Temperament changes on the fly

Thank you, EvilDragon, that was very useful! Assigning MIDI events to temperament changes gives a rather good result, but not perfect. A held note is affected by the temperament change but the sound itself is continuous, if not absolutely even. But the note does sometimes stop sounding altogether, so there's a little pause. It seems that this happens when the new temperament tunes that note too far from the original tuning.
In fact, this is the same behavior as in my earlier test - sometimes there are little glitches and sometimes a pause. I think my way of changing patches, with *everything* except the one single parameter - the temperament preset - freezed, is in effect the same parameter change as the MIDI-event-controlled Temperament selection.

So it seems that Pianoteq is not able to perform the way I'd need it to, which makes this post a feature request.

Yes, I tested it too, and indeed changing Temperament presets with MIDI notes works rather well, and it's quite handy to be able to change tunings independently of Instrument patches. However, the small glitches and interruptions are still there and it's true that MTS would allow smooth changes even when notes are sounding.

But one would think the tuning implementation of Pianoteq could be improved to make any Temperament changes instantaneous and smooth, and not affect simultaneously sounding, held notes.

Technically it should not be a big problem to make temperament changes not be real-time but applied on next played note. However it would still impact previously sounding notes (i.e. sympathetic resonance, or if sustain pedal is depressed), because the model runs in realtime... It's like, literally detuning all the strings at the same time with the piano tuning wrench. Of course you'd need absolute silence if you want to change temperaments without ANY sort of artifacting or messing with existing output.

At this point, just use multiple instances in a DAW and switch between them? That is going to be guaranteed glitch-free transitions.

"Technically it should not be a big problem to make temperament changes not be real-time but applied on next played note. However it would still impact previously sounding notes."
Yes, that makes sense. But assuming no sustain pedal, most of the Temperament changes I tested were in fact instantaneous; I just would prefer Pianoteq to wait for NoteOff for all sounding notes before making the change. I can live with this, though. However, sometimes there is an interruption in the sound and some noise. I am not 100% sure why this occurs but as I wrote above, I believe it happens when the new tuning is too distant from the old one.

Switching between multiple instances within a DAW works, sure, but it's not a good solution in my case, as this is for performing, and there are tens of Temperaments that the player keeps changing via MIDI. One possibility would be to have only two instances of Pianoteq and change the Temperament in the one that's not being played, in advance, and then change the MIDI Channel at the point of the required Temperament change in the music, then change the Temperament in the mute instance again, and so on,  but this seems complicated.

Do you have a suggestion? I was thinking that a program could be made in Max/MSP that follows what midi notes the pianist is playing, and changes the tuning of the mute channel in advance, while the pianist would still make the channel changes. But a much simpler solution would be to use some other instrument for the piano sounds than Pianoteq, one that does the tuning changes without glitches - Kontakt, for example.

Sure, I do want to use Pianoteq.
This is for live performance, preferably without DAW, perhaps with a softsynth host like MainStage. In a DAW project (recording), there is no problem, everything can be handled the way you suggest.

Can we assign A/B switch via midi events ?

If you haven't found an ideal solution to this yet, look into GigPerformer, it can do what you want. Its "patch persist" function is what you'll need to use.

I use it for switching between many different instruments seamlessly, and it works very nicely.