Re: glissando lines and accidentals

[Aaron Hill]

On 2020-08-11 5:05 am, Aaron Hill wrote:
> On 2020-08-11 3:43 am, Werner LEMBERG wrote:
> > Very nice, thanks!  The only thing now is to get the two lines
> > parallel, which looks like a lot of additional work...
>
> Oof... I am almost certainly overlooking something easier, but this
> appears to work:
>
> %%%%
> . . .
> %%%%

Wait.  How did that code even work?  Surely the change to 
end-on-accidental should happen in before-line-breaking before 
ly:line-spanner::calc-bound-info has been called.  I had to switch to 
after-line-breaking so that left-bound-info and right-bound-info were 
valid, but somehow even a seemingly late change to end-on-accidental 
makes a difference.  ??!

Here is a theoretically better version of the code, along with an 
improved test case so that the slopes are more easily compared:

%%%%
\version "2.20.0"

{
  \override Glissando.before-line-breaking =
  #(lambda (grob)
    (let* ((nh (ly:spanner-bound grob RIGHT))
           (acc (ly:grob-object nh 'accidental-grob)))
      (ly:grob-set-nested-property!
        grob
        '(bound-details right end-on-accidental)
        (ly:grob? acc))))
  \override Glissando.after-line-breaking =
  #(lambda (grob)
    (let* ((nh (ly:spanner-bound grob RIGHT))
           (acc (ly:grob-object nh 'accidental-grob)))
      (if (ly:grob? acc)
        (let* ((refp (ly:grob-common-refpoint nh acc X))
               (nh-xex (ly:grob-robust-relative-extent nh refp X))
               (acc-xex (ly:grob-robust-relative-extent acc refp X))
               (dx (- (interval-index acc-xex LEFT)
                      (interval-index nh-xex LEFT)))
               (lbi (ly:grob-property grob 'left-bound-info))
               (rbi (ly:grob-property grob 'right-bound-info))
               (left-x (ly:assoc-get 'X lbi))
               (left-y (ly:assoc-get 'Y lbi))
               (right-x (ly:assoc-get 'X rbi))
               (right-y (ly:assoc-get 'Y rbi))
               (max-x (- right-x dx)))
          (ly:grob-set-nested-property!
            grob
            '(right-bound-info Y)
            (+ left-y
               (* (- right-y left-y)
                  (/ (- right-x left-x)
                     (- max-x left-x)))))))))

  \time 6/1 \stemUp
  <a' f''>1\glissando <d' bes'>
  <a' f''>1\glissando <d' b'>
  <a' f''>1\glissando <d' bis'>
}
%%%%

-- Aaron Hill

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