Suppose I use darktable with a scene referred workflow and I am using sigmoid.
Suppose I adjusted the middle grey of my picture with the exposure module, before entering sigmoid, as advised in the user manual.
Most of the time, when you apply sigmoid, the output dynamic range of the picture does not match the one of the screen. The histogram starts after the completely black and stops before the completely white.
Now, how should the user do to make the output histogram reach the full black and full white?
One idea I had was to change the “target white” and “target black” of the sigmoid. But it is said in the user manual that we should not touch these sliders. One other problem I found is that the sliders don’t go high enough. For instance I need to manually type a value of 150% for “target white” in order to make the top of the histogram reach the maximum white of my screen. So, these sliders don’t seem to be made for that.
One other idea I had was to use the tone equalizer. And artificially expand the dynamic range of the picture before sigmoid. But doing this doesn’t feel right. And I also heard that tone equalizer is made to decrease the dynamic range of the pictures (bring up the shadows or down the highlights) rather than increase it.
So, what is the solution for my problem?
Note: I have just started using sigmoid. Before, I was using filmic RGB. And with filmic what I was doing was to set the black and white point by adjusting “white relative exposure” and “black relative exposure” in filmic RGB. This is what Aurélien Pierre was doing in his youtube tutorials.
On a side note, in case you are trying to achieve pure black to pure white for each and every image: not every image has those characteristics, and some will look a bit strange if you stretch the dynamic range like that.
Also, just to add: the manual is a guide but not gospel. So, you can adjust target white and target black in Sigmoid, and use Tone Equalizer to increase dynamic range, even though they may not always be recommended.
When these things are “not recommended”, it’s because there is usually a technically better way to achieve what you want to do. But there is nothing stopping you from using any of Darktable’s tools however you want creatively. Just try to understand what you are doing so that you can work within the limits of the tools.
Regarding your main question, I would first echo what @elGordo said and make sure that you do actually want pure black and pure white. Don’t just do this automatically for every picture. If you do decide that you want to stretch the histogram, then Tone Equalizer used as a simple tone curve (preserve details = “no”) can be used to darken and brighten the extremes. Or you can just use the shadows and highlights sliders of “Brilliance” in Color Balance RGB.
There are actually various other ways of doing it, but these are probably the most recommended.
I agree that you will like AgX which I am using in DT 5.3 weekly builds for Windows. However, I feel your editing will be flawed if you try to paint by numbers and use the histogram to stretch from pure black to pure white. Photography is about the look. But setting the histogram to log rather than linear may reveal it is already stretched further than you realised. See screen grabs of histogram for same image below
What do you set your histogram profile to?? If your monitor is closer to srgb and you were to leave it on the default of rec2020 the histogram is also represented differently wrt dnr… so srgb might be a closer representation to your monitor for dnr… but I think also if you set middle gray in a good zone then most times sigmoid will be pulling the dnr down to fit in the display referred space…
I don’t mean log vs linear I mean the color profile…there is a setting for the histogram that allows you to select what it represents and it is what determines what values that you will see from the color picker…
No nothing bad on you…I might not have been clear…DT has several profiles that can be configured…
I dont think its changed but linear rec2020 used to be the default for the histogram profile…so in that case for just this random edit I had been working on you would see this histogram…
