awaforge

tableGpos

AGPL-3.0-onlyRelease 2L4

Table GPOS — Glyph Positioning (OT §6.5.8).

Module tableGpos | Source packages/front/office/fonts/src/table/gpos.js | Deps fontErrors, fontReader, tableGsubScriptFeatureList, tableGposValueRecord, tableGposType1, tableGposType2, tableGposType3, tableGposType46, tableGposType78, tableGposType9 | Worker-safe yes

Header identical to GSUB. Every lookup subtable type 1–9 is decoded: Type 1 (Single Adjustment), Type 2 (Pair Adjustment — OT kerning), Type 3 (Cursive Adjustment), Types 4–6 (Mark-to-Base / Mark-to-Ligature / Mark-to-Mark attachment), Types 7–8 (Context / Chaining Context Positioning, formats 1–3), Type 9 (Extension Positioning). Only lookup types outside 1–9 fall back to metadata only ({ type, parsed: false }).

Exports

Symbol Type Description
VALUE_FORMAT const Bitfield { X_PLACEMENT, Y_PLACEMENT, X_ADVANCE, Y_ADVANCE, ... DEVICE_OFFSET bits }.
parseGpos function (bytes) => { majorVersion, minorVersion, scripts, features, lookups }.
buildKerningTable function (gpos) => Map<number, number> — memoised kerning map keyed by (leftGid<<16)|rightGid, built from every Type 2 (Pair Adjustment) subtable's value1.xAdvance. Callers cache the result themselves.
parseGposSubtable function (type, bytes) => subtable — dispatches to the per-type decoder; unknown types return { type, parsed: false }.
valueRecordSize function Computes a ValueRecord's byte size from its valueFormat bitfield.
tableGpos 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 { parseGpos } = fw.runtime.resolve('tableGpos');
const gpos = parseGpos(sfnt.tables.GPOS.bytes);
const kern = gpos.features.find(f => f.tag === 'kern');

Notes

  • ValueRecord is variable-size depending on valueFormat — decoded byte-by-byte.
  • Reuses parseScriptList / parseFeatureList / parseLookupList from GSUB.
  • buildKerningTable only captures value1.xAdvance — consumers needing richer ValueRecords should walk gpos.lookups directly.
  • Sub-lookup logic is split across ./gpos/value-record.js, ./gpos/type1-single.js, ./gpos/type2-pair.js, ./gpos/type3-cursive.js, ./gpos/type4-6-mark.js, ./gpos/type7-8-context.js, ./gpos/type9-extension.js.

See also