Hi, 

Great solution, thanks a lot!

cheers

Maurits


> On 2020-06-25 7:01 am, Maurits Lamers wrote:
> Hi all,
> 
> I am trying to build a system based on the listener system which can
> identify voices in a piece of music.
> I use a listener to the Voice context to give me the note events.
> 
> In the following passage I can use (ly:context-id
> (ly:translator-context engraver) to get the id, which is empty for c4,
> "1" for the first voice, "2" for the second voice, and empty again for
> d4 at the end.
> 
> time 4/4 \relative c' {  c4 << { c8 d e4 } \\ { c8 b c4 } >> d4 | }
> 
> 
> However in the following passage the context-id cannot be used because
> it is not set.
> 
> \relative c' {
>       \new Voice {
>         c4
>         <<
>           \new Voice {
>             c8 d e4
>           }
>           \new Voice {
>             c8 b c4
>           }
>         >>
>         d4
>       }
>     }
> 
> I am currently using object properties to follow these voices by
> setting a unique value for that voice context object, but that doesn't
> help me
> to detect properly the start and end of the polyphonic passage. The
> main voice gets 1, the first nested voice will be 2, the second nested
> voice will be 3.
> For transcription purposes it would be very helpful to detect the
> start and end of the polyphonic passage. Is this possible?
> The initialize and finalize procedures of an engraver will run when the 
> context in which it is \consisted begins and ends. Consider:
> %%%%
> \version "2.20.0"
> 
> #(define custom-uid
>   (let ((custom-uid-prop (make-object-property))
>         (last-uid 0))
>     (lambda (obj)
>       (or (custom-uid-prop obj)
>           (begin
>             (set! last-uid (1+ last-uid))
>             (set! (custom-uid-prop obj) last-uid)
>             last-uid)))))
> 
> handle-init-and-final =
> #(lambda (context)
>   (make-engraver
>     ((initialize engraver)
>       (let* ((ctxt (ly:translator-context engraver))
>              (id (ly:context-id ctxt))
>              (uid (custom-uid ctxt))
>              (now (ly:context-now ctxt)))
>         (format #t "\ninitialize: ~s, id=~s, uid=~s, now=~s"
>           ctxt id uid now)))
>     ((finalize engraver)
>       (let* ((ctxt (ly:translator-context engraver))
>              (id (ly:context-id ctxt))
>              (uid (custom-uid ctxt))
>              (now (ly:context-now ctxt)))
>         (format #t "\nfinalize: ~s, id=~s, uid=~s, now=~s"
>           ctxt id uid now)))))
> 
> \layout { \context { \Voice \consists \handle-init-and-final } }
> 
> { \relative c' { \new Voice { c4
>   << \new Voice = foo { \voiceOne c8 d e4 }
>      \new Voice { \voiceTwo c8 b c4 } >> d4 } } }
> %%%%
> 
> ====
> . . .
> Parsing...
> Interpreting music...
> initialize: #<Context Voice () >, id="", uid=1, now=#<Mom -infinity>
> initialize: #<Context Voice=foo () >, id="foo", uid=2, now=#<Mom 1/4>
> initialize: #<Context Voice () >, id="", uid=3, now=#<Mom 1/4>
> finalize: #<Context Voice=foo () >, id="foo", uid=2, now=#<Mom 3/4>
> finalize: #<Context Voice () >, id="", uid=3, now=#<Mom 3/4>
> finalize: #<Context Voice () >, id="", uid=1, now=#<Mom 1>
> Preprocessing graphical objects...
> . . .
> ====
> 
> NOTE: I am using Scheme object properties to attach a custom unique 
> identifier to the contexts as they are seen, so it is possible to discern 
> which context is which even when they are not named.
> 
> -- Aaron Hill
> 
> 

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