### 3.2.45 grob-interface

A grob represents a piece of music notation.

All grobs have an X and Y position on the page. These X and Y positions are stored in a relative format, thus they can easily be combined by stacking them, hanging one grob to the side of another, or coupling them into grouping objects.

Each grob has a reference point (a.k.a. parent): The position of a grob is stored relative to that reference point. For example, the X reference point of a staccato dot usually is the note head that it applies to. When the note head is moved, the staccato dot moves along automatically.

A grob is often associated with a symbol, but some grobs do not print any symbols. They take care of grouping objects. For example, there is a separate grob that stacks staves vertically. The NoteCollision object is also an abstract grob: It only moves around chords, but doesn’t print anything.

Grobs have properties (Scheme variables) that can be read and set. Two types of them exist: immutable and mutable. Immutable variables define the default style and behavior. They are shared between many objects. They can be changed using `\override` and `\revert`. Mutable properties are variables that are specific to one grob. Typically, lists of other objects, or results from computations are stored in mutable properties. In particular, every call to `ly:grob-set-property!` (or its C++ equivalent) sets a mutable property.

The properties `after-line-breaking` and `before-line-breaking` are dummies that are not user-serviceable.

#### User settable properties:

> `X-extent` (pair of numbers)

>   Extent (size) in the X direction, measured in staff-space units, relative to object’s reference point.

> `X-offset` (number)

>   The horizontal amount that this object is moved relative to its X-parent.

> `Y-extent` (pair of numbers)

>   Extent (size) in the Y direction, measured in staff-space units, relative to object’s reference point.

> `Y-offset` (number)

>   The vertical amount that this object is moved relative to its Y-parent.

> `after-line-breaking` (boolean)

>   Dummy property, used to trigger callback for `after-line-breaking`.

> `avoid-slur` (symbol)

>   Method of handling slur collisions. Choices are `inside`, `outside`, `around`, and `ignore`. `inside` adjusts the slur if needed to keep the grob inside the slur. `outside` moves the grob vertically to the outside of the slur. `around` moves the grob vertically to the outside of the slur only if there is a collision. `ignore` does not move either. In grobs whose notational significance depends on vertical position (such as accidentals, clefs, etc.), `outside` and `around` behave like `ignore`.

> `before-line-breaking` (boolean)

>   Dummy property, used to trigger a callback function.

> `color` (color)

>   The color of this grob.

> `id` (string)

>   An id string for the grob. Depending on the typestting backend being used, this id will be assigned to a group containing all of the stencils that comprise a given grob. For example, in the svg backend, the string will be assigned to the `id` attribute of a group (<g>) that encloses the stencils that comprise the grob. In the Postscript backend, as there is no way to group items, the setting of the id property will have no effect.

> `extra-offset` (pair of numbers)

>   A pair representing an offset. This offset is added just before outputting the symbol, so the typesetting engine is completely oblivious to it. The values are measured in `staff-space` units of the staff’s `StaffSymbol`.

> `footnote-music` (music)

>   Music creating a footnote.

> `forced-spacing` (number)

>   Spacing forced between grobs, used in various ligature engravers.

> `horizontal-skylines` (pair of skylines)

>   Two skylines, one to the left and one to the right of this grob.

> `layer` (integer)

>   An integer which determines the order of printing objects. Objects with the lowest value of layer are drawn first, then objects with progressively higher values are drawn, so objects with higher values overwrite objects with lower values. By default most objects are assigned a layer value of 1.

> `minimum-X-extent` (pair of numbers)

>   Minimum size of an object in X dimension, measured in `staff-space` units.

> `minimum-Y-extent` (pair of numbers)

>   Minimum size of an object in Y dimension, measured in `staff-space` units.

> `outside-staff-horizontal-padding` (number)

>   By default, an outside-staff-object can be placed so that is it very close to another grob horizontally. If this property is set, the outside-staff-object is raised so that it is not so close to its neighbor.

> `outside-staff-padding` (number)

>   The padding to place between grobs when spacing according to `outside-staff-priority`. Two grobs with different `outside-staff-padding` values have the larger value of padding between them.

> `outside-staff-priority` (number)

>   If set, the grob is positioned outside the staff in such a way as to avoid all collisions. In case of a potential collision, the grob with the smaller `outside-staff-priority` is closer to the staff.

> `rotation` (list)

>   Number of degrees to rotate this object, and what point to rotate around. For example, `'(45 0 0)` rotates by 45 degrees around the center of this object.

> `skyline-horizontal-padding` (number)

>   For determining the vertical distance between two staves, it is possible to have a configuration which would result in a tight interleaving of grobs from the top staff and the bottom staff. The larger this parameter is, the farther apart the staves are placed in such a configuration.

