Re: [frogs] Chord-name-engraver plus capo - pitches

["Trevor Daniels" <t.daniels@xxxxxxxxxxx>]

Wols Lists wrote Friday, August 27, 2010 12:42 AM


> Next glitch - I'm trying to actually do the transpose ... and I 
> don't
> understand this Rational stuff :-(
>
> The code is below:
>
>      if (capo) {
>        Pitch *p = unsmob_pitch (n->get_property ("pitch"));
>        Pitch orig = p->transposed (Pitch (0, 0, -capo));
>        capo_pitches = scm_cons ( orig.smobbed_copy (), 
> capo_pitches);
>      }
>
> In my code, capo happens to be set to 3 at the moment. In other 
> words, I
> want to transpose down by three semitones. I've already discovered 
> that
> the first argument to Pitch( 0, 0, 0) takes it down three octaves. 
> The
> second takes it down three notenames. So I'm guessing that I need 
> to
> pass something in the third argument. The only problem is, what? 
> capo is
> an int and the above code simply bombs. I've found rational.hh, 
> but my
> C++fu isn't up to understanding what on earth is going on.
>
> So, simply put, how do I describe 3 semitones as a rational, so I 
> can
> get my transposition to work?

Good files to study:

lily/pitch.cc
lily/include/pitch.hh
flower/rational.cc
flower/include/rational.hh

You'll see Pitch::transpose (Pitch delta) is a private
method of the Pitch class, called by Pitch::transposed(d).
Like you, I think this is what you need.

The delta is a pitch, and the alteration is a rational
specified in tones (normally used for sharps and flats)
so I'd guess you'd use

transposedPitch = originalPitch->transposed (Pitch (0, 0, Rational 
(-capo/2)));

Caveat: untried and remember I'm also a novice at this.

Trevor



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