Re: Help on handling both rests and notes in one engraver

[Carl D. Sorensen]

On 5/9/09 1:58 PM, "Neil Puttock" <address@hidden> wrote:

> 2009/5/9 Carl D. Sorensen <address@hidden>:
>> Over the past few months, several people have asked for a "N.C." notation to
>> be part of the ChordNames context.
>> 
>> It occurred to me that using r to generate a "N.C." would be logically
>> consistent -- the music gets a rest, the chord name gets a "N.C.".  And if a
>> user wants to put space in without generating a "N.C." chordname, a spacer
>> rest would work.  Of course, there would also be a property to disable the
>> N.C. notation.
> 
> If you have a property (like chordNameSeparator) which returns a
> markup, you won't need another property to disable the `N.C.: you can
> simply return without creating a ChordName if the property isn't set.
> 

Right, I could create a property like noChordSymbol, if it is #f, then we
don't print anything for a rest.

>> 
>> I put the following code in:
>> 
>> IMPLEMENT_TRANSLATOR_LISTENER (Chord_name_engraver, rest);
>> void
>> Chord_name_engraver::listen_rest (Stream_event *ev)
>> {
>>  new_event_found_ = true;
>>  rest_event_ = ev;
>> }
>> 
>> I think this makes the context accept a rest event.
>> 
> 
> This should probably be set once using the macro ASSIGN_EVENT_ONCE (),
> though I can't envisage many situations with chord names where
> somebody might have polyphonic rests. :)

Similar to the  Bass_Figure_engraver, I don't think I want to use
ASSIGN_EVENT_ONCE (), because even if I have polyphonic rests, I just want
one N.C. symbol.

> 
>> In bass_figure_engraver.cc, there is a start_translation_timestep that sets
>> rest_event_ = 0.
>> 
>> In chord_name_engraver.cc, there is no start_translation_timestep.
>> 
>> Question 1: What do start_translation_timestep and stop_translation_timestep
>> do?  When are they called?  Why do some engravers have
>> start_translation_timestep and some don't?
> 
> I'd also like to know this.
> 
> I get the impression that stop_translation_timestep () is called after
> music has been processed at each timestep, whereas
> start_translation_timestep () will be called right at the beginning,
> even if there's no music to process, then at the start of each
> timestep.
> 
> It appears that the engravers which use both tend to make typesetting
> decisions based on start and stop moments rather than just
> acknowledging grobs or listening to events.
> 
> In the case of the Chord_name_engraver, there's no need to have
> start_translation_timestep, since it waits for note events to come
> along at each timestep, then creates a ChordName grob if it's gathered
> any. Once it's done the processing work, stop_translation_timestep ()
> ensures that everything's clear before the next set of notes arrives.
> 
>> My thought was that in Chord_name_engraver::process_music, I would first
>> check to see if rest_event_ was != 0.  If so, I'd make the markup be "N.C.",
>> set last_chord_ to some unique value (perhaps a scheme identifier 'no-chord,
>> although I'm not sure yet how to do that, but I can find it in the Guile
>> reference docs), set rest_event_=0, create a ChordName item and set its text
>> property to the "N.C." markup, and return.
> 
> Are you proposing that the chordNameFunction would return the markup?

No, I'm proposing that the N.C. markup would be a ChordName grob.  When I
look at Chord_name_engraver::process_music, the markup is added to the
ChordName grob.  If it's a rest, I'd just add the N.C. markup, instead of
the ChordName markup.
> 
> It would be simpler to have a property read by the engraver, e.g.
> noChordMarkup, which would be read instead of calling the markup
> procedure when a rest is found.  Then you can rely on the default
> consing of the values for last_chord_ instead of having to set a
> symbol identifying it as "no-chord".

I still want to set a symbol setting last_chord_ as a rest, because  if
chordChanges is true, I want to have a chord name printed following a NC,
even if it's the same as the previous chord.  And I don't want to NC printed
twice for successive rests.  Hence, I need to have last_chord_ set to
something to indicate a rest -- and I can't use chord pitches because there
aren't any for the rest.

Thanks for your input, Neil.  I think I'm ready to try now, since you've
confirmed my understanding.

Carl