Beam direction property

[Andrew Bernard]

Hello all,

While writing my right hand side slash function for beams, when modifying the beam stencil to add a slash on the right I need to determine if the beam is up or down (on top or below the notes) and the beam slope, for my geometry calculations to work. I had thought that the ‘direction’ property of a Beam indicates whether it is up or down, but on careful examination I appear to be getting values I cannot understand. My minimal working example to debug this works perfectly, but my scheme code seems to produce unpredictable (to me) results. So the first question I have is: what is Beam.direction, in point of fact? I I set Beam.direction to 1, -1, or 0 with an override beams move an amount that I cannot understand but the beam stays on top or on the bottom.

The IR says:

direction (direction):

ly:beam::calc-direction

If side-axis is 0 (or X), then this property determines whether the object is placed LEFT, CENTER or RIGHT with respect to the other object. Otherwise, it determines whether the object is placed UP, CENTER or DOWN. Numerical values may also be used: UP=1, DOWN=-1, LEFT=-1, RIGHT=1, CENTER=0.

But what does this really mean for beams? Using \override Beam.direction = 1 or -1 or 0 produces an error:

\version "2.19.45"

{

  c''8[ e'' g'' c'']

  \override Beam.direction = -1

  c''8[ e'' g'' c'']

}

…

Parsing...

Interpreting music...

Preprocessing graphical objects...

programming error: Grob direction requested while calculation in progress.

continuing, cross fingers

programming error: Grob direction requested while calculation in progress.

continuing, cross fingers

programming error: Grob direction requested while calculation in progress.

continuing, cross fingers

programming error: Grob direction requested while calculation in progress.

continuing, cross fingers

Finding the ideal number of pages...

Fitting music on 1 page...

Drawing systems...

Layout output to `/tmp/lilypond-rOxJao'…

…

Andrew