Do you know whether the results of Log Tone Mapping can also be achieved with the curves?
You can’t compress dynamic range with curves*. Curves just shuffle around what’s between those endpoints. You can’t so to speak move the right part of the curve even further to the right if you see what I mean. Log pushes the data beyond the “ends of the curves” into the scope of the curves to that you can shuffle the tones around.
The exposure tool will move that end point but only in one direction at a time. Increase exposure > loose highlights. Log and some other tools allow you to push data from both the black and the white side into the “view” of the curves.
*not great wording actually but simplified
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Wonderfully explained, I understood it perfectly.
With sigmoid, you can adjust the contrast and slide the skew.
As @nosle explained, you can use e.g. a log curve to map the unbounded brightness range to 0 … 100% (screen brightness) range, and then use a ‘bounded curve’ (one mapping 0…100% to 0…100%) to distribute it tonal range any way you want.
Sigmoid and darktable’s filmic do both (that’s why they have settings for contrast and related controls like ‘skew’ (sigmoid) or ‘latitude’ and ‘shadow - highlights balance’ (filmic).
Bottom: your ‘sigmoid’ image; top: your ‘log’ image with the tone curve on the right applied in darktable. Not a 100% match, but it could be refined. I hope this provides a visual explanation.
In DT this is what the latitude is for…you can introduce a strait segment to the curve and adjust the slope of that… the compression each way happens outside those boundaries… I think you can use the regularization a bit in ART for some local contrast but I think you have to add it back with a supplementary tone curve after to areas or a zone where you want contrast…
Have you really placed the two grayscales of mine on top of each other here? They look practically identical.
Which tool did you use to match them so perfectly?
He has just put a tone curve on it… using the picker on your image bars you could adjust points to match visually once you have the spot on the curve from the picker…
A tone curve that you see on the right, next to the two grey scales.
Oh yes, dt’s ability to work with these contrast equalizers is fantastic.
But I don’t want to complain, ART offers me so many good options that I haven’t exhausted yet.
latitude is a parameter of filmic, not of contrast equalizer. Please read our replies a bit more carefully.
It’s all a bit too much for me. You experts may take all this for granted. I had never heard of latitude before.
I just wanted to express my delight that I have finally discovered a significant difference between Sigmoid and Log Tone Mapping in ART. And if my statement is not correct, I would be happy to be proven wrong.
It is well known that dt can do great things and works differently. But that goes a little beyond the scope of this topic.
The point is Sigmoid has a straight linear portion and something similar can be introduced in filmic and it explains some of what you are commenting on about contrast. It is is not part of log tone mapping in ART and so for that you also might need to use an addition tone curve… it wasn’t meant to promote one program over the other… just noting this for clarity
You’re getting lots of replies ignoring the ART tag and that you’re interested in ART. People are talking about a different software altogether without even explaining why. In addition they are intentionally or not using convoluted language that should be reserved for development discussions. Just note the names and remember when not to worry to much about their posts.
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We brought examples from another software because sigmoid exists there, too. I do respect that this is an ART topic. The use of the term latitude only came up because that is what one of the filmic parameters is called. I think filmic was mentioned only to point out that the two approaches (a log mapping and then a separate contrast curve vs an all-in-one parametric curve) are not all that different. None of that was done to confuse anyone or to promote darktable in an ART topic.
Just note the names and remember when not to worry to much about their posts.
I think that was uncalled for. Everyone who responded had good intentions.
I’m on the phone now. Once I get to the PC, I’ll try to create a simple, illustrated explanation.
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What wonderful words to my ears.
I often don’t understand what these people are saying and I don’t understand why they are saying it. What you say here relieves me enormously.
Thank you very much for your comment.
Of course I am also open to new ideas.
If someone says to me: Listen, what you want to do here with ART works much better with xy, why don’t you try it out? Look, I’ll show you how it works, then of course I’d be interested in taking a look.
Even if I would prefer to stay with ART. I am deeply convinced that ART can do much more than I can. I’m also convinced that it’s much more important to photograph better than to develop even better.
The sine qua non of photography ! 95% should be done when you press the shutter button. The last 5% can be done in software.
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I’ve seen many examples of various software (FOSS and proprietary) made to “walk and talk” by highly skilled operators. But as impressive as that is, I always have to wonder if / why the shot wasn’t better exposed / composed. Who knows, there may have been good reasons.
Then again, it’s more interesting to watch a miracle being performed than watch ordinary stuff, so… 
That said, I’m glad there are “miraculous” tools available, given how I shoot sometimes! LOL
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If someone exposes badly or chooses an unfavorable section, it is rarely intentional, but almost always an inability. Either the photographer couldn’t do any better or the situation made it impossible to make the necessary adjustments. But sometimes there are also situations where there is no better way if you don’t want to miss the shot.
Good software should and can help you get the best out of your photos.
And that brings us back to an old topic:
ART, RawTherapee and darktable: all three are very good. And if poor developments are made, it is certainly always because the user was too sloppy or too inexperienced. But as far as I know today, it is completely impossible to achieve significantly better results with one of the three than with the others.
The crucial question is: How do I personally cope with the software?
I’ve admired darktable for years - but if I take a break for a few weeks, I’ve forgotten a lot of important things. In short: I’ve always struggled with my understanding and mastery of dt.
