Re: Help on Item/Grob creation

[Jan Nieuwenhuizen]

Carl Sorensen writes:

> I've (somewhat) successfully created a FretDiagrams context, with a
> Fret_diagram_engraver.

Great.

> As part of the engraver, I have the code
>
> fret_diagram_ = make_item ("FretDiagram",SCM_EOL); //WHAT GOES HERE?
>
> It appears to me that the "name" of the Item, FretDiagram, is in fact a
> pointer into scm/define-grobs.scm.

Yes.

> Since I want to make a FretDiagram
> Item, I believe I need to make an alist entry for FretDiagram in
> scm/define-grobs.scm.  Is there any documentation for what goes in the
> alist entry?

In the alist entry go the immutable properties that describe a Grob.
Look at other Grob definitions for inspiration.

> Also, as part of making the item, I need to have a "cause", which is
> SCM_EOL in this particular example.  It appears from some of the
> internal documentation that the cause is a music expression, generally
> the one responsible for creating the Item/Grob.  What is the cause used
> for?

Cause is used for constructing error and warning messages to the user.
You use the music request form try_music () that caused the engraver
to create the Item.

> Also, what is the difference between an Item and a Grob?  From some
> earlier descriptions, I think that an Item is one type of Grob; that is,
> all Items are Grobs, but not all Grobs are Items (i.e. some Grobs can be
> Spanners, etc.).  Is this correct?

Yes, that's correct.

Jan.

-- 
Jan Nieuwenhuizen <address@hidden> | GNU LilyPond - The music typesetter
http://www.xs4all.nl/~jantien       | http://www.lilypond.org