is the Piano Sympathetic Resonance real if there are no special samples that represent the "singing" corresponding non-struck strings? I guess the "singing" strings notes are just samples who were original played by an hammer and not by vibration. To make it more real would it be necessary to record notes generated by vibration and not by hammers?
"[..]The principle is simple: when a key is pressed on an acoustic piano, the strings for the keys which are already pressed vibrate sympathetically. The fundamental and overtones of the struck string generate vibrations in the corresponding non-struck strings that start to "sing". Every note played is going to excite all the other open strings and change their overtones. Previously every note on a sampled piano always used to stand alone and play the same every time.
Sympathetic resonance in PMI piano libraries is achieved by using scripts. These scripts have an interface which allows ultimate control over each generated harmonic as well as integration with sustain, sostenuto and soft pedals.
[..]" (excerpt from http://www.record-producer.com/learn.cfm?a=2602)
Yes. The samples used for sympathetic resonance are the same as played by hammers, but I think Kornel's script don't use the 'attack' of the samples, so you only hear the vibration of the string. And the volume of all overtones excited are accurately adjust. So, the realism is amazing.
It's only my opinion about how I think it works for what I listen. Anyway, Kornel is the only one who gives you the right answer.