Re: skylines and custom-code

[Thomas Morley]

Am Do., 8. Apr. 2021 um 17:52 Uhr schrieb Jean Abou Samra <jean@abou-samra.fr>:
>
>
> Le 08/04/2021 à 16:21, Kevin Barry a écrit :
> > Hi Harm,
> >
> >> Ok, another 5 days passed working allday on the problem, progress: zero.
> >> I give up.
> >>
> >> Some insights:
> >> (1)
> >> It would have been helpful to understand how Stems fit into a Beam,
> >> then I could use this knowledge to probably do similar with glissando
> >> and Stems.
> >> I was not able to figure it out.
> >> (2)
> >> Obviously the Stems needs to be positioned after Beam (or Glissando) is 
> >> done.
> >> An unpure-pure-container looked promising.
> >> Though, I was not able to code even the simplest via native scheme.
> >> This leads to the conclusion that unpure-pure-containers are not
> >> accessible/customizable for users like me.
> >> At least unless much more working examples are done in the docs.
> > I haven't had time to try and understand your issue, but having worked
> > on problems with Stems and Beams in the past I think it's an area of the
> > code that could be improved, because there is an effective circular
> > dependency:
> > - Stems check Beams in order to determine their direction/position etc
> > - Beams check where the first and last noteheads are - compared to the
> >    whole system - to try and figure out on which side they should be on
> >    or if the beam should be kneed, etc
> > - Checking where noteheads are in relation to the whole system triggers
> >    lots of other grob calculations. If any of these calculations check
> >    the Beam or the Stem you get a circular dependency (I fixed at least
> >    one regression in this area where DynamicText was triggering the
> >    circular dependency).
> > - If you try to dive into all the consequences of the code that
> >    calculates a Stem's direction it is almost impossible to understand.
> >    It's a long chain of pure/unpure calls - maybe 50 stack frames of just
> >    that stuff (IIRC); completely impenetrable (for me at any rate)
> > - In my opinion, the fix is to find a way to figure out notehead
> >    positions without calculating everything else, then Beams, then Stems
> >
> > I doubt that is any help to you, but perhaps someone else will read this
> > and know more.
> >
> > Kevin
>
>
> Hi Harm and Kevin,
>
> My apologies, I got swamped in other things and didn't look
> at the problem since I unsuccessfully tried when you sent
> the code.
>
> One thought: it's not very pretty, and maybe I'm just being
> dumb, but why not remove the skylines completely?
>
>    \temporary \override NoteHead.vertical-skylines = ##f

Well, I need correct skylines, at least when it comes to Script,
TextScript, Fingerings etc

> It looks like the glissandi are enough to provide their
> own skylines.
>
> On the general issue of purity: indeed, it is euphemism
> to say that it is thorny. Today I've been filling
> https://extending-lilypond.readthedocs.io/en/latest/backend.html#unpure-pure-containers
> and I left it with a bunch of TODOs because I just
> have no clue.

I'll study the section.
For some other details I may have some suggestions.
Do you prefer a (private) mail or issue/MR at GitLab?

>
> To comment on Kevin's insights -- thank you! --, the enormous
> dependency graph is the big problem with pure functions in my
> opinion. There are so many possible paths that it is extremely
> easy to trigger cyclic dependencies.
>
> The other big problem is that the caller, not the callee,
> determines whether the pure or the unpure version of a
> callback is used. I stumbled upon this:
>
> {
>    \override NoteHead.Y-offset =
>      #(ly:make-unpure-pure-container
>         (lambda (grob)
>           (ly:message "UNPURE")
>           0)
>         (lambda (grob start end)
>           (ly:message "PURE")
>           0))
>    c'1
> }
>
> Guess what? It prints "UNPURE". Before "Finding the ideal
> number of pages.".

Well, yes, I noticed too, but how could I say whether it's correct or not.
unpure-pure-containers are a black box, maybe containing a black hole ;)

> That does not sound correct. The default for
> NoteHead.Y-offset is
>
> (define-public staff-symbol-referencer::callback
>    (ly:make-unpure-pure-container ly:staff-symbol-referencer::callback))
>
> Something is triggering the unpure callback while
> it should use the pure one.  Go figure.  At any
> rate, this means it is impossible to make separate
> pure and unpure callbacks for NoteHead.Y-offset.
>
> Sigh.
>
> Best,
> Jean
>

Thanks,
  Harm