Re: Getting \tempo params

[David Kastrup]

Paolo Prete <paolopr976@gmail.com> writes:

> On Thu, Oct 21, 2021 at 2:01 PM David Kastrup <dak@gnu.org> wrote:
>
>> Paolo Prete <paolopr976@gmail.com> writes:
>>
>> > On Thu, Oct 21, 2021 at 1:22 PM David Kastrup <dak@gnu.org> wrote:
>> >
>> >> Paolo Prete <paolopr976@gmail.com> writes:
>> >>
>> >> > Hello,
>> >> >
>> >> > after a \tempo X = Y is set, is there a way (a scheme function or
>> >> > variable?) to get X and Y?
>> >>
>> >> What does "after" mean and "is set"?  At a later point in the source
>> >> text, at a later time in musical execution?
>> >>
>> >>
>> > yes
>>
>> Those are two different things entirely.  "yes" as an answer is not
>> suitable.
>>
>
>
> Sorry, I used a bad quote of the text. With "yes" I mean "at a later point
> in the source text".

And then you go on to give an example where clearly "at a later time in
musical execution" is desired.

>> > unfortunately this doesn't sound trivial to achieve, and I presume
>> > that tempoWholesPerMinute is not in the API.
>>
>> I have no idea what you mean by "is not in the API".  What is the API
>> according to your definition?
>>
>>
> API is the public interface exposed to the user. I assumed that
> tempoWholesPerMinute was internal stuff,


> but I just checked that it IS in the API:
> https://lilypond.org/doc/v2.22/Documentation/internals/tempo_005fperformer

Given that the manual is called "Internals", you may need to adjust your
terminology to that other LilyPond users work with.

> Anyway, I don't know if it fits what I need. More specifically, I'm
> trying to obtain in the midi output the behavior of growing beams
> (Beam.grow-direction). For example, a sort of accelerando from a fixed
> metronome (which is not the previous metronome mark).  After the beam,
> the metronome must be reset to the initial value.  For example:
>
> %%%%%%%%%%%%%%%%%%%
> {
>
> \tempo 4 = 120
>
> c'4
>
> \override Beam.grow-direction = #RIGHT
> %start from \tempo 4 = 40
> c'16[
> % \tempo 4 = 40 + 20%
> c'
> % \tempo 4 = 40 + 40%
> c'
> % \tempo 4 = 40 + 60%
> c']
>
> %reset \tempo to 4 = 120
> c'4
>
> }
> %%%%%%%%%%%%%%%%%%%
>
> I already wrote a Scheme function that increases a tempo by 20% for each
> note grouped by the beam, starting from a chosen value, but I don't know
> how to reset the tempo after the beam.

Here's a cut&paste of something I currently use in scores of mine for
doing things like accelerando, ritardando, and so on.

tempoChange =
#(define-music-function (interval endscale thenscale music)
   (ly:duration? scale? (scale? 1) ly:music?)
  "Make a gradual tempo change over @var{music}, essentially changing speed 
after
every duration of @var{interval}, approaching a factor of speed of 
@var{endscale}
compared to the start.  Afterwards, tempo is switched to @var{thenscale} of the
original speed (default 1).  If @var{thenscale} is 0, the speed reached at the
end is just maintained and can be overriden with an explicit @samp{\\tempo}
command if required."
   (define (scaletempo oldscale newscale)
     (make-apply-context
      (lambda (ctx)
        (set! (ly:context-property ctx 'tempoWholesPerMinute)
         (ly:moment-mul (ly:context-property ctx 'tempoWholesPerMinute)
                 (ly:make-moment (/ newscale oldscale)))))))

   (let* ((muslen (ly:moment-main (ly:music-length music)))
          (intlen (ly:moment-main (ly:duration-length interval)))
          (steps (/ muslen intlen))
          (endfactor (scale->factor endscale))
          (thenfactor (scale->factor thenscale)))
     (make-simultaneous-music
      (list music
            (context-spec-music
             (make-sequential-music
              (let loop ((rsteplst (iota (1+ steps) endfactor (/ (- 1 
endfactor) steps)))
                         (res (if (positive? thenfactor)
                                  (list (scaletempo endfactor thenfactor))
                                  (list))))
                (if (null? (cdr rsteplst))
                    res
                    (loop (cdr rsteplst)
                          (cons* (scaletempo (cadr rsteplst) (car rsteplst))
                                 (make-skip-music (ly:make-duration 0 0 intlen))
                                 res)))))
             'Score)))))

See if that helps you achieve your objective.

-- 
David Kastrup