Widgets
Sliders
Most settings are adjusted with a slider ranging from left (minimum)l to right (maximum).
You can either drag & drop the slider, use the scroll wheel to move the value left (down) or right (up), or click somewhere on the line to change the value anywhere in one click.
Darktable has a special variation for each slider, if you right click on the line it will transform the slider into curves to change the distance between values.
Tabs
Some tools have tabs, they manifest advanced options with splines, or variation in histogram displays. Tabs are also found to adjust painting and layers options.
Selects
Select are used mostly for presets but are also found in many other places to choose variations of one parameter, such as the type of blur, type of mask, or RGB channel.
Histograms
A Histogram is a graphic where you can visualize exposure or other parameters from left which is 0% or minimum value, to the right for 100% of the value. In some cases several curves are pictured overlapping on one histogram, with different colors, to represent different parameters variations.
The most basic histogram tool is found in the top right square of the darktable window and it represents the repartition of light across RGB colors. Basically, this tool allows you to increase and decrease the global exposure by clicking and dragging it respectively toward the right, or the left. If you are in a hurry it is OK to use it, but I strongly discourage its use as it is not very accurate and difficult to roll back changes, or even to finely adjust it, compared to the other tools parameters you will find below.
Levels
Levels are tools shown over histograms, where you can move three bars in order to change the range of exposure from black to white or the range of RGB colors. By range, I mean reducing the quantity of black, white or rgb colors starting from pixels containing the most and least of them. I will explain more about them in their dedicated sections.
Curves
Curves are similar to levels because they are shown over a histogram, however manipulating a curve gives you much control over the scope it is applied to, because you control not only the curve but also the number of points from which you can bend it.
Splines
In some cases, it is possible to move points on a spline to change parameters over a histogram. Darktable has spline histograms based tools to adjust parameters for the following:
Splines are used instead of sliders for all the tools listed above, plus the global rgb exposure adjustment, which is OK to slightly move at the beginning of a your edit, so that you won’t have any trouble to roll back changes and any subsequent impact of lighting effects applied will be easier to read.
A spline is somehow easier to move than a curve, without distorting too much of the image, because your points are always starting from the current base value, whether you increase or decrease the value along the Y axis (usually lightness, saturation, noise).
Double click on a spline to reset its position to the default horizontal line.