awaforge

tableGsub

AGPL-3.0-onlyRelease 2L4

Table GSUB — Glyph Substitution (OT §6.5.7).

Module tableGsub | Source packages/front/office/fonts/src/table/gsub.js | Deps fontErrors, fontReader, tableGsubScriptFeatureList, tableGsubTypes14, tableGsubTypes57 | Worker-safe yes

Decodes the Script/Feature/Lookup lists and every lookup subtable type 1–8: Type 1 (Single), Type 2 (Multiple), Type 3 (Alternate), Type 4 (Ligature), Type 5 (Context), Type 6 (Chaining Context), Type 7 (Extension), Type 8 (Reverse Chaining Contextual Single). Lookup types outside 1–8 fall back to metadata only ({ type, parsed: false }).

Exports

Symbol Type Description
parseGsub function (bytes) => { majorVersion, minorVersion, scripts, features, lookups }.
parseGsubSubtable function (type, bytes) => subtable — dispatches to the per-type decoder; unknown types return { type, parsed: false }.
parseScriptList function (bytes, offset) => Script[].
parseFeatureList function (bytes, offset) => Feature[].
parseLookupList function (bytes, offset, parseSubtableFn) => Lookup[].
tableGsub factory Factory.

Usage

import fw from '@awacloud/fw';
import { fw_require, modules } from '@awacloud/fonts';

for (const m of [...fw_require, ...modules]) fw.runtime.register(m);
const { parseGsub } = fw.runtime.resolve('tableGsub');
const gsub = parseGsub(sfnt.tables.GSUB.bytes);
const liga = gsub.features.find(f => f.tag === 'liga');

Notes

  • Type 4 ligatures decoded into { ligGlyph, components } array.
  • parseScriptList/parseFeatureList/parseLookupList exported for reuse by GPOS.
  • Type 8 (ReverseChainSingleSubstFormat1) decodes to { type: 8, format: 1, coverage, backtrackCoverages, lookaheadCoverages, substitutes }: the three coverages are decoded Coverage objects (the two arrays in file order, possibly empty) and substitutes is the number[] of substitute glyph IDs. Any other type 8 format throws a ParseError with code fonts/gsub-reverse-format. Substitutions are decoded, not applied. Type 8 is verified against synthetic subtables built from the specification: no font in the test corpus carries a type 8 lookup.
  • Sub-lookup logic is split across ./gsub/script-feature-list.js, ./gsub/types-1-4.js (Single/Multiple/Alternate/Ligature), ./gsub/types-5-7.js (Context/Chaining/Extension/Reverse Chaining).

See also