HDR-SDR using ffMpeg...

[Selur]

but I have no idea what these filters are doing exactly and how they differ in what they do.

tone mapping is an automated color grading approach trying to produce a similar representation HDR conten offers in SDR.
If you want to understand it correctly you will have to read up on the algorithm which will require a lot of general understanding of math around image processing. https://en.wikipedia.org/wiki/Tone_mapping might give a general idea,..

could you explain the difference between say the way Mobius would handle a conversion as opposed to Reinhard?

Mobius transform allows the user to decide the trade-off between color accuracy and detail preservation on a continuous scale. The default of 0.3 is already far more accurate than `reinhard` while also being reasonably good at preserving highlights, without suffering from the overall
brightness drop and color distortion of `hable`. But depending on the source and settings (not just the method choice) one or the other method might preserve more details or highlights. Remember something gotta give, since HDR has wider color gamut(range), wider dynamic range and higher brightness.
There is no way general method X is better in XY.

Also, what is the desaturation -2 about? Is that just arbitrary or is there a good reason for that?

Just something the tonemap authors preferred and chose as default.

May I ask what exactly does 'HDR10 to SDR (DG)' and 'ToneMap Placebo' mean? Is that a specific setting on Hybrid?

Yes.
'HDR10 to SDR (DG)' is basically Reinhard with some more controls.
'ToneMap Placebo' is tone mapping offerec by lib placebo which uses it's own approach https://github.com/haasn/libplacebo/blob...options.md.

I do like a very detailed and saturated picture, even if it may not be absolutely 'Natural' looking. The first few times I tried to do this I used Handbrake just using the BT709 colour timing. But, I thought the results came out a bit on the dark side. When I tried the Clever FFmpeg GUI it seemed to come out more vibrant and lighter, but I have NO idea why.

Unless you do a HDR to SDR conversion simply switching to BT709 will drop all the HDR info.
I suspect FFmpeg GUI by default automatically uses some HDR to SDR conversion an Handbrake does not, but you would have to ask the authors of these tools to know for sure, since I don't use them.


Cu Selur