> `springs-and-rods` (boolean)

>   Dummy variable for triggering spacing routines.

> `stencil` (stencil)

>   The symbol to print.

> `transparent` (boolean)

>   This makes the grob invisible.

> `vertical-skylines` (pair of skylines)

>   Two skylines, one above and one below this grob.

> `whiteout` (boolean)

>   If true, the grob is printed over a white background to white-out underlying material, if the grob is visible. Usually #f by default.

#### Internal properties:

> `axis-group-parent-X` (graphical (layout) object)

>   Containing X axis group.

> `axis-group-parent-Y` (graphical (layout) object)

>   Containing Y axis group.

> `cause` (any type)

>   Any kind of causation objects (i.e., music, or perhaps translator) that was the cause for this grob.

> `cross-staff` (boolean)

>   True for grobs whose `Y-extent` depends on inter-staff spacing. The extent is measured relative to the grobs’s parent staff (more generally, its `VerticalAxisGroup`) so this boolean flags grobs that are not rigidly fixed to their parent staff. Beams that join notes from two staves are `cross-staff`. Grobs that are positioned around such beams are also `cross-staff`. Grobs that are grouping objects, however, like `VerticalAxisGroups` will not in general be marked `cross-staff` when some of the members of the group are `cross-staff`.

> `interfaces` (list)

>   A list of symbols indicating the interfaces supported by this object. It is initialized from the `meta` field.

> `meta` (list)

>   Provide meta information. It is an alist with the entries `name` and `interfaces`.

> `pure-Y-offset-in-progress` (boolean)

>   A debugging aid for catching cyclic dependencies.

> `staff-symbol` (graphical (layout) object)

>   The staff symbol grob that we are in.

This grob interface is used in the following graphical object(s): Accidental, AccidentalCautionary, AccidentalPlacement, AccidentalSuggestion, Ambitus, AmbitusAccidental, AmbitusLine, AmbitusNoteHead, Arpeggio, BalloonTextItem, BarLine, BarNumber, BassFigure, BassFigureAlignment, BassFigureAlignmentPositioning, BassFigureBracket, BassFigureContinuation, BassFigureLine, Beam, BendAfter, BreakAlignGroup, BreakAlignment, BreathingSign, ChordName, Clef, ClefModifier, ClusterSpanner, ClusterSpannerBeacon, CombineTextScript, CueClef, CueEndClef, Custos, DotColumn, Dots, DoublePercentRepeat, DoublePercentRepeatCounter, DoubleRepeatSlash, DynamicLineSpanner, DynamicText, DynamicTextSpanner, Episema, Fingering, FingeringColumn, Flag, FootnoteItem, FootnoteSpanner, FretBoard, Glissando, GraceSpacing, GridLine, GridPoint, Hairpin, HorizontalBracket, InstrumentName, InstrumentSwitch, KeyCancellation, KeySignature, KievanLigature, LaissezVibrerTie, LaissezVibrerTieColumn, LedgerLineSpanner, LeftEdge, LigatureBracket, LyricExtender, LyricHyphen, LyricSpace, LyricText, MeasureCounter, MeasureGrouping, MelodyItem, MensuralLigature, MetronomeMark, MultiMeasureRest, MultiMeasureRestNumber, MultiMeasureRestText, NonMusicalPaperColumn, NoteCollision, NoteColumn, NoteHead, NoteName, NoteSpacing, OttavaBracket, PaperColumn, ParenthesesItem, PercentRepeat, PercentRepeatCounter, PhrasingSlur, PianoPedalBracket, RehearsalMark, RepeatSlash, RepeatTie, RepeatTieColumn, Rest, RestCollision, Script, ScriptColumn, ScriptRow, Slur, SostenutoPedal, SostenutoPedalLineSpanner, SpacingSpanner, SpanBar, SpanBarStub, StaffGrouper, StaffSpacing, StaffSymbol, StanzaNumber, Stem, StemStub, StemTremolo, StringNumber, StrokeFinger, SustainPedal, SustainPedalLineSpanner, System, SystemStartBar, SystemStartBrace, SystemStartBracket, SystemStartSquare, TabNoteHead, TextScript, TextSpanner, Tie, TieColumn, TimeSignature, TrillPitchAccidental, TrillPitchGroup, TrillPitchHead, TrillSpanner, TupletBracket, TupletNumber, UnaCordaPedal, UnaCordaPedalLineSpanner, VaticanaLigature, VerticalAlignment, VerticalAxisGroup, VoiceFollower, VoltaBracket and VoltaBracketSpanner.

#### Internals Reference v2.18.2 (稳定分支).