Re: Cross-staff stem engraver (was: New frog in an empty pond?)

[Pavel Roskin]

Quoting Joe Neeman <address@hidden>:

> The problem is where you ask for the stem Y-extent relative to yref (the
> VerticalAlignment): in order to calculate the vertical distance between the
> various staves and the VerticalAlignment, line breaking needs to be done
> first. (And line breaking doesn't happen until after all the engravers are
> done.)

I see.  It means that using an engraver it not a solution per se.   
Good to know.

> > I guess I'll need to redefine the stencil for stems.  Or maybe you mean
> > some other callback?
>
> You could redefine stencil, but I think redefining length would be easier.

Of course, that's why I tried it first :)

> Can you be more specific about the Y-extent not being calculated yet? When
> you ask for the Y-extent, the calculation should be triggered if it hasn't
> already happened.

Now I understand what's going on!  Apparently this triggering is not  
transitive (which is reasonable).  I'm attaching the minimal example  
when I get

grob Y-extent: (+inf.0 . -inf.0)

I think it should be possible to work around.  I'll be perfectly fine  
if my code doesn't work with chords spanning three and more staves if  
that limitation needs to exist.

> As a general approach, setting length sounds like a reasonable way forward.
> In addition, I'd suggest creating a new stem in process-acknowledged, then
> doing
>
> (set! (ly:grob-object new-stem 'spanned-stems) old-stems)
> (set! (ly:grob-property new-stem 'length) your-length-callback)
>
> In your length callback, you read back the spanned-stems object and do your
> calculations. Is this similar to what you tried?

I never used an engraver and the length override at the save time.   
When doing the length override, I tried this approach:

- find PaperColumn among the grob parents
- scan its elements for stems
- use stems extents to find stems connectable to the given stem
- find the closest stem
- use the length that would extend the given stem to the closest stem

It actually worked, except some corner cases.  Now I understand why.

Using an engraver to catch the stems sounds like a good approach.  I  
could even do some filtering in the engraver.  And it should be  
possible to replace stems with one rather than connect them together.   
It would simplify SVG output, and I suspect the connect point could be  
seen in some renderings of the postscript output.

Thank you for your help!  I'm getting really close!

--
Regards,
Pavel Roskin

stemconnect.ly
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