I have seen this question several times these past few weeks:
If you open darktable modules like tone curve, rgb tone curve, tone equalizer, levels, rgb levels etc. the inset histogram doesn’t look the same as the global histogram (at the top of the right sidebar). Why ?
The top right histogram is the non-linear RGB histogram of the full pipeline output. darktable stacks modules in a sequence named a pipeline:
RAW -> [demosaic] -> [...] -> [input color profile] -> [...] -> [output colour profile] -> DISPLAY
The histogram you see in the top right corner is the data sent to the display, after the full pipeline is processed, and (in darktable 3.0) you can select in which RGB space you want it (“histogram profile” option when you open the softproofing menu):
The inset histograms you see in modules like tone curve represents the input of the current module, in the colour space it is working (Lab for tone curve and levels, linear RGB for RGB tone curve and RGB levels, luminance for tone equalizer). That’s all you care about at the module level, when you set it up: local input.
- global histogram : global output of the full pixelpipe
- modules inset histograms: local input of the concerned module
Same applies to colour pickers (global vs. in-module).