosity mask into a “U” shape (or an inverted “U” shape depending on your curve display settings), known as a “midtone mask” or “endpoint mask” so that only the midtones are revealed (or using layer blend if sliders to do similar)
• Optionally soften/blur the mask a little bit
• Apply USM to the masked image using a small amount and large radius

P.S. It should also be noted that the results of using the Camera Raw filter on rendered data is quite different when compared to raw camera data.

https://forums.adobe.com/people/Barbara+Ash  wrote Clarity increases contrast, mostly in the midtones.

You beat me to it Barbara, except I was going to say 'local contrast'.  It stretches out the histogram dropping the black point and brightening whites, but tries not to blow highlights.  But if you watch the histogram while dragging the clarity and contrast sliders to the right, the effect is very similar. 

A trick some people miss is negative Clarity, which is a super easy, and very effective tool for cleaning skin blemishes.  It _tries_ not to soften the blacks too much when doing this, but it is best to do it on a copied layer and mask out the eyes, lashes, hair, lips etc.