I’m new to darktable, trying to learn it and trying to do so using AgX primarily since I keep hearing good things about it. But when I edit my photos, my curves keep getting very flat, like this, causing the module to complain at me.
I think this is because I am setting the white and black points too close together. However, I’m just doing so by selecting something that I want to be white and something that I want to be black.
Am I using this in the intended way? Is something about the way I am exposing my photos making it not play nicely with this module? Do others get this?
Here’s a screenshot of the full photo, for reference:
This happens frequently when adjusting the black/white relative exposure on photos with little inherent contrast. This is not an issue per se but you could try increasing the contrast in modules ahead of AgX in the Pipeline - e.g. with the contrast slider of “colour balance rgb” or tone equalizer.
Aside from that: Beautiful photo! If you liked you could share this as a playraw and see how others go about it.
It is not a problem, as long as the image looks good. The module is not ‘complaining’, there’s nothing you have to ‘fix’. It just tells you (hover over the warning) that this is not an S-curve, so shoulder (and/or toe) power cannot be applied.
See also: AgX auto tune levels help
I have that regularly for scenes with relatively low dynamic range: no sky, fairly diffuse light, no deep shadows with relevant detail.
Those may or may not need a tone mapper, filmic curve becomes almost a straight line. But the extra contrast you can apply in the midtones can come in handy.
At least that makes for good contrast in the brighter and darker parts of the image: the tone mappers basically work by lowering highlight and shadow contrast…
It is a curve, a function, distributing the available output tonal range over the (selected) input range. It’s for you to decide how you want that distribution to occur.
There only thing special slot tone mappers is that they take a theoretically unlimited dynamic range as input, and create finite output. If your input tonal range is small enough, output = input may be a good option. If your input is very flat-looking, you may want to increase contrast, which you can do with any curves-based tool. If your input covers a high final range, you have to reduce it somehow: using a tone mapper curve, or a tool like tone equalizer.
I select the whole image for auto tune levels and it seems to correctly identify what is white and what is black. I was also impressed with how it handled a very low contrast misty scene. I have only found one image taken under UV light when I felt the auto tune levels didn’t do a good job.
Kofa supplied a sigmoid like default preset for AgX which I have adopted as my default AgX starting point. It has a bit more contrast and saturation because of the extra contrast. This is the preset that was supplied if you want to try it. In the preferences for DT the preset option includes an import option to install it.
Your input exposure has no value and your output is mapped to 18 percent…This may be intended but also is partially why the curve is like that… You can use the second picker ie the output one and then adjust it to taste afterwards…Kofa has outlined the default workflow in the documentation…I have actually started more often to use the input picker to get my starting point but that is just how I am working …In any case these two settings will impact the curve…and often when adjusted will return your s curve
I have also started doing the same for low contrast images as @mino suggests.
I generally use the contrast slider (for general global contrast and to “fill out” the histogram if needed) and the Perceptual Brilliance section (for highlight and shadow management) in Color Balance RGB module, then I use the auto set white/black relative exposure in AgX to automatically find the white and black point.
This has several benefits: It makes the AgX have a more predictable effect, it almost always keeps the curve in AgX looking nice (not required, but I like it), and it allows me to work through the modules in a more or less linear manner (not having to jump around between the beginning and end of the pixelpipe).
If you are using contrast there it should be noted that the fulcrum for the contrast can be set in the right most tab where the mask settings are. This effectively lets you select the pivot in the image for what gets darker and what gets lighter when you add or subtract contrast. I think most people just move the slider without setting this but it can really improve the result …even just setting it with the auto picker…
