Re: accessing accidentals with \tweak

[Carl Sorensen]

On 7/14/09 2:15 PM, "Mark Polesky" <address@hidden> wrote:

> 
> 
> In NR 5.3.4 "The \tweak command", it says this:
> 
> Notably the \tweak command cannot be used to modify stems, beams
> or accidentals, since these are generated later by note heads,
> rather than by music elements in the input stream.
> 
> http://kainhofer.com/~lilypond/Documentation/user/lilypond/The-tweak-command.h
> tml
> 
> But I *am* able to abuse the notehead's stencil property to tweak
> the accidental:
> 
> \version "2.13.2"
> 
> #(define (color-accidental grob)
>   ;; notehead stencil callback
>   (let ((stil       (ly:note-head::print grob))
>         (accidental (ly:grob-object grob 'accidental-grob)))
>     (ly:grob-set-property! accidental 'color red)
>    stil))
>   
> #(define (suppress-accidental grob)
>   ;; notehead stencil callback
>   (let ((stil       (ly:note-head::print grob))
>         (accidental (ly:grob-object grob 'accidental-grob)))
>     (ly:grob-suicide! accidental)
>    stil))
> 
> \relative {
>   <c! \tweak #'stencil #color-accidental e! g!>
>   <c! \tweak #'stencil #suppress-accidental e! g!>
> }
> 
> Questions:
> 
> 1) are the results of this technique undefined? Was it just luck
>    that these worked?

This is well-defined.  If you do a \displayMusic of your expression, you'll
see that the 
procedure gets put into the parse tree.

(make-music
  'EventChord
  'elements
  (list (make-music
          'NoteEvent
          'duration
          (ly:make-duration 2 0 1 1)
          'force-accidental
          #t
          'pitch
          (ly:make-pitch -1 0 0))
        (make-music
          'NoteEvent
          'duration
          (ly:make-duration 2 0 1 1)
          'tweaks
          (list (cons (quote stencil) color-accidental))
          'force-accidental
          #t
          'pitch
          (ly:make-pitch -1 2 0))
        (make-music
          'NoteEvent
          'duration
          (ly:make-duration 2 0 1 1)
          'force-accidental
          #t
          'pitch
          (ly:make-pitch -1 4 0))
        (make-music
          'SlurEvent
          'span-direction
          1)))

So color-accidental is going to get evaluated and the result passed as the
stencil of the note.
>   
> 2) This seems so kludgy. I presume I can only access the
>    NoteHead's 'accidental-grob property through its standard
>    settings (duration-log, stem-attachment, stencil, X-offset, and
>    Y-offset). I wish I could do something like this instead (this
>    of course doesn't work) ---
>   
>    #(define (color-accidental grob)
>      ;; notehead foo callback
>      (let ((accidental (ly:grob-object grob 'accidental-grob)))
>        (ly:grob-set-property! accidental 'color red)
>        #t))
> 
>    \relative {
>      <c! \tweak #'foo #color-accidental e! g!>
>    }
> 
>    This would imply that I could make up my own dummy property and
>    it would (magically) have access to the note-head-interface
>    internal properties. Then I wouldn't have to bother making sure
>    I return the correct value for whatever standard setting I'm
>    abusing (stencil in the above case). Besides, I'd want to be
>    able to tweak that standard setting independently. As a
>    compromise, I considered the possibility of actually
>    implementing a 'dummy property, but I don't think that would
>    work either, because each subsequent call to \tweak #'dummy
>    would override the previous one (for a given note).
>   
> This bizarre approach might solve a lot of problems for me, but I
> don't like abusing properties like this. What do you guys think?

If you look at the IR, you'll see three dummy grob properties that are used
to trigger callbacks.

What if a property like tweak-callback were added, whose job is just to
trigger the grob callback necessary for what you want to do?

Then you could have

\relative {
  <c! \tweak #'tweak-callback #color-accidental e! g!>
}

I think you could have multiple calls to #'tweak-callback, because
all of the tweaks are stored in a list, and would all be executed.

\displayMusic
\relative {
  <c  
    \tweak #'color #red
    \tweak #'color #yellow
  d>  
}


gives

(make-music
  'RelativeOctaveMusic
  'element
  (make-music
    'SequentialMusic
    'elements
    (list (make-music
            'EventChord
            'elements
            (list (make-music
                    'NoteEvent
                    'duration
                    (ly:make-duration 2 0 1 1)
                    'pitch
                    (ly:make-pitch 0 0 0))
                  (make-music
                    'NoteEvent
                    'duration
                    (ly:make-duration 2 0 1 1)
                    'tweaks
                    (list (list (quote color) 1.0 0.0 0.0)
                          (list (quote color) 1.0 1.0 0.0))
                    'pitch
                    (ly:make-pitch 0 1 0)))))))


Personally, I think this is a cool idea!

Carl