I think filmic is quite easy, once one understands a few key points.
It is a tone curve. Its input is the linear (twice the number → twice the physical amount of light) view of the image; its output is closer to the world of the display. The linear side is ‘HDR’ in the sense that the camera typically handles more dynamic range than the display (it needs not be a HDR composite image). Its current output is low dynamic range, but with new display technologies, that may change. The important difference is the encoding: in the output, twice the number does not mean twice the physical amount of light.
scene tab: the curve is centred around the orange dot, representing mid-grey. Even though visually it moves around when you alter the black and white relative exposures, it still represents the point around which filmic is built, and it always maps mid-grey to mid-grey (the real behaviour, keeping mid-grey at a fixed position, and re-drawing the whole graph around it, would have been rather inconvenient). The X-axis is logarithmic (values are in EV, a change of 1 unit means doubling or halving the amount of light). The white and black offsets simply describe what input values will be mapped to black and white output, respectively. Even if one of the previous modules seemingly pushed highlights to clipping, those can be recovered, given that the raw sensor input was not clipped. Negative shadow values cannot be recovered; in exposure, one may use the black level to make sure no value is pushed below zero.
look tab: contrast defines the steepness (contrast) around mid-grey. Latitude defines the range in which this contrast is maintained. The same latitude is used for retention of colour saturation: outside the range, contrast and saturation both drop: shadows and highlights will be desaturated so black and white will be neutral; contrast will drop as shown by the shoulder and toe of the curve. Shadow/highlight balance allows you to slide the range higher or lower. If the graph misbehaves (becomes non-monotonic, which means contrast is inverted; such parts are shown in orange), you can:
- reduce contrast
- reduce latitude
- slide the range.
The first two always work (if you reduce them enough), the third one may or may not work, depending on how much the curve is messed up.
This should be enough to get started.
Create a black-to-white ramp in the Gimp (save as a PNG or JPG), load it into darktable, move exposure, turn on filmic, place a few colour samplers on the image, and adjust settings. This will help understand filmic. See Darktable - Workflow approaches - Custom Base curves, LUTS, Filmic - #20 by kofa