RawTherapee was my absolute favorite for years. I could do anything with it and would be happy with it for the next 100 years, but not when the project stalled like this, that worried me a little, especially in terms of support for new cameras or lenses.
And that’s where ART came in: Fresh, clear and intuitive. After several attempts, I manage to achieve the quality of RT. And ART is thoroughly likeable. And then there’s Alberto. I can’t believe how much and how quickly he helps everyone, even beginners. And as soon as someone draws his attention to a bug, it is usually fixed very quickly.
What more could you want? I don’t need any more, ART offers me everything I can only dream of.
You probably already know the term mid-grey, or midtones: stuff that we perceive as neither ‘dark’ nor ‘bright’: the sky, away from the Sun; grass; many wooden surfaces. When Ansel Adams developed his ‘zone system’, he centred it around mid-grey (German Wikipedia: Neutralgrau – Wikipedia), which is defined as a surface that reflects 18% of the light. Here 100% would be defined as a surface that reflects all the light in a diffuse way, like white paper or a white wall; not like a mirror. However, in the world that we photograph, there are areas that are brighter than 100% diffuse reflections: light sources, or surfaces such as metal, water etc. that are almost like mirrors (called ‘specular’ reflections (German Wikipedia link: Reflexion (Physik) – Wikipedia)).
While our displays clearly emit light, so they are light sources, they cannot be as bright as the Sun, or the surface of the filament in a traditional light bulb. When printing, the situation is even worse, as paper is clearly a reflective surface, it cannot be brighter than 100%, by definition.
If we tried to represent light in a linear way, without compression, we may end up with something like this. Parts that are brighter than the display can handle are ‘burnt out’.
What we want is to be able to create a picture between 0% and 100% brightness that somehow resembles reality, even though in reality the contrast between the darkest and brightest parts of a scene is much larger than what paper or a display can produce.
If one were to simply scale the values equally (multiply pixel values by a number < 1), making sure that the brightest part of an image becomes 100% white on the display or the paper, the scene would be too dark. Here’s a screenshot using ART:
We could then add a curve on top to bring up the shadows. That is the traditional ‘base curve’/‘camera curve’/‘tone curve’ approach. A very bad attempt:
Luckily, our senses (sight as well as hearing) are (as far as I know, and probably only approximately) logarithmic in nature. That means, when the signal (light or volume) is multiplied by an amount (for example, doubles), we sense it increased by some amount (we perceive addition), not multiplication (for example, doubling). When the signal doubles again, we again perceive an increase by the same amount. So, it went from ‘10’ (whatever the unit is) to ‘20’, then from ‘20’ to ‘40’ (so it increased first by ‘10 units’, then by ‘20 units’), we feel it changed ‘x’ both times.
For sound volume, you are probably familiar with dB. That is also a logarithmic scale; the threshold of our hearing is defined as the base line 0 dB; a quiet room is 30 dB, a normal conversation is about 60 dB, a hairdrier 90 dB, a rock concert 120 or above. Even though physically the power levels moving the air are not 1:4 in ratio, we feel that if we take the difference between the threshold of human hearing to a quiet room, we then go again ‘as much’ from the quiet room to 60 dB (about the loudness of a normal conversation), then again to 90 dB (the sound level of a blender or hair drier), and once more to 120 dB, the rock concert.
Here is a plot of the ‘natural’ logarithm function:
At x = 10, the value is 2.3; at 20, about 3, so it increased by 0.7. At x = 40, the curve reads about 3.7; again, it increased by 0.7; at 80, it’s at 4.4, so it went up by 0.7 again.
With log tone mapping, you get this gentle curve that becomes less and less steep. There is more maths involved, but that is not so important.
Instead of darkening the image as shown above, we can apply the log tone mapping to keep the shadows visible, and map those bright parts (the sky, which is a light source) into the displayable/printable range:
However, you can also see that it also means you lose contrast. There is only a small portion where contrast is close to 1 (the black line is contrast = 1, the blue is the log):
.
One solution is to use a traditional S-curve, like the one I gave above, to place contrast wherever you need it. The other is a parametric curve, like Sigmoid. (filmic is another such curve, originating in the animation software Blender, I think, and also used in darktable in a modified form.)
If you open the curve explorer (also posted above), and only plot sigmoid (log-logistic), using contrast you can control how steep the straight part of the curve is. Contrast 1 vs 2:
With skew, you can control how much the shadows and the highlights are compressed. Notice when the line ‘leaves the ground’ and ‘hits the ceiling’. Skew: -2 vs 1:
(Edit: contrast is contrast (slope of a tangent, a straight line touching but not crossing the curve) at 0 EV; the slope at other points depends on the base contrast one sets, but also on the skew.)
How is that different from a traditional S-curve? The tricky part is that the y axis is in screen/paper brightness percent, but the x, the input, is in EV (so, at the bottom, you don’t see 1%, 2%, 4%, 8%, 16%, 32%, 64%, 128%, 256% and so on; 0 EV is mid-grey, the others are the given number of EV above or below it, so for each step the amount of light doubles or halves). That is where you can see the logarithm at play: a difference of 1 EV is doubling the amount of light, which we perceive as ‘it got brighter by the same amount each time’.
I do not know if this helped or confused you even more. Please let me know.
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