Re: What is a voice context?

[Urs Liska]

Am 23.04.2015 um 18:37 schrieb Carl Sorensen:
> On 4/23/15 12:02 AM, "Urs Liska" <address@hidden> wrote:
>
> > Am 23.04.2015 um 06:07 schrieb Carl Sorensen:
> > > So now I'm going to jump in on what I think a Voice context is.
> > >
> > > A Voice context is a context, which means it can contain engravers,
> > > performers, and music expressions.
> > > ...
> > >
> > > I'd be happy to try to answer any questions others might have about a
> > > Voice.
> > >
> > > Thanks,
> > >
> > > Carl
> >
> > I think this is a good explanation, but it requires some prerequisites
> > (and that is one of the points I find very hard on the LilyPond learning
> > curve: There are so many things where you have to understand other
> > things before, and these again have there required pre-understanding
> > a.s.o. Sometimes you get the feeling there are short-circuits of the
> > kind that in order to understand a part of documentation you'd have to
> > have that much knowledge that you don't need that documentation anyway).
> >
> > What is a context?
> > I mean, when I run LilyPond, when does a context "enter" and what is its
> > purpose?
>
> I think I answered that in my other thread:
> https://lists.gnu.org/archive/html/lilypond-user/2015-04/msg00837.html

Yes, I'm sorry I had only read that later (so much going on 
simultaneously ATM ...)

> "Contexts are LilyPond objects that contain properties, music expressions,
> and translators.  Some contexts, like Score and Staff, can contain other
> contexts as well.  Other contexts, like Voice, are bottom contexts and
> cannot contain other
> contexts.
>
> Contexts are responsible for controlling the output (printed or midi) of
> the music they contain.  They do this by calling translators in the
> environment of the context properties."
>
>
> > What is an engraver?
> > When reading this word I think of a human with a certain profession.
> > When I try to translate this to LilyPond I think of some module that
> > "prints" items.
>
> An engraver is an object that is responsible for creating printed output
> of a certain type as controlled within a context.
>
> The way an engraver works is to listen for events and create grobs based
> on relevant events.  When it creates a grob, it announces that creation to
> the context.
>
> The events that engravers listen for can be either stream events or grobs
> announced by other engravers.
>
> I think these details of how engravers work are necessary for developers,
> but not for users.

Not necessarily. There is a certain point where you *could* be able to 
write your own solutions. I mean, writing an engraver is essentially 
just one step beyond writing a complex music function. But as it stands 
it is very difficult to get the information together to do that. 
Recently I had some success building upon functions that I received from 
David Nalesnik as ready-made engraver stubs. But I wouldn't have ever 
had a chance to start them from scratch.

Which is directly related to your effort of gathering an explanation of 
the mental model ...

Best
Urs

> > But as far as I've learned recently an engraver is first an entity that
> > waits for items to be passed to it from the "context" and that can then
> > do something with this item, e.g. printing it, modifying it before
> > printing, writing the event to a logfile or sending an email on its
> > behalf.
> > I'm still not really sure about how that fits into the architecture.
> >
> > What is a performer?
>
> A performer is an object that creates MIDI output, by following the exact
> same process an engraver follows to create printed output.
>
> HTH,
>
> Carl


--
Urs Liska
www.openlilylib.org