Pianoteq source for its simulation

Good points, especially about AI generative synthesis.
My point with this first post in the forum is that typically we know the math formula for a vibrating string, incorporating timbre and those parameters are 'on paper' for sure. But how Modartt team gets their sources of actual physical strings off of a piano to 'match' their model to those? This is crucial for the whole intended development of the simulated piano sound!

• is it from a microphone recording in an anechoic or real studio room chamber? (if yes this is ok but it is wrong)
• is it from this or that piano brand and model (if yes, then why not buliding a sort of "neutral" or scientific setup as I would have done it? But I understand they are not a piano manufacturer and brand that makes pianos; they are scientists and piano and music lovers)
• is it from a collection of sources and sort of 'averaging' the output in order to fit the parameters?
• and the actual engine's piano parameters are not quite 'available to be tinkered' by the end user it seems otherwise how would they sell all those add-ons branded with the respective logo of a real piano manufacturers as those add-ons seems to do exactly that – preset tweaks not available from the UI?

How I would have done it:
1. Place linear pickups (such as Cycfi Nu modular pickups) under every string of a neutral build of a piano frame\soundboard and maybe a few hammers and test keys
2. Attach piezo-pickups (piezoelectric materials produce an electrical signal when a stress\vibration is applied to certain spots on the soundboard non-destructively (per test or capture)
3. Setup with one string per note (real 'una corda') and another setup with up to three two strings with two aliquot strings for the higher register (not like the current pianos!)
4. Possibly use of a few solenoids triggers to get a few dynamic pushes to the keys mentioned to get various loudness and dynamic captures form all the above 'sensors'
5. All that in an semi-anechoic or full anechoic environment with or without piano cabinet (probably without as it is terrible by current construction)

Then I can say I have a close to neutral source sounds to compare the models against them and to try and match them.
Wavetable synthesis eventually to be incorporated for exploring the morphing of corresponding source material samples.

Again, that is my own opinion and how I would have attempted such endeavour. Then Math and Physics, AI and ML  models are generated for the purpose.

I think you are making a lot of incorrect assumptions. I recommend the reading of the following article written by Juliette Chabassier , PHD, who was involved in the development of pianoteq.
The document is public domain .

https://inria.hal.science/hal-01894219/file/hal.pdf

yeah, I think the degree of difficulty here is magnitudes above (and I'm a keys guy with at least 10 guitars) guitar amp modeling.

multiple strings per note, and they interact (and this interaction are also parameters in the models).
the soundboard is > than an amp
the micing is > room simulation.

let alone that for guitar (amp) modeling, or even stomp and effects modeling, there's still the core instrument itself - okay Variax is an actual model of various guitars, as is a few Fender models, but I doubt anybody really commits those "naked" as solo instruments yet folks (including some A-list pros) actually do that with Pianoteq.

I've been with Pianoteq since v1 - and I would say "it was an idea" then, but I could use it for pop pianos as a synth. It *still* has all of the synth capabilities. But...  Today? I'll also use it as a naked solo piano in a typical piano audio chain, meaning compression, EQ, verbs are all external to what is in native Pianoteq - because I love the models, can make the micing work for me, but just prefer better EFX.  I own more than enough premium (priced!) piano libraries and Pianoteq really is for me "looking at the challenge from the other side", and most times it's the better side.

I will *def* give the new micing setup in v9 a serious workout.

I just think this is quite a bit more modeling than an entire audio path on a guitar, it'd be closer if we model the guitar itself too.  If you have any doubt just how deep these models are?  switch down to a single mic, and move it what would approximate to just a few centimeters and hear the effect.  It's like the everywhere, not only on the virtual model of the instrument, but in every spot in the space shown.  It's stunning.  Now take that by 8x stereo. Voila, not just a model of the piano, but the piano in the space it's in.  That's huge. :-)

Accurately simulating the behavior of a piano's sound production is one thing, but running the simulation in real-time for playing is another. In industrial simulations, it is common for large servers to run for dozens of hours to produce results that last just a few seconds. If we take every detail into account as much as possible, real-time playing would become impossible. Therefore, approximations must have been used here. I believe the Modartt team has put in a great deal of effort to balance accuracy and real-time performance. However, it would be desirable to have an "accuracy" mode for offline rendering.