Re: [frogs] bend implementation

[Marc Hohl <marc@xxxxxxxxxx>]

Trevor Daniels schrieb:
> Marc Hohl wrote Tuesday, January 26, 2010 8:42 AM
> > I think there will be more questions arising during the work, mostly
> > silly ones (from a programmer's point of view), so I wonder whether I 
> > should fill up the
> > frogs list with these questions, or does it make more sense to handle 
> > this similar to
> > Graham's mentor proposals, so I fill up the mail inbox of just *one* 
> > person ;-)
>
> Far better to keep the discussion on the frogs list.  That way we
> all learn and others are able to correct any misconceptions.  I've
> learnt quite a lot thinking about questions posed on -frogs.
>
> And please don't worry about asking questions you think might
> be silly.  We all should be more willing to do this.  It's often the
> silly questions that stump us the longest.
>
> Trevor
So here I am again. I have established a new string-bend-event
by editing

scm/define-music-types.scm
scm/define-event-classes.scm and
ly/declarations-init.ly

Moreover, I created lily/string-bend-engraver.cc which has
mostly empty function declarations in it.

So far, I can compile a file containing stuff like

c \startBend d e f \stopBend   %(1)

(It looks the same as c d e f, of course, but the infrastructure seems 
to be working.)

For clarification: the output of (1) should look like
c /\ d /\ e /\ f
with pointed slurs over (or under) the note heads. I don't want to code 
the string bends
between every pair of notes, but rather mark a note range with 
\startBend ... \stopBend.

Now I looked at slur-engraver.cc, tie-engraver.cc, tuplet-engraver.cc and
episema-engraver.cc (which is on rietveld, but it is a small file which 
I think I understand
roughly). I wanted to find out what has to be acknowledged.

If I understand correctly, the engraver is called when \startBend is 
processed,
and he does the stuff defined in String_bend_engraver::listen_string_bend.

Then I want the engraver to draw a pointed slur from the previous note 
(if there is any)
to the current note. Therefore, I have to use the ACKNOWLEDGER mechanism.
So I add (amongst the initialisation stuff) 
String_bend_engraver::acknowledge_note_column.
The routines defined here are called at any time the spanner has not 
been terminated
yet and a note_column is processes. Is this correct? From here, I get 
the info about the note heads.
So if the d in example (1) is processed, I  can get information about it 
via this mechanism.

Then, process_music() is called, and that's the place for telling the 
engraver
"hey, there is a new note; draw your pointed slur to the previous
one, save the current note and go on."

Then, we are at note e, and there it goes again, acknowledge_note_column 
is called,
process_music () afterwards, and so on, until we reach the f.

What I don't understand is the need for start/stop_translation_timestep 
(). Some
engravers use the one, some the other function, but it seems that they 
aren't mandatory.
Reading CG 9.9.2, it seems to be as follows:

* an event is read
* the acknowledge_* (9 functions (if any) are called
* start_translation_timestep () is called before doing any property 
operations,
* then the property operations (if any) are done
* process_music () is called
* stop_translation_timestep () is called
* the next event is read

Is this true?

One final question (for now). Is note_column appropriate in my approach? 
I am not sure
whether I understand the difference between note_column and note_head 
correctly.
note_column includes stems and in < ...> constructs all the 
corresponding note heads,
is that right?

Phooey, this is a lot of stuff.

Thanks in advance for your explanations and your patience

Marc




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