Re: Understanding Lilypond

[Urs Liska]

Am 16.01.2015 um 13:35 schrieb David Sumbler:
> As I start to gain experience in setting music in Lilypond I am trying
> to understand more about how it works internally.  As well as personal
> satisfaction, this obviously has a practical aim: it will make it easier
> for me to modify or correct things without having to ask so many
> questions on this forum, and will also perhaps eventually mean that I
> can help by answering others' questions.
>
> However, despite having read the documentation - some of it several
> times - I do find understanding some aspects of the structure of
> Lilypond extremely difficult.  One of the manuals likens a Lilypond file
> to source code in a computer language, but I find that understanding the
> structure of a coding language is perfectly straightforward compared to
> getting my head around Lilypond.  (I have learnt several languages over
> the years, although not, I admit, Lisp or Scheme; however, I have no
> reason to suppose that understanding their structure is any more
> difficult than other languages).
>
> For instance, in Lilypond there is a sensible difference in the default
> handling of time- and key-signatures.  Using the \key command a key is
> defined for the current Staff.  But using the \time command sets the
> time signature for every staff.  If a different time signature is
> required for a particular staff, then timeSignatureFraction has to be
> changed.
>
>  From the Internals Reference I see that the 2 layout objects
> KeySignature and TimeSignature both exist, by default, in a Staff
> context, which makes perfect sense.

Yes, that makes sense because they are used to *engrave* the visual 
objects at staff level.

> Clearly, though, when the \time command is used, then not only is
> Staff.timeSignatureFraction set, but so also is some other variable in a
> higher context.

The timing aspects are controlled by the Timing_translator, and this 
engraver by default lives in the Score context.
Which also makes perfect sense because usually the timing is uniform in 
a score from top to bottom.

But this construct is what makes polymetrics so easy to do in LilyPond. 
You can move the Timing_translator from the Score to Staff level to get 
independent timing in the different staves.
But you should not forget to also move the Default_bar_line_engraver 
that should live at the same level as Timing_translator.

With key signatures it is much more common that different instruments 
*show* different keys.
Nevertheless it *is* sort of an inconsistency that you *always* have to 
specify the key for each staff separately (or in a global variable, 
which is not much better). It would be more consistent to have the key 
also live in the Score context by default and give the ability to 
sepcify contexts with different keys through their context properties.
I think this has been discussed recently.

HTH
Urs

> What I can't seem to find (although it may well be in the documentation
> somewhere) is a clear explanation of this.  Can somebody point me in the
> right direction?
>
> David
>
>
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