Re: Issue 3945: fix (De)crescendo with unspecified starting volume in MIDI (issue 294700043 by address@hidden)

[ht . lilypond . development]

I understand from this patch that you are in favor of removing the
warning altogether, which - I agree - works very inconsistently in
current versions (as demonstrated by the examples in the issues about
the warning).  As you yourself have noted on a discussion attached to
issue 4104, however, the behavior used to be different in LilyPond
2.16...


It appears that the changes in behavior (the introduction of the warning
as part of issue 2707, and the changes that made the behavior to I what
think is still current, see issue 3234) are a result of code changes by
developers with opposite views about how serious a problem it is to
start a new Voice without explicitly specifying an explicit MIDI volume
for the first note inside the Voice.

As I am to blame for originally adding the warning to the code, let me
explain here why I did it :-)

During my initial learning phase on using LilyPond I spent a lot of time
transcribing music with LilyPond in order to produce MIDIs (for my choir
practice).  However, on more than one occasion, after successfully
finishing a piece (with orchestral parts and everything, including all
expressive marks for the typeset output, and keyboard parts written in a
happy and carefree way using lots of temporary Voices and simultaneous
music without really understanding the MIDI implications of using \\
between simultaneous music expressions), rendering the LilyPond input to
MIDI gave surprising and even disappointing results - at the boundaries
of Voices, there could be sudden jumps in the MIDI volume without any
explicit dynamic markings in LilyPond input.

After learning more about LilyPond contexts, I eventually came to
realize that the observed behavior was actually not a bug, but was
caused by LilyPond's default context settings where every new Voice
context has its own Dynamic_performer (which does not inherit any
properties - such as the "current" note velocity level - from its
syntactically enclosing Voice context, since Voice contexts, even if
syntactically nested, do not nest in the default LilyPond context
hierarchy).  Therefore, a Voice context syntactically enclosed within
another Voice context will have its own Dynamic_performer with the
initial note velocity in MIDI possibly set to something different from
what could be expected by looking at the typeset output, where Voice
contexts are not distinguishable in an unambiguous way in the general
case.

This realization then led to a lot of additional work on refactoring all
of the write-only LilyPond code that I had managed to produce to get rid
of the temporary Voice contexts (or in some cases move the
Dynamic_performer from the Voice context to the Staff context), and a
rule of thumb to make sure that every Voice context always begins with
an explicit MIDI dynamic.  To try to help myself (and others) avoid
making the mistake of creating new Voices without an explicit initial
MIDI dynamic in the future, I thought it useful to add a warning to the
code to alert about cases where the generated MIDI output might fail to
match the user's expectations (similar to the warnings that had already
been given about "Impossible (de)crescendos in MIDI").  (However, at
this point I can't explain at all why I then decided to focus only on
(de)crescendos - I think it's just as bad to begin a new Voice with just
a single note without a dynamic indication if one cares so much about
MIDI volume.)

It appears that the author of the patch for issue 3234, after having
apparently discovered the same above feature of Voice contexts,
approached the handling of initial MIDI velocity in new Voice contexts
from a different angle - instead of giving warnings about cases where
the MIDI velocity might get "reset to the default value" upon the
creation of a Dynamic_performer (which could result in MIDI volume
changes failing to match the user's expectations, so the user should
probably check them in the source), the goal of the patch appears to
specifically have been to make the Dynamic_performer silently use
default velocity values in more scenarios than it used to.  I believe
this patch is the reason for the change in behavior between 2.16 and
2.18.


After following the discussion on the mailing lists in the years after
writing the patch, I've since come to the conclusion that many people
end up being just confused by this warning, and most users do not see -
or even care about - the subtleties of keeping in full control of the
MIDI volume when crossing Voice boundaries.  Even though I'd hoped there
to have been more users like myself who find these kind of details
important, not caring about such details is of course understandable:
after all, it's not likely to be possible for any computer program that
only follows simple fixed rules to produce a good interpretation of the
dynamic markings of an arbitrary piece of typeset music automatically
anyway.  Since I have personally also learned to always take extra
precautions with Voices if I need to care about MIDI velocity, I won't
mind if the warning gets removed from the code.

(With the warning removed, what I'd nevertheless like to have is a
warning in the documentation about the implications of creating
temporary Voices on MIDI velocity, maybe already in the section where
temporary voices are introduced.  Writing such a patch for documentation
has been for several years on my to-do list, but I just never seem to
find the time to write one...)


https://codereview.appspot.com/294700043/