Re: Get default Gliss endpoint Y value and add intelligently a value on top of it

[Aaron Hill]

On 2021-03-19 12:30 pm, Dimitris Marinakis wrote:
> I don't like manually setting Y values of glissando endpoints. I'd
> appreciate a more thoughtful approach.
> Is it possible to reference the calculation Lilypond already does and 
> add a
> value to it without trying to guesstimate what the actual value is from
> scratch? The default placement is sometimes 95% fine but I'd like to 
> nudge
> it a bit upwards or downwards without tweaking the padding.
>
> This makes cross-staff glissando editing really annoying. If my 
> thinking
> makes sense, this would solve the problem partially since Lilypond will 
> do
> its best each time the staff spacing gets changed but the tweak will 
> help
> on top of that.

Normally, this would be a job for \offset.  However, \offset does not 
work with all properties, in particular, nested properties like the 
bound-details of the line-spanner-interface.

But you can effect the same change by using extra-dy and Y-offset with a 
little bit of arithmetic:

%%%%
\version "2.22.0"

offsetGlissando =
#(begin
  (define (number-or-number-list? arg)
   (or (number? arg) (number-list? arg)))
  (define ((indexed lst) grob)
   (list-ref lst (ly:grob-property grob 'glissando-index)))
  (define-event-function
   (left-dy right-dy)
   (number-or-number-list? number-or-number-list?)
   (if (number? left-dy) (set! left-dy (list left-dy)))
   (if (number? right-dy) (set! right-dy (list right-dy)))
   (let ((extra-dy (map (lambda (l r) (- r l)) left-dy right-dy))
         (Y-offset (map (lambda (l r) (/ (+ l r) 2)) left-dy right-dy)))
    #{ \tweak extra-dy #(indexed extra-dy)
       \tweak Y-offset #(indexed Y-offset)
       \glissando #})))

\paper { indent = 0 line-width = 10\cm}

{ f'2 \glissando c''2
  f'2 \offsetGlissando #-1 #-1 c''2
  f'2 \offsetGlissando #1 #1 c''2
  f'2 \offsetGlissando #-2 #0 c''2
  f'2 \offsetGlissando #2 #0 c''2 \break
  f'2 \offsetGlissando #0 #-2 c''2
  f'2 \offsetGlissando #0 #2 c''2 \bar "||"

  \override Glissando.before-line-breaking =
   #(lambda (grob)
     (ly:grob-set-property! grob 'color
      (list-ref (list black red blue)
                (ly:grob-property grob 'glissando-index))))
  <f' a' c''>2 \glissando <c'' g' e'>2
  <f' a' c''>2 \offsetGlissando #'(1 1 -2) #'(-1.5 1.5 0)
  <c'' g' e'>2 \bar "|." }
%%%%

The parameters to \offsetGlissando are delta-Y values for the left and 
right bounds respectively.  If there are multiple glissandi, then the 
arguments should be lists of numbers, each value corresponding to a 
particular glissando.  Shown above is an untangling of the crossing 
glissandi, which the coloring should make clearer.

P.S.  I am ignoring the possibility of broken glissandi, as they are not 
supported by default.


-- Aaron Hill

offset-glissando.cropped.png
Description: PNG image