Re: [Lilypond-auto] Issue 2801 in lilypond: Patch: Approximates cross-staff slurs in VerticalAxisGroup vertical-skylines.

[lilypond]

Comment #69 on issue 2801 by address@hidden: Patch: Approximates  
cross-staff slurs in VerticalAxisGroup vertical-skylines.
http://code.google.com/p/lilypond/issues/detail?id=2801

It is more complicated than that for a couple reasons:

1) If grobs with outside staff priority are placed above the cross-staff  
slur (like a tempo indication), they will need to be re-placed as well.
2) If the cross staff slur happens on the interior of a system (say from  
staves 2 to 3 in a 4-staff system), then the staves on either side will  
need respacing.  Furthermore, if another cross-staff slur exists from staff  
1 to 4, it will depend on the height of the interior one.

Neither of these are prohibitively difficult but they require a  
sophisticated sort of rewinding mechanism so that the solutions to the  
spring-and-rods problem used in Page_layout_problem::find_system_offsets  
can become the new heights off of which a new round of calculations are  
based.  Objects whose Y-offsets depend on the slur (stuff with  
outside-staff-priority or stuff with avoid-slur set to outside or around  
come to mind) and objects that depend on these objects need to have their  
offsets and extents wiped, their original callback functions restored, and  
the calculations redone.  Of course, if any of this becomes more  
complicated through user tweaks (i.e. if the color of a grob depends on the  
extent of a cross-staff-slur) these need to be redone as well.  So a system  
would need to be put in place where property-dependency was completely  
accounted for and any property dependent on the Y-offset of a cross-staff  
grob could be reset.

You'd then have to run this however many times such that the values in the  
solution_ vector of the Page_layout_problem diverge by less than epsilon  
between successive runs.  Of course this risks becoming cyclic, so LilyPond  
would need to perhaps say that if after X runs the spacing hasn't been  
found, use the widest spacing generated so far.