Basically I think, that it is a good idea to test and learn really ALL tools in Darktable with not only real world photos, but with PNG test images. Especially filmic RGB can be understood very good with adequate test images.
Please find attached one of my test images. With such image you can clearly see, what happens in most (at least a lot of) Darktable modules.
The four rows are:
- Just 10 areas from 8 bit RGB 0, 28, 56, 85, 113, 141, 170, 198, 226 to 255.
- 255 areas (each with 5 pixel width) from RGB 0 to 255
- Vice versa of row 2
- Similar to row 3, but with small variations of area 2, 4, 6 …
The row 4 can be used to see what happens, when the contrast in specific brightness areas is changed.