hhea

The hhea table, short for horizontal header, is a component of TrueType and OpenType font files. It stores font-wide horizontal metrics and line-related measurements that layout engines use when rendering text. Located within the font’s sfnt structure, hhea provides the essential data needed to place glyphs horizontally and to determine line spacing.

Key fields in the hhea table include version, ascent, descent, and lineGap. These vertical metrics describe the font’s extents in font units and influence the vertical space allocated for lines of text. The table also records horizontal bounds such as advanceWidthMax, minLeftSideBearing, minRightSideBearing, and xMaxExtent. Additional fields include caretSlopeRise, caretSlopeRun, and caretOffset, which describe the slope and position of the text caret for italic and obliqued fonts. The metricDataFormat field is typically zero, and numberOfHMetrics specifies how many horizontal metrics entries exist in the hmtx table.

Relation to hmtx: The hmtx (horizontal metrics) table provides per-glyph data for advance width and left side bearing for the first numberOfHMetrics glyphs. Glyphs beyond this count use the last advance width defined in hmtx for their horizontal advance, while their left side bearing may still be present. The hhea’s numberOfHMetrics informs how to interpret hmtx data.

Use in rendering: Rendering engines consult hhea to determine line height and to compute glyph placement on each line. The ascent and descent influence vertical layout, while the advance widths and bearings affect horizontal positioning. The caret metrics support proper cursor rendering in italic fonts.

Conformance: hhea is a mandatory table in TrueType fonts and a fundamental part of OpenType font structures, working alongside head, hmtx, maxp, os/2, and other tables.