- the "voices" in the score, as in the first video below (Bach),
and - harmonic information, as in the second video below
(Chopin) Johann Sebastian Bach, Toccata and Fugue in D Minor:
Frederic Chopin, Etude, opus 10 #7: The harmonic coloration is
based on the circle of fifths, which is an interesting solution
to the problem that notes which are close together in pitch are
not close together in some sort of "harmonic space". I find
myself wondering if the coloring based on voice could be
automated. This would be trivial for things written entirely in,
say, standard four-part voice leading as taught in an
introductory music theory class, because there are always
exactly four notes at any given time and the voices don't cross;
it would be very nontrivial for actual music. (This isn't just a
musical problem, believe it or not. A related problem is as
follows: a baseball team has five starting pitchers, which it
uses in a pitching rotation: ideally pitcher n pitches on days
n, n+5, n+10, ... But there are off days