Re: [frogs] Another new tadpole?

[Carl Sorensen <c_sorensen@xxxxxxx>]

On 8/9/10 5:18 PM, "Trevor Daniels" <t.daniels@xxxxxxxxxxx> wrote:

> 
> 
> Wols Lists wrote Monday, August 09, 2010 9:30 PM
> 
> Let me say up-front I'm getting a little out of my depth
> now, having never written an engraver myself ... and
> Carl hasn't showed up yet.  And I'm not a guitar player,
> so I may be missing something obvious in my comments,
> But here goes - if anyone else can offer better advice
> please step in.

I'm back, but my Lilypond time will be extremely limited over the next three
weeks, so any help that others can give will be greatly appreciated.  And I
appreciate your helping out Wol and William in my absence.

> 
> 
>> On 09/08/10 00:05, Trevor Daniels wrote:
>>> 
>>> Yes, diving into the chords engraver is the next step.
>>> 
>>>  When you have a
>>> feel for what is happening you can begin to plan what you
>>> need to do.
>>> 
>> Okay. I've looked through chord-name-engraver.cc, and I think I've
>> understood roughly what's going on. It looks like the main thing
>> I've
>> got to change is Chord_name_engraver::process_music(). As far as I
>> can
>> see, this is building up a list of note pitches in the variable
>> "pitches" and then passing it to a function to convert it to a
>> chord name.
> 
> This sounds right to me.

Yes, this is so.

> 
>> So what I need to do, is create a new variable "capo_pitches", and
>> just
>> as pitches are assigned to "pitches", I need to transpose them and
>> assign them to "capo_pitches". I can then create "capo_markup",
>> which
>> will (if I've understood the code correctly) contain a text
>> description
>> of the transposed chord. So I now have - hopefully - "markup" ==
>> eg "Gm"
>> giving me "capo_markup" == "Em" if capo is 3. I now need
>> chord_name_->set_property("text", "markup ( capo_markup )" )

Not quite.  It's not a text description, but actually a markup description.
The property name is "text", but the result from calling the scheme
procedure stored in the context property chordNameFunction returns a markup..

Better I think to just have line 95 change to something like

SCM raw_pitch = n->get_property ("pitch");
SCM p = raw_pitch->transposed (Pitch (0, 0, capo_semitones));

Now you have p transposed, and everything should work properly, IIUC.

> 
> So far so good, but I'm beginning to wonder if this is not
> duplicating something...
> 
>> The other thing I need to do is write a new engraver ...
> 
> Hhm.  Why is that?  Would that be better than modifying
> the existing chord name engraver to be aware of capos (capi?).
> 

I'm guessing that you're wanting a capo_indicating_engraver to write the
statement that tells you what fret to put the capo on, and indicates the key
in the "capo" worldview.  I don't think you should need to do the key
signature engraver.  Something like the metronome engraver is probably a
better pattern.

>> I've tried to look at the key signature engraver, but that doesn't
>> seem
>> to be so simple (I'm not surprised ...). What I need to do is have
>> some
>> lilypond syntax "\capo 3 vertical|horizontal", with it defaulting
>> to
>> horizontal.

If you really need a new engraver, then you'll also need to create a new
event (a capoEvent) that both the Capo engraver and the ChordName engraver
will need to listen to.  You might want to consider Trevor's suggestion, of
just defining a new context property.

> 
> This sounds like a new context property set in the ChordName
> context and used by the ChordName engraver to decide whether
> to transpose and by how much.
> 
>> This will then need to get the key signature and print
>> (assuming a key of G minor, the example in front of me) "Capo 3
>> (Em)",
>> transposing the key down three semitones. It'll also need to leave
>> the 3
>> somewhere (along with vertical/horizontal), so the chord engraver
>> knows
>> what to do. Wherever it stores the transposition, should that
>> default to
>> null or 0?

No, you don't "leave the 3 somewhere".  You have an event that both the Capo
engraver and the ChordName engraver listen to.  Then the ChordName engraver
puts the 3 in the appropriate place.  And the Capo engraver creates the
appropriate output.

Just out of curiosity, what does vertical/horizontal do?


> 
> I'm getting puzzled now.  \transpose can already be invoked to
> change chord names, can't it?  What functionality does
> it lack that you need?  (Sorry if the answer's obvious - I'm not
> a guitar player.)  I can see the capo is not fully supported in
> fret diagrams.  AFAICS this currently has to be provided for every
> fret diagram.  But if you only want chord names perhaps a slightly
> different way of invoking \transpose would suffice?
> 
>> Okay. Where do I start with writing my new engraver? And have I
>> got my
>> changes to chord-name-engraver close? One of the things that gets
>> me
>> (dunno if it's common practice, it isn't to me) is the almost
>> complete
>> lack of comments in the code. What I'll probably do is write up
>> what I'm
>> doing as I do it, and heavily comment my engraver so that even if
>> the
>> comments don't make it into production code, they can make
>> documentation
>> as a "sample engraver".
> 
> Assuming you really do need a new engraver this is where
> I have to hand over to someone more competent.  But a commented
> engraver would be a useful addition to the CG.

I agree wholeheartedly.   The real problem (IMO) with engravers is that he
general structure of an engraver is not documented anywhere very well
(although we're adding some stuff to the CG).  Most of the comments that
would help to understand an engraver are unneeded once you understand an
engraver structure (e.g. we don't really want to have a comment in every
engraver describing what stop_translation_timestep (), process_music (),
etc. do -- that would just clutter up the code).  Once you understand
process_music (), you only really want comments related to the *specific*
procsess_for this engraver, and with good variable names, it's not really
that hard in most cases to figure most of it out.

But the overall structure is *very* hard to get figured out, in my
experience.

Thanks,

Carl


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