Sidebearing arithmetics

Sidebearing formulas let you link spacing instead of typing the same numbers into every related glyph. Define spacing once, then let derived glyphs follow that logic while still showing the resolved numeric result.

Where to editlink

With nothing selected on the canvas, the editor property panel shows left and right sidebearings, plus LKG and RKG chips. Left (LKG) uses orange; right (RKG) uses blue, the same colors as the Second and First chips in LTR text-mode kerning. Each side can have one group. A placeholder + chip stands in when a side has no group; click it to add. x removes a group.

Each field takes a number or a metrics key. The resolved value sits next to the input, so you can keep =n+10 in the field and still see the spacing that is applied.

An empty field clears the metrics key for that side. On a fully auto-aligned component layer with no key, the field stays empty.

Notationlink

These are the forms the metrics-key parser accepts. Spaces are not part of the syntax.

Plain numberslink

A number stores that sidebearing and clears any metrics key:

40
-15
+20

Glyph-wide keys (`=`) versus layer-local keys (`==`)link

A leading = stores a glyph-wide metrics key: every layer of that glyph uses it unless a layer overrides it.

A leading == stores the same kind of key only on the current layer. After the extra =, the rest of the string is parsed the same way. Use == when most layers should follow the glyph-wide key but one master needs an exception.

==n, ==+10, ==-20, and ==|H-10 are the layer-local counterparts of =n, =+10, =-20, and =|H-10.

Constants (`=40`)link

= followed by an unsigned number is a constant formula. The sidebearing stays that value when the outline later changes:

=40

Unsigned constants are rejected on fully auto-aligned component layers. For a signed pin such as -20 on a normal layer, use =-20 (below).

Signed formulas (`=+10`, `=-15`)link

= or == followed immediately by a signed number (has + or - sign) is one syntax with two meanings:

=+10, =-15
==+10, ==-15 (layer-local)

On a normal layer, =+40 pins the sidebearing to that number and keeps the formula after later outline edits. =-20 means the sidebearing is always -20. A plain -20 (no =) also sets -20, but clears the formula.

On a fully auto-aligned component layer, the same syntax is an offset: auto-aligned spacing plus that delta. Those layers accept only this signed form (=+… / =-… / ==+… / ==-…). References, unsigned constants, and plain numbers are rejected there.

Glyph Referenceslink

A reference reads another glyph’s corresponding sidebearing (or a measured slice of it), then optionally mirrors, samples at a height, and applies one arithmetic suffix.

The leading = is optional for references, so n is accepted as well as =n.

Form	Meaning
`=n` or `n`	Same side of glyph `n`
`=H+20`, `=o-10`	Referenced sidebearing plus or minus a value
`=H*2`, `=n/2`	Multiply or divide the referenced sidebearing
`=\|n`, `=\|H-10`	Opposite side of the referenced glyph, then optional arithmetic
`=\|`	Opposite side of this layer (nothing after the pipe symbol)
`=c@200`	Measure this outline and `c` at y = 200, then apply the difference (not a copy of `c`’s stored sidebearing)
`=o@300+15`, `=c@-50`	Height sample, then optional `+` `-` `*` `/` number. Height may be signed.
`=\|c@200+15`	Mirror, height, and arithmetic together

The arithmetic suffix is exactly one operator and a number: +, -, *, or /, then a signed or unsigned value (decimals allowed). /0 does not resolve.

Combinations with `==`link

Any reference form can be layer-local:

==n
==|H-10
==c@200
==|c@200+15

Resolution and errorslink

Dependents recompute when a source glyph’s spacing changes. If a formula cannot resolve — unknown glyph name, bad suffix, a height that misses the outline, division by zero, or a cycle — the field is marked invalid until you fix it.

Best practicelink

A useful pattern: set spacing on a few base glyphs, formula-link similar-looking other base glyphs on either side, use == only for a layer that must break the shared rule, and use @height when the optical relationship lives at a vertical slice rather than the overall margin.

Automatically composed base glyphs inherit the base glyph’s sidebearings by default. Set sidebearings on composed glyphs only for overrides.