Yes, I also thought about asking him.
I also thought that to avoid any risk of problems in a two instances process one should also probably need two perfectly complimentary masks, which should mean no soft edges – which likely will create some kind of unwanted lines in the image, wouldn’t it?
It seems to look ok, from a quick test… this is two identical instances of sigmoid, split vertically roughly down the centre by a mask.
There’s a small funny transition at the border near the top, but apart from that seems ok.
This is a zero feathering mask - both instances using the same mask just polarity toggled.
All files licenced CC0.
IMG_0188.CR2 (17.3 MB)
IMG_0188.CR2.xmp (13.5 KB)
MMm. I tried feathering the single mask from above example, and sure enough, something’s not quite right.
I think @s7habo once used two instances of filmic with an edit. It was from a Fuji camera, if I recall correctly, shot in a forest, with a huge dynamic range. I’m not sure if there were even masks. If I can find the image, I’ll link it here for inspiration, or maybe Boris knows what I’m talking about, and will link it himself.
I don’t think that was me. I have played with several instances of filmic but mostly in conjunction with blend modes and that would be an atypical edit for me. But, maybe you’re right. Who knows, I’m getting old and forgetful. 
Now I will say, knowing the restrictions of the sigmoid, you can tweak things in post-formation (after tonnemaping).
Here is original photo with smooth preset of Sigmoid:
You can, for example, first adjust the contrast and skew in the sigmoid until you have nice contrasts in the shadows and mid-grays:
Then you can use ton equalizer to reduce the brightness of the area in highlights that is later in pixelpipe strongly influenced by sigmoid:
To adjust contrasts in highlights only, you can bring color balance RGB module over the sigmoid and use briliance sliders:
You can use a new instance of tone equalizer without detail preservation for the same purpose (simple tone curve preset):
You just have to bear in mind that you no longer have gamut protection after sigmoid and have to be correspondingly careful. For example, I use the waveform as a guide to how far I can go.
It was. I remembered it because it was so unusual:
OMG, yes!
That’s how it looks now 
:
I misused the second instance to increase the contrast with multiply blendmode. Please don’t do that!
Indeed. But then it means there’s a problem with backwards compatibility.
Short answer is no.
Sure you could have a binary mask and invert that but you would have get a weird seam between the two.
You could however do it if you are willing to blend the two results externally from darktable:
You can apply multiple global tone mappers in parallel and merge the results but not sequentially. darktable only does sequential processing so you shouldn’t use two tone mappers in darktable. Node based editors such as vkdt opens opens up for this kind of workflow!
I think this is not a good idea at all. It’s much better to use sigmoid OR filmic to get to a base starting point and then use curves and other tools to get to where you want. If you use RGB curve instead you’ll get much more precise control over your shot.
indeed.
consider this graph with two film curves, i.e. 01 exposing for the foreground:
and 02 exposing for the sky:
there’s a special blend mode for exposure fusion, it is somewhat parameterised with the opacity parameter that instructs it to keep more of the low or high frequencies.
resulting in this final render, ymmv:
it’s often a good idea to set blend mode to over and set opacity to 0 or 1 to see one or the other filmcurv/whatever module have you in between.
I am glad I asked this question. I had never before used two instances of sigmoid, but I had an image which did benefit from doing this. But it seemed wrong to do this as surely the input going into the second instance was already transformed by the first instance. But it looked good. Hence my question.
I had fallen into a trap and depended too much on the skew slider in sigmoid to achieve the desired look. I was neglecting the contribution of the tone equalizer module to achieve the shadow/highlight balance that I desired. The examples shown by Boris showed me that I could achieve the effect I was desired by using the tone equalizer module as it was intended. Boris also demonstrated how the color balance rgb module could further improve the contrast of the highlights.
Thanks all for your great responses.
Sigmoid has that nice fall off to white but it also in the process really destroys the detail so I think the tone eq is the way to go to offset that. Skew can help a bit but actually dropping contrast seems to be the best way for the some of the highlight details to come back a bit and then the look changes again and may be less desirable to some. Filmic does it as well but to my eye much less… I usually have tone eq local contrast and diffuse and sharpen if the mix and often I will not even use one of the tone mappers unless I have to because of DNR… tone eq and rgb color balance with local contrast and diffuse and sharpen…maybe dehaze will provide a result with more details when you can go this route because you are not needing to manage DNR with a tone mapper…
I often ignore this part of the manual. Not because it’s wrong, but because I find Sigmoid to be a really good module for artistic purposes, especially with the new Primaries section. Honestly, I can almost finish an image just using Sigmoid these days, along with the basic modules of exposure, WB, etc.
You can set contrast, skew, colour grade, add an old-film fade… it really is a great module and very robust in my experience.
The manual also talks about how you can adjust Target Black in Sigmoid, but it’s best done with Global Offset in colour balance RGB. Again, not saying the manual is wrong, but I have experimented with both and I can’t tell any difference visually. Maybe the math behind it all works better in the way the manual recommends, but my eyes can’t usually see these differences.
In sum, don’t be afraid to break the “rules” if breaking them produces the results you want and like. Darktable is a toolbox, and it’s great to just experiment and play around with it. That said, I would always recommend to people to read the manual, especially beginners.
If you use the waveform you can see that shifting the black in sigmoid actually also impacts the highlights and you get a sort of compression towards the middle whereas the CB is pretty much a black offset lifting the baseline like a black compensation… but visually in many images you are likely right you might not really see a difference…
it’s the result that counts - if its achieved after several deviation from darktables recommendations then it’s completely ok.
darktable doesn’t protect you from shooting into your feet - but if someone likes that pretty gunshot wound then why not?
Not being a DT user, I am struggling to follow this interesting thread. May I ask:
Does “sigmoid” refer to an S-shaped mapping?
If so, does the skew slider move the sigmoid function inflection point i.e. to the right, more highlight contrast and to the left, more shadow contrast?
Hi @xpatUSA, I am not an expect on this so I defer to anyone more knowledgeable that answers. However, I would say yes you are correct. I often move the skew slider to the left to add more contrast to the shadows and give more room for the highlights to retain detail. Moving the skew slider to the right increases contrast in the highlights but means you have to be careful not to bleach out the brightest parts of the image.
Thanks for the speedy response, Terry.
I’m a GIMPster, so I imagine that the Curves tool set to smooth would do something similar and that moving a point of maximum slope would have a similar effect.
But of course more adjustment steps than the DT sigmoid function …
I would suggest Sigmoid and Filmic do functions beyond the curves in GIMP, Photoshop, Lightroom and other programs. Darktable also has curves.
I too am a GIMP user but I see the big difference is that DT is predominantly a RAW file editor despite being able to work JPG and TIFF files. GIMP is for JPG and TIFFS and doesn’t do RAW files.
Since I mainly shoot in RAW I have become a dedicated DT user. GIMP I mainly use for photo restoration work.
