d2lang/d2 · warning

unsupported kern table coverage 0x%04x

Error message

unsupported kern table coverage 0x%04x

What it means

The parser only supports kern subtables with coverage/format 0x0001 (format 0, horizontal kerning values). Any other coverage bitfield (vertical kerning, cross-stream, format 1/2/3, Apple format variants) is rejected.

Source

Thrown at lib/textmeasure/fontface.go:171

func parseLegacyKern(src []byte) ([]kernPair, error) {
	table, err := sfntTable(src, "kern")
	if err != nil || table == nil {
		return nil, err
	}
	if len(table) < 18 {
		return nil, fmt.Errorf("invalid kern table: too short")
	}
	if binary.BigEndian.Uint16(table[0:2]) != 0 {
		return nil, fmt.Errorf("unsupported kern table version")
	}
	if binary.BigEndian.Uint16(table[2:4]) == 0 {
		return nil, fmt.Errorf("invalid kern table: no subtables")
	}

	length := int(binary.BigEndian.Uint16(table[6:8]))
	coverage := binary.BigEndian.Uint16(table[8:10])
	if coverage != 0x0001 {
		return nil, fmt.Errorf("unsupported kern table coverage 0x%04x", coverage)
	}
	n := int(binary.BigEndian.Uint16(table[10:12]))
	if length < 14 || 6*n != length-14 || 4+length > len(table) {
		return nil, fmt.Errorf("invalid kern table length")
	}

	pairs := make([]kernPair, n)
	for i := range pairs {
		offset := 18 + 6*i
		pairs[i] = kernPair{
			key:   binary.BigEndian.Uint32(table[offset : offset+4]),
			value: int16(binary.BigEndian.Uint16(table[offset+4 : offset+6])),
		}
	}
	return pairs, nil
}

func sfntTable(src []byte, tag string) ([]byte, error) {

View on GitHub (pinned to 0d69dca6f5)

Solutions

1. Convert the font with fonttools/otf2ttf so kerning is stored as OpenType GPOS pairs or a format-0 kern subtable.
2. Choose a different variant of the font that uses standard format-0 horizontal kerning.
3. Accept the error as signaling reduced kerning: strip the kern table so the library proceeds without kerning.

// coverage := binary.BigEndian.Uint16(kern[8:10]); usable := coverage == 0x0001

pairs, err := parseLegacyKern(fontData)
if err != nil {
    log.Printf("kerning unavailable (%v), continuing without", err)
    pairs = nil
}

When it happens

Trigger: parseFont encounters a font whose kern subtable coverage field (bytes 8–10) is not exactly 0x0001 — e.g. format 2 class-based kerning, vertical kerning, or Apple format 1 kerning lists.

Common situations: Apple system fonts using AAT format-1 kerning; fonts with vertical-writing kern tables (Japanese/CJK fonts); cross-stream kerning tables.

Related errors

• invalid kern table: too short
• unsupported kern table version
• invalid kern table: no subtables
• invalid kern table length
• checksum error in table: %v

AI-assisted analysis of d2lang/d2@0d69dca6f5 (2026-08-31). Data as JSON: /api/errors/08405dfa1e5406cb. Report an issue: GitHub.