Re: Distance of a grob from its reference point

[Aaron Hill]

On 2020-01-15 1:57 pm, Paolo Prete wrote:
> I quote Carl's words:
>
> "So offset applies to the *estimated* position, and then the spacing 
> engine
> works on the offset+estimated postion and ends up putting things where 
> it
> thinks they belong"
>
> I checked that with the examples made before and it seems absolutely 
> true.
>
> Then: you have to offset an estimated position. How could you do that 
> if
> you don't know this estimated position?

\offset is a simple tool that merely adds a number to a number.  Well to 
be fair, it has some complexity behind the scenes as it can handle 
offsetting numbers, pairs of numbers, and lists of numbers.  The problem 
is unlikely with \offset; rather, it is the property you are modifying 
not having the final say.

Consider: Y-offset is a desired distance between the reference points of 
a grob and its parent.  OttavaBrackets use a procedure to determine this 
initial desired distance.  And because Y-axis spacing issues are tricky, 
the unpure-pure container system exists to assist.  The pure value is an 
estimate used before line breaking, and the unpure value consulted after 
line breaking.

But even with the issues of unpure-pure containers aside, Y-offset is 
not the sole arbiter of positioning a bracket.  You can see this here:

%%%%
\version "2.19.83"

visualizeYOffset = #(define-music-function (grob-path) (symbol-list?)
  (define (proc grob)
    (let* ((orig (ly:grob-property grob 'stencil))
           (yoff (ly:grob-property grob 'Y-offset))
           (y (- yoff))
           (sten (grob-interpret-markup grob #{ \markup
             \with-dimensions-from \stencil #orig
             \overlay {
               \stencil #orig
               \with-color #red \path #0.1 #`(
                 (moveto -1 0) (lineto 1 0)
                 (moveto 0 0) (lineto 0 ,y)
                 (moveto -0.4 -1) (lineto 0 0) (lineto 0.4 -1)
                 (moveto -1 ,y) (lineto 1 ,y))
               \translate #(cons -0.8 (/ y 2))
               \general-align #X #RIGHT \vcenter \rotate #90
               \whiteout \pad-around #0.2
               \with-color #red \normal-text \fontsize #-6
               \line { Y-offset: #(format #f "~,3f" yoff) }
             } #})))
      (ly:grob-set-property! grob 'layer 1000)
      (ly:grob-set-property! grob 'stencil sten)))
  #{ \override $grob-path .after-line-breaking = #proc #})

notes = \fixed c''' { \ottava 1 a8( 8) \ottava 0 a,4 }

{
  \visualizeYOffset Staff.OttavaBracket

  \notes
  \once \offset Y-offset 0.4 Staff.OttavaBracket
  \notes
  \once \override Staff.OttavaBracket.Y-offset = 6
  \notes
  \once \offset Y-offset 1.6 Staff.OttavaBracket
  \notes
  \once \override Staff.OttavaBracket.outside-staff-padding = 1
  \notes
}
%%%%

The slur exerts influence on the final bracket position that goes beyond 
the bracket's Y-offset.  The first \offset is not enough to overcome 
this; but the explicit value and larger \offset are enough to show that 
modifying Y-offset can work.

The alternate approach would be to not use Y-offset at all but rather 
control something like outside-staff-padding, as shown in the final 
example.


-- Aaron Hill

ottavabracket-y-offset.cropped.png
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