hhea — Horizontal Header Table

This table contains information for horizontal layout. The values in the minRightSidebearing, minLeftSideBearing and xMaxExtent should be computed using only glyphs that have contours. Glyphs with no contours should be ignored for the purposes of these calculations. All reserved areas must be set to 0.

Type	Name	Description
uint16	majorVersion	Major version number of the horizontal header table — set to 1.
uint16	minorVersion	Minor version number of the horizontal header table — set to 0.
FWORD	ascender	Typographic ascent—see remarks below.
FWORD	descender	Typographic descent—see remarks below.
FWORD	lineGap	Typographic line gap. Negative lineGap values are treated as zero in some legacy platform implementations.
UFWORD	advanceWidthMax	Maximum advance width value in 'hmtx' table.
FWORD	minLeftSideBearing	Minimum left sidebearing value in 'hmtx' table for glyphs with contours (empty glyphs should be ignored).
FWORD	minRightSideBearing	Minimum right sidebearing value; calculated as min(aw - (lsb + xMax - xMin)) for glyphs with contours (empty glyphs should be ignored).
FWORD	xMaxExtent	Max(lsb + (xMax - xMin)).
int16	caretSlopeRise	Used to calculate the slope of the cursor (rise/run); 1 for vertical.
int16	caretSlopeRun	0 for vertical.
int16	caretOffset	The amount by which a slanted highlight on a glyph needs to be shifted to produce the best appearance. Set to 0 for non-slanted fonts
int16	(reserved)	set to 0
int16	(reserved)	set to 0
int16	(reserved)	set to 0
int16	(reserved)	set to 0
int16	metricDataFormat	0 for current format.
uint16	numberOfHMetrics	Number of hMetric entries in 'hmtx' table

The ascender, descender and linegap values in this table are Apple specific; see Apple's specification for details regarding Apple platforms. The sTypoAscender, sTypoDescender and sTypoLineGap fields in the OS/2 table are used on the Windows platform, and are recommended for new text-layout implementations. Font developers should evaluate behavior in target applications that may use fields in this table or in the OS/2 table to ensure consistent layout. See the descriptions of the OS/2 fields for additional details.

'hhea' Table and OpenType Font Variations

In a variable font, various font-metric values within the horizontal header table may need to be adjusted for different variation instances. Variation data for 'hhea' entries can be provided in the metrics variations (MVAR) table. Different 'hhea' entries are associated with particular variation data in the MVAR table using value tags, as follows:

'hhea' entry	Tag
caretOffset	'hcof'
caretSlopeRise	'hcrs'
caretSlopeRun	'hcrn'

For general information on OpenType Font Variations, see the chapter, OpenType Font Variations Overview.