THotPDF GSUB Lookup Methods

THotPDF API reference

Documents the capability-level GSUB lookup APIs for single, multiple, alternate, ligature, contextual, chained contextual, reverse chained, contextual-feature, and localized-form substitutions

Delphi syntax

function GetSingleSubstituteGlyph(InputGID: Word; const FeatureTag: AnsiString): Word;

function GetMultipleSubstituteGlyphs(InputGID: Word; const FeatureTag: AnsiString; var OutGIDs: TGSUBLookupArray): Boolean;

function GetAlternateGlyphCount(InputGID: Word; const FeatureTag: AnsiString): Integer;

function GetAlternateGlyph(InputGID: Word; const FeatureTag: AnsiString; AlternateIndex: Integer): Word;

function ApplyLigatureSubstitution(const InputGIDs: array of Word; StartIndex: Integer; const FeatureTag: AnsiString; out OutGID: Word; out ConsumedCount: Integer): Boolean;

function ApplyContextualSubst(const InputGIDs: array of Word; StartIndex: Integer; const FeatureTag: AnsiString; var OutGIDs: TGSUBLookupArray; out ConsumedLen: Integer): Boolean;

function ApplyReverseChainedContextualSubst(const InputGIDs: array of Word; StartIndex: Integer; const FeatureTag: AnsiString; out OutGID: Word): Boolean;

function ApplyGSUBContextualFeature(const Wide: UnicodeString; const AFeatureTag: AnsiString): UnicodeString;

function ApplyLocalizedForms(const Wide: UnicodeString): UnicodeString;

Covered APIs

API	Usage notes
`GetSingleSubstituteGlyph`	Queries GSUB LookupType 1 single substitution and returns the input GID when no match exists
`GetMultipleSubstituteGlyphs`	Queries GSUB LookupType 2 multiple substitution and returns the replacement glyph sequence
`GetAlternateGlyphCount`	Counts GSUB LookupType 3 alternate glyphs for a covered input glyph
`GetAlternateGlyph`	Returns a selected alternate glyph by zero-based alternate index
`ApplyLigatureSubstitution`	Queries GSUB LookupType 4 ligature substitution for a glyph run starting at a specific index
`ApplyContextualSubst`	Applies GSUB contextual or chained contextual substitution where a rule can replace a run segment
`ApplyReverseChainedContextualSubst`	Queries GSUB reverse chained contextual single substitution for right-to-left compatible shaping rules
`ApplyGSUBContextualFeature`	Applies a named GSUB contextual feature such as rclt or calt to a Unicode run and maps substitutions safely
`ApplyLocalizedForms`	Applies GSUB locl substitutions under the active script and language while preserving text extraction mappings
`MarkUnicodeGlyphUsed`	Adds a returned substitute GID to the embedded font subset closure
`sfContextualAlternates`	Applies required contextual alternates through the rclt-capable GSUB contextual lookup path
`sfLocalizedForms`	Applies GSUB locl localized-form substitutions under the active script and language

Usage notes

These methods answer what the font's GSUB table would do. ApplyGSUBContextualFeature is the shared path for contextual features such as rclt and calt; ApplyLocalizedForms applies locl under the active GSUB script and language. If the caller emits returned substitute GIDs manually, MarkUnicodeGlyphUsed or a synthetic-codepoint mapping must keep those glyphs inside the embedded subset and ToUnicode map