OpenType

A TypeScript library for parsing OpenType and TrueType fonts, designed primarily for the needs of PDF document generation. The library provides access to the most commonly used OpenType tables and supports font subsetting.

Features

Font Parsing

Access to essential OpenType font tables:

• head (Font Header)
• hhea (Horizontal Header)
• hmtx (Horizontal Metrics)
• maxp (Maximum Profile)
• cmap (Character to Glyph Mapping)
• glyf (Glyph Data)
• loca (Index to Location)
• name (Naming Table)
• OS/2 (OS/2 and Windows Metrics)
• post (PostScript Information)
• GPOS (Glyph Positioning)
• GSUB (Glyph Substitution)

Font Subsetting

Build subset fonts containing only required glyphs for embedding in PDF documents

Installation

npm install @ralfstx/opentype

Usage

Loading Fonts

import { Font } from '@ralfstx/opentype';
import { readFile } from 'node:fs/promises';

// Load a font file
const fontData = await readFile('path/to/font.ttf');
const font = Font.fromData(fontData);

// Access font properties
console.log(font.fontName); // Full font name
console.log(font.numGlyphs); // Number of glyphs
console.log(font.unitsPerEm); // Units per em square

Accessing Font Tables

// Access specific tables
const headTable = font.tables.head;
const cmapTable = font.tables.cmap;
const glyfTable = font.tables.glyf;

// Get glyph ID for a character
const glyphId = font.glyphMapping?.get('A'.codePointAt(0)!);

// Get horizontal metrics
const hmtx = font.tables.hmtx;
const metric = hmtx?.getMetric(glyphId);
console.log(metric?.advanceWidth);

Text Shaping

Text shaping converts Unicode text into positioned glyphs, applying OpenType features like ligatures and kerning. The result is a GlyphRun containing ShapedGlyph objects ready for rendering.

import { TextShaper } from '@ralfstx/opentype';

// Create a text shaper for Latin script, English language
const shaper = new TextShaper(font, 'latn', 'en');

// Shape text into a glyph run
const glyphRun = shaper.shape('Hello');

// Each glyph has positioning information
for (const glyph of glyphRun.glyphs) {
  console.log({
    glyphId: glyph.glyphId, // Glyph ID in the font
    codePoints: glyph.codePoints, // Unicode code points (for ligatures: multiple)
    advance: glyph.advance, // Horizontal advance (font units)
    advanceAdjust: glyph.advanceAdjust, // Kerning adjustment (if any)
  });
}

Script and language tags:

Common script tags: latn (Latin), cyrl (Cyrillic), arab (Arabic), hans (Simplified Chinese). See the OpenType script tags for a complete list.

Default features applied:

• GSUB: locl, ccmp, rlig, liga, clig, calt
• GPOS: kern

Optional features:

Common typographic features that can be enabled:

• smcp: Small caps
• c2sc: Caps to small caps
• subs: Subscript
• sups: Superscript
• frac: Fractions

// Shape text with small caps feature
const smallCaps = shaper.shape('Hello', { features: ['smcp'] });

Font Subsetting

The createSubset function builds a subset font containing only the specified glyphs, suitable for embedding in PDF documents.

import { createSubset } from '@ralfstx/opentype';

// Get glyph IDs for specific characters
const text = 'Hello, World!';
const glyphIds = Array.from(text)
  .map((char) => font.glyphMapping?.get(char.codePointAt(0)!))
  .filter((id) => id !== undefined);

// Create a subset font
const { fontData, glyphIdMapping } = createSubset(font, glyphIds);

// Save the subset font
await writeFile('subset.ttf', new Uint8Array(fontData));

License

MIT

Resources

Specifications

• OpenType Specification
• TrueType Reference Manual

Related Libraries

• opentype.js - JavaScript OpenType parser
• fontkit - Advanced font toolkit for Node
• fonttools - Python font manipulation library
• Fontations - Rust font parsing library