Has anyone tried to replicate in darktable the effects of black mist filters? How would you go about making highlights glow like that? In my opinion, the most notable difference wrt to the bloom effect is the hue of the glow around the highlights. For me, it’s very difficult to emulate.
(in case you are not familiar with such filters I recommend this video)
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What I tried is to use the bloom module and combine it with a separate instance of color balance rgb to add the hue in the highlights combined with some draw+parametric masks. The results were mostly okay, but all tedious manual adjustments are needed every time (hue and masks). It would be awesome to have something easier.
I would try the diffuse or sharpen module: darktable 4.0 user manual - diffuse or sharpen
If you want the most pain free way… Just buy the filter 
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I’d have a go with the lowpass filter trying differents overlay mode (lighten only) with high radius, high contrast and play with saturation …
If you want the most pain free way… Just buy the filter
I am a fan of Nisi filters, but they are not cheap. Here in the UK the 1/4 stop, circular black mist filter for my standard lens is £95…
From here https://www.liftgammagain.com/forum/index.php?threads/halation-and-gate-weave.13056/
For me, the way to do this effect has been to observe the way in which it happens naturally on film and then emulate that process. I observe that:
1. The halation has the characteristic of flaring, and not of brightening, around bright objects. This is why it is most noticeable when the bright object is surrounded by black (i.e. traffic light at night). When the background gets brighter (i.e. traffic light at day), the halation is less or not visible. The "spillover" effect still happens, only we don't see it (the overexposed silver halide doesn't "know" if the ones next to it are also overexposed).
2. The brightness of an object modulates both the luminance and the width of the halo.
It follows then that:
1. The emulation needs to be processed in scene-linear (gamma 1.0) to represent the flare.
2. Its intensity is dynamic in terms of nits and pixels. That means neither qualifiers, nor edge detection will be useful.
3. It's alright if it affects the whole image, as long as it's proportional to the intensity of the light and the darkness of the surrounding pixels.
Here is my recipe:
1. Convert the image from the working space curve to linear (gamut stays the same).
2. With curves, force all except for the super bright spots (the "halo emitters") to 0. Being in linear-light, the curve has to be quite aggressive because the highlights are way outside the usual 0.0-1.0 range (see image).
3. Give it the orange tint.
4. Blur. The blur amount is the maximum flare width.
5. Add the result back to the original (linear) image.
6. Convert from linear to working space curve.
original
Reddish glow
I’ve used gimp and I have inverted step 3 and 4 , I’ve used rgb curved for the orange tint
Another interesting discussion
https://www.eoshd.com/comments/topic/27604-simulating-a-tiffen-black-pro-mist-filter-in-post/page/3/
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Thank you very much, that is very useful! I will respond in this thread if I succeed in replicating this formula in darktable
No success? I’m super curious.
I thought that this is also a nice demonstration of the artistic value of those filters: Tiffen Black Pro Mist Review - Is This The One? | Alik Griffin
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I don’t understand why you don’t use the diffuse and sharpen module as @paperdigits recommended.
To see exactly what this filter does you would need a photo with a raw file taken without the filter and one where this filter was used as a comparison. And then you could recreate it with diffuse and sharpen module.
The example photos in the page you offered are relatively easy to reproduce in my opinion.
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It would be nice to see it done. 
Actually, now I’ve written that, I might try myself. Just need to find a suitable photo…
Yes, that is the problem.
Found this one I took… not enough different color lights I think. Still looking and thinking.
Edit… I’m stumped! I found a couple of Christmas light images that I thought would but they’re from my JPEG-only days - no raw. I think with highlights involved it’s not going to be the same thing, working from a jpg.
Here is another one from https://www.signatureedits.com/free-raw-photos:
With simple diffusion of highlights:
Easy diffusion of highlights with preservation of edges:
Soft focus:
Soft focus with edge preservation:
Soft focus with softening shadows:
And so it can be continued endlessly.
To influence the color of the diffusion you can also use the color channel blend modes.
Here for example diffusion of highlights with red channel. This will make the diffusion (halo) a little more reddish:
With parametric mask you can regulate which brightness range is influenced by the coloration.
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I suppose it would be good to have a shot with an actual filter and the same shot without and subtract one from the other to show the difference. Or maybe I’m overthinking it
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There are so many “mist” filters out there, with different strengths. So I would just start from the end goal: what is “look” the photographer is aiming for? Then see if it can be done with d&s (99%: yes). This is more about artistic choice than exact replication of a particular filter or halation of a specific film.
FWIW, I prefer to use d&s for softening like this selectively, usually with a drawn mask, to de-emphasize details which are not central to a composition but appear very crisp with modern lenses.
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Funny, I’ve been using a Tiffen Black Pro-Mist 1/8 for a couple of years and I’ve been thinking about doing a Play Paw where I provide pictures of the same subject, with and without the filter. The play part would be trying to replicate the pictures that has the physical filter from a non-filtered picture with diffuse and sharpen.
Can’t say I’ve explored diffuse and sharpen enough, but I’ve used the bloom preset and just dialed down the x order speeds from 50% to ~10%. I pretty confident it should be possible to get something good out of it with some more tweaking.
It will be impossible replicate the effect perfectly. Someone on DPReview described the challenge:
Due to clipping information is lost. You have to guess what is in that overexposed area. And what is in that clipped area is what creates the most bloom.
Due to noise (both in the sensor but also photon noise) you do not have the full information in the capture to make a correct and smooth bloom.
A full computation of effect of the filter is very complex. Often programs only do some more advanced version of Gaussian blur. So - maybe there exist such softwares, but they are not found in Lightroom or Photoshop. And the calculation will take time.
The optical filter is put in the optical path and affects the optical rays in a way that is totally lost if you only have an image. That is why it is impossible to simulate old not perfect lenses in post. You have to use those lenses to get the effect correct.
These clips below could also be helpful if you’re trying to replicate specific filters. They show unfiltered input and filtered output:
The version on Vimeo is of higher quality (but you may need to sign in to see it):
Rip the clips with yt-dlp.
I also want to emphasis that these filters are not only for enhancing your night time city shots. They’re are very nice way of taking the digital edge of our modern sensors and optics. If I could do it well in post, I’d prefer doing that.
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In the title card of the video, the filter also makes the left woman smile. Is that why people like these filters? 
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I use a Tiffen Glimmer Glass and it’s wonderful on the X100V. I find the lens to be too sharp sometimes, and the filter really gives it some wanted softness while preserving sharpness where it matters.
_DSF0199.RAF (32.1 MB)
_DSF0198.RAF (30.1 MB)
License: CC BY-NC 4.0 Deed | Attribution-NonCommercial 4.0 International | Creative Commons
Not great pictures but enough to run some tests I hope.
With/Without glimmer glass, iirc it’s a 1/4 or 1/8… unsure. White balance, and exposure should be the same between the two pics, unfortunately no tripod so there are some differences. Both are focused at the same place.
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Row file without filter:
Raw file with filter:
Rowfile without filter. Filter imitation with diffuse and sharpen module:
_DSF0199.RAF.xmp (17,5 KB)
darktable 4.5.0+812~gf8afa6480
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