My understanding of that "inform plugins" Reaper setting is that it means "since we are rendering offline, I will wait for you to compute your audio, no matter how long you take".

(speaking as a programmer) All plugins should (and do) work like this anyway, none should attempt to work real-time during a render and drop out/glitch if there isn't sufficient CPU available to sustain real-time.  I have heard this was true of some old plugs, but it's a horrible bug.

No, Reaper (and others?) can inform the plugin that they're doing an offline render, which the plug can optionally use to do higher quality processing (as was suggested).  For example Voxengo's plugs allow setting different oversampling factors for offline.  During offline rendering their plugs actually flash a 'rendering offline' status message.

EDIT: another use is meter plugins that can disable their processing for offline renders to save CPU.

I know that at least with some plugins, if I do not enable this setting, the render will have glitches/dropouts.  I asked in the Reaper forum why one would ever not enable this setting. and why it isn't on by default, and the answer seems to be that it's because of the way Reaper development is done, and to be polite to the users, just in case the introduction of the setting causes any problems.

Greg.

Pianoteq has "high-quality settings" that can be applied for exporting wav. But if I use a DAW, how to enable this settings when rendering output sound file?

Reaper has an option "Inform plug-ins of offline rendering state". I think other DAWs also have this option. Does this force pianoteq to use high-quality settings?

Not automatically, that is only for plugins that allow the user to create different settings for playing and rendering (eg FXPansion D-Cam plugins, 2C Audio Reverbs)

I don't think Pianoteq has such a built in switch so you would have to set this in the host itself by increasing the samplerate before rendering.

Reaper has an option "Inform plug-ins of offline rendering state". I think other DAWs also have this option. Does this force pianoteq to use high-quality settings?

A possibility is that the VST (here, Pianoteq) is informed that it is not interesting to limit the number of voices depending of the CPU usage since the DAW will wait for the computation. I don't know if Pianoteq do use this information.

The bit-depth internally is at least 24 bits already, maybe even 32 bits??

Something different is, of course, that you can make a lot of settings in Reaper when it comes to exporting a recorded session.

Then you can even make it mp3 with whatever settings you like.

I think, setting WAV output with 48kHz/24 bits makes the best match, and that it does not get any better when using much higher settings when rendering.

Greetings,

Geert

Reaper has an option "Inform plug-ins of offline rendering state". I think other DAWs also have this option. Does this force pianoteq to use high-quality settings?