LilyPond: how do I visualize Bezier control points?

Question

More often than not, the LilyPond score's final touch requires meticulous slur adjustment. It would be nice to see the control points while modifying the shape. I noticed a special engraver to accomplish this task, Show_control_points_engraver. However, when I added it to the score context, nothing changed. What am I missing?

Here is my code:

\version "2.24.3"
\language "italiano"

global = {
    \key reb \major
    \time 9/8
}

right = \new Staff = "right" {
    \global
    \relative do'' {
        \shape #'((0 . 0) (0 . 0) (0 . 0) (0 . 0)) PhrasingSlur
        do8 \(sib do sib mib sib lab sib lab\)
    }
}

left = \new Staff = "left" {
    \global
    \relative do' {
        <<{mib2.}\\{reb2.}>>
        <do mib>4.
    }
}

\score {
    \new PianoStaff {
        <<
            \right
            \left
        >>
    }
    \layout {
        \context {
            \Score
            \consists Show_control_points_engraver
        }
    }
}

This is the result:

Answer 1

This whole thing is quite undocumented. This engraver does not need to be added, it is already on by default. But it will only act if the show-control-points property is true (see here https://lilypond.org/doc/v2.24/Documentation/internals/bezier_002dcurve_002dinterface).

So you need to either override or tweak that property for the PhrasingSlur:

do8-\tweak show-control-points ##t -\(

Note that there is a \vshape command which does the same as \shape, but will also tweak this parameter to #t. So in your case you’d simply need to replace \shape by \vshape.