Re: [Lilypond-auto] Issue 3669 in lilypond: vertical spacing messed up when trying to access the grob stencil for a cross-staff glissando

[lilypond]

Updates:
        Labels: -Type-Ugly Type-Enhancement Needs-design

Comment #1 on issue 3669 by address@hidden: vertical spacing messed up  
when trying to access the grob stencil for a cross-staff glissando
http://code.google.com/p/lilypond/issues/detail?id=3669

Internals Reference 3.2.45 says:
"The properties after-line-breaking and before-line-breaking are dummies  
that are not user-serviceable."

On the other hand, Notation Reference 2.9 says:
"The after-line-breaking callback contains the Scheme procedure that is  
called after the line breaks have been determined and layout objects have  
been split over different systems."
and proceeds to change a setting of the grob (and then call the default  
procedure, if one was used for that type of grob).

At this point in time, immediately after line-breaking,
you can change the settings the LilyPond uses to draw or position the  
graphical object,
but the object would not normally have its stencil drawn at this time.

The cross-staff-glissando normally waits until later, after staves are  
spread on the page, to create its stencil.  It seems that calling the  
stencil-drawing function early causes LilyPond to space staves too early,  
which seems to cause other things go to be done wrongly because the second  
staff overlaps the markup that was intended to go below.

It is unlikely that LilyPond will be able to support requests to generate  
stencils of lines that join staves at arbitrary times in processing.
Marked needs-design to get another opinion on whether there is a feasible  
design.  If not, it should be Invalid.

(Looking at the email thread, the desire was to take LilyPond's stencil and  
modify it at the PostScript level.  In that case, you do not want to do the  
action immediately after-line-breaking.  A Scheme programmer can probably  
override the 'stencil command itself with a wrapper function.  Then  
LilyPond will call the wrapper when it is ready to draw the Glissando, you  
can first call the default ly:line-spanner::print, modify what it returns,  
and return the modified stencil to be printed.)


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