Nice work @alexfry!
I’m not sure what the purpose of the “extra” input is. As far as I can tell it is used to pass the original RGB image data to the forwardTonescale function, as well as the JMh data. But the function then appears not to use that data.
I have been experimenting with a Blink conversion between Hellwig J and achromatic luminance, as discussed in the last meeting, to reduce the conversion to a simple 1D function in both directions, to go back and forth to the luminance domain for Daniele tone-mapping without needing to use the whole Hellwig conversion. It is available in my repo as hellwig_ach.nk.
I have plugged that into the “breakout” version of the DRT and can use it in place of the existing J tone-mapping step. With a couple of extra nodes I can also match the “simple chroma compression”. The breakout version is great for this kind of experimentation.