I’ve finished reading the introductory Italian book by Marco Olivotto, Il colore: gestione e correzione, Castello 2ed 2023, where the author presents, among others, two general techniques for color correction, explained for Photoshop, but I’d like to check whether these can be followed in DarkTable.
The first basic technique is to determine the “most significant” white and black points. The author provides as example a picture with the sun in the background. This is the brightest point but not the most significant. He shows, in Photoshop, by adjusting the lever in the threshold panel how the brightest/darkest points can be found, and eventually picks as white point a precise element in the subject. The final adjustment is made with a curve. PiXimperfect describes a similar technique in youtube, this time using the curves also to find the white&black points, in Photoshop: https://www.youtube.com/watch?v=Xt39PClmo4U (There is no mention of the middle point in the Italian book though).
I suspect this can be accomplished in DarkTable with a tone mapper. For instance, in Agx, the eyedroppers automatically select them. How can I inspect/determine where these dark/white points could be so I can drive this selection ?
The second technique in the book is “known colors”. These are heuristics, in the Lab color space, on how our eye expects certain colors:
Neutral colors, a ~ b ~ 0
Skin, a > 0, b > 0, b >= a (with exceptions), small values.
Greenery, |a| <= b <= 3|a|
Sky, -6 <= a <= 3, b < 0
Ground b >= a >= 0.
The workflow requires to first pinpoint and bookmark some specific points in the image, showing their color in Lab space. In Photoshop, this is accomplished with the Info panel. The adjustment then is done with the curves for the single channels in RGB space, but the bookmarked colors in Lab must be visible all time, and updated in real-time, so to drive the correction. How can this technique achieved in DarkTable?
I think I’ve found an approach for this second technique. The color picker in the left panel allows to select & save the colors in Lab space. It can only show one saved color in my screen, not sure how to increase the panel size.
The adjustment can be achieved with the color look-up table, though I don’t understand the concept with multiple color patches.
keep,in mind that there’s a major difference in darktables aproach using scene referred workflow: It’s all about midtones - you don’t need to care about black/whitepoint
Use exposure to bring the midtones (related to your main subject) where you want to have them.
The tone mapper handles black and white point and takes care they won’t clip.
Once you’re done with your midtones you can modify the black and white points as you want them to behave. To play around with darker and brighter areas better use the tone equalizer.
If you try to use darktable like Lightroom or Photoshop it’s quite easy to fail.
Turn clipping indicator (bottom panel, icons right hand side) on, set to luminance mode. This will paint highlights above the threshold the selected colour. Then drag the black and white points until the clipping indicator shows no painted colour.
Same technique works in filmic (an alternative to agx).
Keep in mind in darktable the tone mappers are typically placed last (or near last) in a workflow. It’s possible to adjust the black and white points in a variety of modules, in a variety of ways, before that point.
They’re not fixed, rather, they’re moveable. Perhaps the way I stated it was a bit unclear. It’s possible to adjust tones in a variety of ways before the tone mapper, which could affect the black and white points. Eg. If you set white point in agx, then go back and boost exposure, that in turn will change the brightest point, meaning you will have to reset the white point in agx.