Which modules convert internally the image in prophoto rgb?
-tone curve
-rgb curve
-rgb level
-filmic (only old version?)
-base curve
-other?
For the conversion I’d think “input color profile”, provided the working space is set to “prophoto RGB”.
After that, all modules before “output color profile” should use the working space defined in “input color profile”.
None of them should change the working space, though they might use different representations of the colors to work on.
Looking at the GitHub page seems that some modules use a fixed “working space”
https://github.com/darktable-org/darktable/blob/c1991590bd6c9a0ad7fde544f3f809ca1f6c9ca4/doc/usermanual/darkroom/modules/tone/rgbcurve.xml
The RGB, linked channels, works in ProPhoto RGB and applies the L-channel
curve to all three channels in the RGB color space. Adding contrast in RGB space
is known to desaturate highlights and boost saturation in lowlights, but this has proven to
be the most reliable way to edit contrast, and is the standard way to do it in most
software.