Timing of events in the beginning of bars

[Han-Wen Nienhuys]

address@hidden writes:
> 
> Does this sound sensible to you developers? If it does I'll add it to the bug 
> cvs as a wish.


No, it doesn't sound sensible. suppose you have item A that should be
at grace = -inf and an item B that should be at grace = 0. What do you
do when you get

  { .. B A .. }

B must be processed before A, but A happens earlier.

The grace code works more abstract than what you're describing. The
real problem is as follows. Grace is interpreted at sequential level
(in sequential-iterator.cc). Assume that you have

<<{ c2 \grace e16 c2 }
  { c2            d2 }>>

then, the ds and cs should be aligned. To achieve this, the 1st half
note should start at t = 0, and the second at t = 1/2. Hence, the
grace note should start before t = 1/2, at (t=1/2, grace=-1/16), and
consequently, the 1st c2 in the 1st voice, should also _end_ at 
(t=1/2, grace=-1/16). To achieve this, Sequential_iterator maintains a
Grace_fixup: a substitution that says:

  Items ending at t=1/2 really end at 1/2, grace=-1/16.

This means that in

  { c2 \bar "|:" \grace e16 c2 }  

the "|:" also happens at 1/2,-1/16.

At present, the grace substitution is done per voice. So, in the
example above, in the 2nd voice

  { c2  d2 }

the 2 simply starts at t=1/2, and  in

  { c2 \bar "|:"  d2 }

the |: happens at t=1/2, which causes all the problems with grace
notes and clefs.

The solution that I see is to insert an implicit

  \grace s16

in any voice without a grace,  just before any element with non-zero duration
when there is another voice with grace notes. In other words

  { c2 \bar "|:"  d2 }

is interpreted as

  { c2 \bar "|:" \grace *as-much-grace-as-any-other-voice*
    d2 }


However, I don't see how I can get this to work.  Bear in mind that
getting grace notes to work correctly is pretty hard. Getting

  { c2 \grace e16 d2 }

to work is not so hard, but the problem is that it's recursive.  In a
construction like

  { {
      c2 \bar "|:"
    }
    << { \grace e16 d2 }
       { \grace e32 d4 }
    >>
  }

the c2 should still end at t=1/2,g=-1/16, and the |: should happen
together with the grace notes.

Grace notes seem to be working now, but it was the hardest thing I
ever did in Lily, and I still only understand half of how and why it
works.


--
 Han-Wen Nienhuys   |   address@hidden   |   http://www.xs4all.nl/~hanwen