Font

Tables

Read this section when a subset is refused, when the output lacks something the source had, or when you want to know what HintingPolicy and LayoutPolicy remove. Inside a TrueType, OpenType, WOFF, or WOFF2 file the data is split into tables, each named by a four-letter tag: cmap maps characters to glyphs, glyf holds the outlines, GPOS positions them, fvar declares variation axes. Reading leaves every table untouched. Subsetting rewrites some, keeps others as they are, and refuses a few; which is which decides whether compact numbering works on your font and what the output loses.

  • Hinting: what the instruction tables do at small sizes, and what HintingPolicy::Drop removes.
  • Layout: the tables behind ligatures, kerning, and marks, and what each LayoutPolicy keeps.
  • Variable fonts: what makes one file a whole family, and how to pick a weight.

What every font contains

Every font the package loads has the eight tables the specification requires: cmap, head, hhea, hmtx, maxp, name, OS/2, and post. They provide the character map, the dimensions, the horizontal metrics, the names, and the glyph names.

Outlines must be TrueType: glyf, indexed by loca. CFF and CFF2 are rejected on load, and SVG , COLR, CPAL, sbix, CBDT, CBLC, EBDT, and EBLC glyph data is not rendered; see Formats.

What subsetting keeps, rewrites, or refuses

Writing an unchanged standalone font preserves every table byte for byte. Subsetting with the default GlyphIdPolicy::Preserve keeps glyph slots in place and rewrites only what the selection requires. Compact renumbers the retained glyphs, so every table that stores glyph identifiers is rewritten, dropped, or refused:

  • cmap, hmtx, hhea, maxp, OS/2, and post are rewritten for the selection in both modes. Compact mode switches post to format 3 when glyph names must go.
  • glyf and loca keep unselected glyphs as empty slots in preserve mode. Compact mode renumbers them and remaps compound references.
  • cvt , fpgm, prep, gasp, hdmx, LTSH, VDMX, and cvar are kept with HintingPolicy::Keep and dropped with Drop, in both modes. See Hinting.
  • GSUB, GPOS, and GDEF are kept, reduced, or dropped by LayoutPolicy. Compact mode also remaps them. See Layout.
  • kern format 0 is kept. Compact mode remaps it independently of LayoutPolicy.
  • fvar, avar, gvar, HVAR, vhea, vmtx, and VVAR are kept. Compact mode remaps them. See Variable fonts.
  • DSIG is removed whenever any data is rebuilt.
  • meta is kept in preserve mode and removed in compact mode, because its glyph references cannot be remapped safely.
  • VARC, BASE, MATH, color, bitmap, and unsupported kern formats are kept as opaque bytes in preserve mode and refused with UnsupportedFontException in compact mode.

A refusal happens in subset(), before anything is written. Keep Preserve for that font, or remove the data with HintingPolicy or LayoutPolicy when it belongs to those groups. Remapping vhea, vmtx, and VVAR does not make vertical layout supported while the other vertical tables are not. Read SubsetResult::$warnings after every subset, and validate the output where it will be shaped and rendered.