I hope that Alberto’s explication of this tool will help you to better understand the mysterious part of it.
On the other hand, my opinion is that this log tone mapper isn’t the easiest tool to master. The Auto button often changes nearly nothing, exposure wise, and never gives satisfying results. Apparently this is not an auto-exposure thing, but something else…
I usually correct the exposure of my photos with the Tone Equalizer plus the contrast slider in Tone Curves, and if needed with some exposure compensation in the Exposure tool. Or I use the tools in the fourth tab, Local Editing, to do all.
That said, I exposed your photo by hand with Log Tone Mapping only - except for a slight increase of contrast with the Tone Curve tool.
Hallo @paulmatth,
Yes, I normally do exactly the same as you:
But here I would really like to get to know this “Log Tone Mapping”.
Your example is excellent, because you use it exclusively here, plus just a little “contrast”.
The main sliders here seem to be Target gray point and Gain. The others are presumably used to compensate for their unbalanced effect.
Your image example is very good to show the effect of the tool, even if too much of a good thing has already been done here, so that the cloudy evening mood is lost, but this is excellent for learning.
Unfortunately, you can no longer see the history in the saved .arp file. But, perhaps you could do the same thing again with several snapshots showing each stage of your settings. If it wouldn’t be too much extra work for you. Or you could simply describe with a few lines what you start with, how you recognize that the value found is sufficient and what you then continue with.
I think I will also benefit greatly from this, and so will everyone else who reads this.
So, thank you for this and thank you in advance for what’s to come, if you feel like it.
Micha
PS:
How can you understand that? The contrast is reduced with “Log Tone Mapping” and increased again with “contrast” in Tone Curves. The results are excellent. But isn’t that contradictory? I suspect that contrast is not the same as contrast. What is the difference?
Attached is the arp file with some snapshots. I have no “plan” when using the log tone mapper, I just change sliders, starting with the upper one, then the second one, etc. Then I re-adjust the sliders until something comes out that is reasonable.
Hello Alberto,
are you kidding? … in a simple way?
Yes, there’s probably no easier way to explain why and how Log Tone Mapper works in just a few minutes.
I see the great effort you have made, but for me the numbers are and remain an unsolved mystery:
“black relative exposure” is -13.5, and 0.18 * 2^{-13.5} \approx 00.18∗2−13.5≈0 …
I fear, or hope, that I am not the only one who finds this far too mathematical.
But you didn’t do all this for nothing. I think I have a good understanding, especially because of the video, of what shifting the gray point and the white point means.
What an interesting graphic mid.tif is: It normally contains a lot of white, and with ART (also with Gimp), you can change the tonal values so that the white is gradually drawn again.
I have translated your text into German and copied it into my private ART-manual. I will read it more often and make sure that I understand more bit by bit.
Alberto, you have already made a short video about LTM, albeit in an older version of ART.
1st request: Could you make a video of the development with LTM with a suitable picture?The whole world of ART will certainly thank you.
2nd request: Could you describe in more detail which images give LTM an advantage over other tools such as Tone Equalizer? Or can you achieve the same results with Tone Equalizer as with LTM?
I’m probably only going to expose my shortcomings but this is a non-technical summary of how I interpreted @agriggio’s explanation:
The lightest linear / unbounded value is mapped to bounded (display) white, or 1
The darkest linear / unbounded value is mapped to bounded (display) black, or 0
The “balance” of the grey (gradient) slope between these two values is determined by where the Target gray point slider is positioned.
I.e., using the two equations he provided, the brightest and darkest points are mapped to (display) white and black, with the center grey point being relatively set by the Target gray point slider. He’s basically remapping white and black relative to the Target gray point slider value.
I think one thing right off the bat you could use is the jpg. If its still got deep shadows, bright sky etc then you know it has been tone mapped and processed and still it has DNR issues… Then to use the raw and to manage that and produce a better result your likely going to benefit from the Log tone mapper…
I think also when you look at the Tone eq map … it does appear that the “zones” are nicely feathered generally and I don’t exactly know how they are determined however if you hunt and peck with sliders in the tone eq you might sort of break the image tonality where as the tone mapping (with filmic or this sort of module) is a global edit with smooth gradients and can be used to capture (compress) the dynamic range and set you up for further processing… some of which might be the tone eq to fine tune things…
Hello Alberto,
I very much hope I haven’t upset you. My poor head, which is already 69 years old, is smoking. But for some reason I want to know.
I’ve watched your video over and over again and especially studied your text. Now I understand the two important controls: target gray point and white relative exposure pretty well.
And I took your “recipe, for most images” to start with target gray point to heart. And look: it works very well. For this picture I only use: Target gain +50, Gain -2 and with Tone Equalizer I make the sky a little darker and the result is very fast, just as I imagined.
I just want to tell you not to be annoyed if it takes me so long to understand.
Your efforts are bearing fruit. And for that I thank you and everyone who was involved.
I am sorry if you didn’t understand something. That’s not what triggered me, I just took your reply in the wrong way, and I apologise. I did my best trying to explain how the tool works, and yes, this includes a little bit of math. I tried to keep this at a minimum, and in my view it was more like an extra, the video explanation would have been the main point.
Anyway, it’s all good now!
I am glad that we were able to clear up the misunderstanding.
I never questioned the quality of your explanation. I just realized that the way it works is too “high” for me and I understand so little about these numbers.
I just wanted to know how to apply log tone mapping. It was my mistake for asking how it works - there’s a big difference.
The video was very descriptive, but your text is hugely important to me because now I understand much better what the sliders do. This makes it much easier for me to see what they do in practice, I know what I have to pay attention to in order to see and understand the effect.
It has clarified a lot and now I have enough knowledge to practice and gain my own experience with it.
It’s not the lightest, it’s just the user selected value. The tool doesn’t clip anything, and brighter tones can still be compressed later (e.g. by a lut or by the new “base curve” options in the tone curve).
For me, “Target gray point” changes the mid tones the most, but also the dark tones and the black. Only the white remains untouched. Is that different for you?
It’s not different, it works the same here. But it does not change the black point for you either. In your screenshot, the black point is set to be 13.5 stops below mid gray (that’s what “-13.5 Ev” means in the GUI), but the darkest band in the test picture is only 10 stops below mid gray. If you set “black relative exposure” to -10, you will see that the darkest band will stay at zero no matter what value you set for “target gray point”.
Hello @jonathanBieler
I don’t know if this is the right way, maybe you want to know something completely different: But I manage to make all white levels visible with RT, just with “Dynamic Range Compression” > Amount > Maximum = 100.
but this is a non-technical summary of how I interpreted 
