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Re: Distance of a grob from its reference point

From: Aaron Hill
Subject: Re: Distance of a grob from its reference point
Date: Wed, 15 Jan 2020 15:45:46 -0800
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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

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

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

Attachment: ottavabracket-y-offset.cropped.png
Description: PNG image

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