kern table

Contains legacy kerning pair adjustments for horizontal glyph positioning. For most modern fonts, kerning is handled via the GPOS table instead. Font Flux supports reading and writing all kern formats and can convert between them — see Creating Kerning.

Scope

• Format family: Shared SFNT
• Table tag in JSON: kern

Specs

• OpenType kern: https://learn.microsoft.com/en-us/typography/opentype/spec/kern
• Apple kern: https://developer.apple.com/fonts/TrueType-Reference-Manual/RM06/Chap6kern.html

Supported formats

Font Flux fully parses and writes all common kern subtable formats:

Variant	Format	Description	Notes
OpenType	0	Ordered pair list	Most common legacy format
OpenType	2	Class-based n×m array	Compact for many pairs
Apple	0	Ordered pair list	Same structure as OT format 0
Apple	1	State table (contextual kerning)	Best-effort extraction to pairs
Apple	2	Class-based n×m array	Same structure as OT format 2
Apple	3	Compact class-based (uint8)	Compact; max 256 classes

Unknown subtable formats are preserved as _raw bytes for lossless round-tripping.

JSON Skeleton

OpenType variant

{
	"tables": {
		"kern": {
			"formatVariant": "opentype",
			"version": 0,
			"subtables": [
				{
					"version": 0,
					"format": 0,
					"coverage": 1,
					"nPairs": 2,
					"searchRange": 6,
					"entrySelector": 1,
					"rangeShift": 6,
					"pairs": [
						{ "left": 1, "right": 3, "value": -80 },
						{ "left": 2, "right": 4, "value": -50 }
					]
				}
			]
		}
	}
}

Apple variant

{
	"tables": {
		"kern": {
			"formatVariant": "apple",
			"version": 65536,
			"subtables": [
				{
					"coverage": 0,
					"format": 3,
					"tupleIndex": 0,
					"glyphCount": 5,
					"kernValueCount": 2,
					"leftClassCount": 2,
					"rightClassCount": 2,
					"flags": 0,
					"kernValues": [0, -50],
					"leftClasses": [0, 1, 0, 0, 0],
					"rightClasses": [0, 0, 1, 0, 0],
					"kernIndices": [0, 0, 0, 1]
				}
			]
		}
	}
}

Top-level Fields

• formatVariant — "opentype" or "apple", determined by table version
• version — 0 for OpenType, 0x00010000 (65536) for Apple
• subtables — Array of subtable objects; each has a format field

Simplified kerning

In the simplified representation, all kern formats are extracted into the top-level kerning[] array using glyph names. You don't normally need to work with the raw kern table directly.

fontData.kerning = [
	{ left: 'A', right: 'V', value: -80 },
	{ left: 'T', right: 'o', value: -40 },
];

See Creating Kerning for the full guide on authoring and format conversion.

Notes

• Preserve _checksum for stable round-tripping.
• Unknown subtable formats are preserved as _raw bytes.
• When both kern and GPOS contain kerning, the simplified representation merges both — GPOS values win on conflicts.
• Validate with .validate() after edits.