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

[lilypond]

Comment #58 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

I think I've found the nature of the problem.  I did a run of the regtests  
with a bunch of messages printed to std:err and it looks like every time  
there is a divergence, the order in which the grobs are read changes.  This  
comes from sorting grobs by pointers and then running ::uniq.  In theory  
this shouldn't matter, but in practice...

1) LilyPond requests pure height from grob X.
2) Grob X's pure height is somehow dependent on grob Y.
3) Depending on how a vector has been pointer-sorted...
  3a) Grob Y's actual offset has not yet been calculated because of the way  
a given vector is sorted, in which is pure offset is returned.
  3b) Grob Y's actual offset has been calculated because it was triggered  
first and stashed in dim_cache_. Grob::pure_relative_y_coordinate will  
always return this as opposed to the pure value.

So there you have it.  One way to solve this may be to disallow  
Grob::pure_relative_y_coordinate from ever returning something out of  
dim_cache_.  This has no bearing on the caching in item.cc, so it shouldn't  
result in too much of a slow-down.  I'll run regtests on a patch doing  
this...