Re: Changing clef automatically when pitch falls between a user-defined range of pitches

[Aaron Hill]

On 2018-10-18 7:21 am, elmeri wrote:
> Hey ML,
>
> I asked a similar question on the Music Practice & Theory Stack 
> Exchange
> site, and I was told that the idea of changing clef automatically is - 
> in
> most cases - a bad idea, at least musically/practically speaking.
> Nevertheless, I would like to ask for pointers as to how to accomplish 
> this
> with pre-existing LilyPond capabilities or just try to implement it 
> myself
> -- I am not at all familiar with the codebase.
>
> The ideal situation would be that there's some kind of \clefAutoChange 
> or
> similar command, and a mechanism to override the changing points would 
> also
> be provided. This also needs to work for \transpose'd music. For 
> example, a
> cellist told me that when there's a passage that lingers long enough 
> above
> E1 (but not too much above A1 or so), the music is usually written with 
> the
> tenor clef. A similar rule (of thumb) applies to viola music and its 
> use of
> the treble clef, bassoon music etc.
>
> Any pointers or thoughts on this?

Hi Elmo,

Virtually anything can be done with custom Scheme functions.  It sounds 
like what you want is something akin to how \relative takes care of 
octaves except for automatically inserting \clef commands in the 
specified music.

The challenge here is deciding upon the interface for the so-called 
"rules of thumb".  I would probably start by defining a global alist 
where a clef can be associated with a range of pitches.  This could look 
something like this:

%%%%
#(define auto-clef-pitch-range (list
  (cons (cons '(-1 5 0) '(1 0 0)) 'bass)
  (cons (cons '( 0 5 0) '(2 0 0)) 'treble)))
%%%%

Here, as an example, I am referencing specific pitches by the values one 
would pass to ly:make-pitch to define a range.  So the bass clef is 
associated with notes from a, to c' and the treble clef for notes from a 
to c''.

The idea is to provide some overlap in the ranges so as to not trigger 
clef changes too often.  So, as long as a note falls within the range, 
the current clef would be continued.  But once the next note is outside 
that range, then a better clef is inserted.

You could introduce some temporal tolerance (i.e. going outside the 
range for a short enough moment is permitted) and that will require 
looking ahead beyond simply the next note.  Similarly, if you only 
wanted clefs to change at the beginning of measures, so that the 
majority of the measure is within the range of the clef, that would be 
another approach to try to avoid excessive clef changes.

Finally, the usage could be something like this:

%%%%
\autoClef \relative g {
  g8 a b c b a g4 |
  g8 b d g fis g d4 |
  c8 b a g d b g4
}
%%%%

Which, assuming the above-mentioned pitch ranges but without any of the 
temporal restrictions, could translate to:

%%%%
\relative g {
  \clef bass g8 a b c b a g4 |
  g8 b \clef treble d g fis g d4 |
  c8 b a \clef bass g d b g4
}
%%%%

NOTE: I have not written any of the underlying Scheme.  This is just a 
mock-up, but I do not suspect that it would be too hard to write a 
function that can iterate some input music and insert \clef changes 
conditionally.

-- Aaron Hill