Re: Notes on wrong side of stem in triads

[Paul]

On 10/18/2016 12:23 PM, David Kastrup wrote:

> Mark Knoop <address@hidden> writes:
> > At 17:05 on 18 Oct 2016, Davide Liessi wrote:
> > > I reproduced the issue with this:
> > >
> > > \version "2.19.47"
> > > \layout { #(layout-set-staff-size 19) }
> > > { <c'' e'' g''> }
> > >
> > > Using #(set-global-staff-size 19) instead works as expected.
> > > I couldn't find a bug report for this.
> > > Any ideas?
> > Very weird, and certainly a bug. This works as expected also:
> >
> > \version "2.19.47"
> > \layout { #(layout-set-staff-size 19.000000000001) }
> > { <c'' e'' g''> }
> Well, _that_ looks like the distance between noteheads is used as a
> factor for deciding about the arrangement in a manner that is
> numerically fragile.  Like looking whether their distance is smaller
> than 1.0 staff line distances.

Indeed, the distance between note heads is calculated (via calls to 
Staff_symbol_referencer::get_position for each note head) and that 
distance determines whether a note head is moved to the other side of 
the stem.  See stem.cc line 553 and following where "dy" is the distance 
between note heads:
http://git.savannah.gnu.org/cgit/lilypond.git/tree/lily/stem.cc#n553

I tried editing line 565 (the comparison involving "dy" that leads to 
moving a note head) and if you remove the 0.1 "safety margin" this fixes 
the problem (while also breaking the placement for some notes that are 
only a second interval apart).  So it does appear to be a mysterious 
problem of numerical fragility.

Some weirdness: if you add \stemUp or \stemDown this fixes the problem, 
but only for chords where the command reverses the default direction of 
the stem.  (!?)

Also, the problem (at size 19 at least) seems to only happen for notes 
at certain staff positions, for example on treble staff, notes at E 
line, A space, C space, and F line (symmetrical pattern around center / 
zero point of staff).

That's as far as I've gotten.  (Won't have time to do any more for at 
least a week.)

-Paul