The need to re-create physical rolls from scans was always a key
part of the roll scanning initiative, and is not only possible but
is the normal way it's done these days.  It's a very specialised
transformation that only those of us with roll perforators need to do.

Modern software allows exact replication of the punch patterns of
the original roll, not an approximation as was previously the norm --
a massive improvement in accuracy.

MIDI files contain notes, their timing and their loudness (velocity).
Of these, the notes and timing are also necessary to describe a piano
roll.  So, it's possible to use the MIDI file format to store roll-
master data, the rollscanners 'punch master' MIDI file format.  Such
files are not immediately playable because every single perforation
in the roll is separately described.

The advantage of using MIDI is that it's a widely-known format so
that archival files can be read and interpreted later, if anybody else
wishes to do so.  MIDI text events can store details of the original
roll, who scanned it, and so on, and there are suggested standards
for doing this.

And of course, playable MIDI files can be made from these replica
roll masters.  They will always be more accurate than playable MIDI
files made without recreating the punch pattern, as long as the job
is carefully done with due regards to how a player plays a roll --
ignoring the bridges, accelerating as the paper builds up, and of
course using any punches that control dynamics.  It must be said
that this is rarely done, very rather time-consuming.

I wrote all this up many years ago, and it can be found on my
website, at http://www.pianorolls.co.uk/rollcopying.htm 

Julian Dyer

 [ Thanks, Julian, that's an excellent explanation.  -- Robbie