\version "2.17.3"
% ulLibrary/curvesToolbox/debugCurves.ily


#(define (make-cross-stencil coords)
 (ly:stencil-add
   (make-line-stencil 0.1 (- (car coords) 0.2) (- (cdr coords) 0.2)
  (+ (car coords) 0.2) (+ (cdr coords) 0.2))
   (make-line-stencil 0.1 (- (car coords) 0.2) (+ (cdr coords) 0.2)
  (+ (car coords) 0.2) (- (cdr coords) 0.2))))

#(define (display-control-points)
 (lambda (grob)
   (let* ((grob-name (lambda (x) (assq-ref (ly:grob-property x 'meta) 'name)))
          (name (grob-name grob))
          (stil (cond ((or (eq? name 'Slur)(eq? name 'PhrasingSlur))(ly:slur::print grob))
                      ((eq? name 'Tie)(ly:tie::print grob))))
          (cps (ly:grob-property grob 'control-points)))

          (ly:stencil-add stil
            (ly:stencil-in-color
              (ly:stencil-add
                (make-cross-stencil (second cps))
                (make-cross-stencil (third cps))
                )
            1 0 0) ; color is hard-coded here (R G B).
                   ; modify to change the color of the crosses
 
          (ly:stencil-in-color
            (ly:stencil-add
            (make-line-stencil 0.05 
                (car (first cps)) (cdr (first cps))
                (car (second cps))  (cdr (second cps)))
            ;(make-line-stencil 0.05 (car (second cps)) (cdr (second cps)) (car (third cps))  (cdr (third cps)))
            (make-line-stencil 0.05 
                (car (third cps)) (cdr (third cps))
                (car (fourth cps))  (cdr (fourth cps)))
            )
            1 0 0) ; color is hard-coded here (R G B).
                   ; modify to change the color of the lines
          empty-stencil)
     )
     ))

% The following functions work in the current context
% So you can place it in the music input and modify the
% following music.

% Switch on the display of control-points for the current Voice
displayControlPoints = {
  \override Slur #'stencil = #(display-control-points)
  \override PhrasingSlur #'stencil = #(display-control-points)
  \override Tie #'stencil = #(display-control-points) 
}

%FIXME: This doesn't seem to work
% How can I set it back to normal behaviour?
% 
% Switch the display of control-points off again
hideControlPoints = {
  \revert Slur #'stencil 
  \revert PhrasingSlur #'stencil
}

% display the control-points of the following curve
% (Slur, PhrasingSlur or Tie).
% This is probably the most common function,
% place immediately before the start of the curve
displayControlPointsOnce = {
  \once \override Slur #'stencil = #(display-control-points)
  \once \override PhrasingSlur #'stencil = #(display-control-points)
  \once \override Tie #'stencil = #(display-control-points) % commenting this out gives an error
}

% Switch on the display of the control-points globally
% Place in a \layout block
debugCurvesOn = \layout {
  \context {
    \Score
    \override Slur #'stencil = #(display-control-points)
    \override PhrasingSlur #'stencil = #(display-control-points)
    \override Tie #'stencil = #(display-control-points) 
  }
}

testDebugCurves = {
  \override Slur #'stencil = #(display-control-points)
  \override PhrasingSlur #'stencil = #(display-control-points)
  \override Tie #'stencil = #(display-control-points) 
}