.WOFF2 File Format – Transformed hmtx

Web Open Font Format 2.0

The hmtx table gives each glyph’s advance width and left side bearing, the gap between the glyph’s origin and the left edge of its outline. In most TrueType fonts the left side bearing equals the xMin already stored in the glyph’s bounding box in glyf, so WOFF2 can leave the bearings out. This is transformation version 1 of hmtx in the table directory; version 0 stores the table as it is. It is optional.

The original table

An OpenType hmtx table has numberOfHMetrics pairs (from the hhea table) of a UInt16 advance width and an Int16 left side bearing, then one more Int16 left side bearing for each remaining glyph, up to numGlyphs (from maxp). The glyphs after the pairs all have the last pair’s advance width.

Transformed structure

TypeNameCountPresentDescription
UInt8flags1alwaysWhich arrays are left out: see flags.
UInt16advanceWidthnumberOfHMetricsalwaysThe advance widths, as in the original pairs.
Int16lsbnumberOfHMetricswhen bit 0 is clearThe left side bearings of the first numberOfHMetrics glyphs.
Int16leftSideBearingnumGlyphs − numberOfHMetricswhen bit 1 is clearThe left side bearings of the remaining glyphs.

Even an array that is kept is stored apart from the advance widths, so like values sit together for the compressor.

Flags

BitMaskMeaning when set
00x01The lsb array is absent; rebuild it from the glyphs’ xMin.
10x02The leftSideBearing array is absent; rebuild it from the glyphs’ xMin.
2 to 70xFCReserved: must be 0.

At least one of bits 0 and 1 must be set; a transformation that leaves out nothing is not allowed. A decoder must reject the font when the flags break either rule.

When it may be used

  • Only in fonts with TrueType outlines, since the bearings come from glyf. A CFF font’s hmtx is always stored as it is.
  • Only when every bearing left out equals its glyph’s xMin, or is 0 for a glyph with no outline. An encoder must check this.
  • In a collection, only when that holds for every combination of hmtx and glyf the fonts share.

Rebuilding the table

The decoder needs the glyphs’ bounding boxes first, so it decodes glyf before hmtx. It then writes the original layout: for each of the first numberOfHMetrics glyphs, the advance width and either the stored lsb or the glyph’s xMin; then for each remaining glyph the stored leftSideBearing or its xMin. A glyph with no outline gets 0.