fontkit

What is fontkit?

Fontkit is a comprehensive library for working with fonts in Node.js and the browser. It supports various font formats and provides a wide range of functionalities for font manipulation, including glyph rendering, font subsetting, and text layout.

What are fontkit's main functionalities?

Loading Fonts

This feature allows you to load a font from a file. The `openSync` method synchronously loads the font, making it ready for further operations.

const fontkit = require('fontkit');
const font = fontkit.openSync('path/to/font-file.ttf');

Glyph Rendering

This feature allows you to render glyphs from the font. You can get the glyph for a specific character and then convert it to an SVG path for rendering.

const glyph = font.glyphForCodePoint(65); // Get the glyph for the character 'A'
const path = glyph.path.toSVG(); // Get the SVG path data for the glyph

Font Subsetting

This feature allows you to create a subset of the font, including only the glyphs you need. This can be useful for reducing the size of the font file.

const subset = font.createSubset();
subset.includeGlyph(font.glyphForCodePoint(65)); // Include the glyph for 'A'
const subsetFont = subset.encode();

Text Layout

This feature allows you to layout text using the font. The `layout` method returns a run of glyphs, which you can then iterate over to get information about each glyph.

const run = font.layout('Hello, world!');
run.glyphs.forEach(glyph => {
  console.log(glyph.id, glyph.advanceWidth);
});

Other packages similar to fontkit

opentype.js

Opentype.js is a JavaScript library for parsing and working with OpenType and TrueType fonts. It provides functionalities for font parsing, glyph rendering, and text layout. Compared to Fontkit, opentype.js is more focused on OpenType and TrueType fonts and may not support as many font formats.

pdf-lib

PDF-lib is a library for creating and modifying PDF documents in JavaScript. It includes functionalities for embedding fonts and rendering text within PDFs. While it is not as specialized in font manipulation as Fontkit, it provides a broader range of PDF-related functionalities.

fonteditor-core

Fonteditor-core is a library for editing font files in JavaScript. It supports various font formats and provides functionalities for font parsing, glyph editing, and font generation. Compared to Fontkit, fonteditor-core is more focused on font editing and manipulation.

README

fontkit

Fontkit is an advanced font engine for Node and the browser, used by PDFKit. It supports many font formats, advanced glyph substitution and layout features, glyph path extraction, color emoji glyphs, font subsetting, and more.

Features

• Suports TrueType (.ttf), OpenType (.otf), WOFF, WOFF2, TrueType Collection (.ttc), and Datafork TrueType (.dfont) font files
• Supports mapping characters to glyphs, including support for ligatures and other advanced substitutions (see below)
• Supports reading glyph metrics and laying out glyphs, including support for kerning and other advanced layout features (see below)
• Advanced OpenType features including glyph substitution (GSUB) and positioning (GPOS)
• Apple Advanced Typography (AAT) glyph substitution features (morx table)
• Support for getting glyph vector paths and converting them to SVG paths, or rendering them to a graphics context
• Support for color glyphs (e.g. emoji), including Apple’s SBIX table, and Microsoft’s COLR table
• Font subsetting support - create a new font including only the specified glyphs

Installation

npm install fontkit

Example

var fontkit = require('fontkit');

// open a font synchronously
var font = fontkit.openSync('font.ttf');

// get some glyphs for a string, and apply ligature substitutions
var glyphs = font.glyphsForString('hello world!', ['liga']);

// get glyph advances, and apply kerning
var advances = font.advancesForGlyphs(glyphs, ['kern']);

// get an SVG path for a glyph
var svg = glyphs[0].path.toSVG();

// create a font subset
var subset = font.createSubset();
glyphs.forEach(function(glyph) {
  subset.includeGlyph(glyph);
});

subset.encodeStream()
      .pipe(fs.createWriteStream('subset.ttf'));

API

fontkit.open(filename, postscriptName = null, callback(err, font))

Opens a font file asynchronously, and calls the callback with a font object. For collection fonts (such as TrueType collection files), you can pass a postscriptName to get that font out of the collection instead of a collection object.

fontkit.openSync(filename, postscriptName = null)

Opens a font file synchronously, and returns a font object. For collection fonts (such as TrueType collection files), you can pass a postscriptName to get that font out of the collection instead of a collection object.

fontkit.create(buffer, postscriptName = null)

Returns a font object for the given buffer. For collection fonts (such as TrueType collection files), you can pass a postscriptName to get that font out of the collection instead of a collection object.

Font objects

There are several different types of font objects that are returned by fontkit depending on the font format. They all inherit from the TTFFont class and have the same public API, described below.

Metadata properties

The following properties are strings (or null if the font does not contain strings for them) describing the font, as specified by the font creator.

• postscriptName
• fullName
• familyName
• subfamilyName
• copyright
• version

Metrics

The following properties describe the general metrics of the font. See here for a good overview of how all of these properties relate to one another.

• scale - the font’s internal scale factor
• unitsPerEm - the size of the font’s internal coordinate grid
• ascent - the font’s ascender
• descent - the font’s descender
• lineGap - the amount of space that should be included between lines
• underlinePosition - the offset from the normal underline position that should be used
• underlineThickness - the weight of the underline that should be used
• italicAngle - if this is an italic font, the angle the cursor should be drawn at to match the font design
• capHeight - the height of capital letters above the baseline. See here for more details.
• xHeight- the height of lower case letters. See here for more details.
• bbox - the font’s bounding box, i.e. the box that encloses all glyphs in the font

Other properties

• numGlyphs - the number of glyphs in the font
• characterSet - an array of all of the unicode code points supported by the font
• availableFeatures - an array of all OpenType feature tags (or mapped AAT tags) supported by the font (see below for a description of this)

Character to glyph mapping

Fontkit includes several methods for character to glyph mapping, including support for advanced OpenType and AAT substitutions.

font.glyphsForString(string, features = null)

This method returns an array of Glyph objects for the given string. This may not be a one to one mapping if OpenType or AAT substitutions are applied.

The features parameter is an array of OpenType feature tags to be applied. If this is an AAT font, the OpenType feature tags are mapped to AAT features. If nothing is passed to the features parameter, a set of default features are applied. To disable features entirely, explicitly pass an empty array to the features parameter.

font.glyphForCodePoint(codePoint)

Maps a single unicode code point (number) to a Glyph object. Does not perform any advanced substitutions (there is no context to do so).

font.hasGlyphForCodePoint(codePoint)

Returns whether there is glyph in the font for the given unicode code point.

Glyph metrics and layout

Fontkit includes several methods for accessing glyph metrics and performing layout, including support for kerning and other advanced OpenType positioning adjustments.

font.advancesForGlyphs(glyphs, features = null)

Returns an array of advances for the given array of Glyph objects. Conceptually, an advance is the distance to move the “pen” after a glyph has been rendered, before the next glyph is rendered.

The features parameter is an array of OpenType feature tags to be applied. If this is an AAT font, only the ‘kern’ feature is supported. If nothing is passed to the features parameter, a set of default features are applied. To disable features entirely, explicitly pass an empty array to the features parameter.

font.widthOfString(string, features = null)

Returns the width of the given string, applying the given features as described above. This is just the sum of advancesForGlyphs(glyphsForString(string)).

font.widthOfGlyph(glyph_id)

Returns the advance width (described above) for a single glyph id.

Other methods

font.getGlyph(glyph_id, codePoints = [])

Returns a glyph object for the given glyph id. You can pass the array of code points this glyph represents for your use later, and it will be stored in the glyph object.

font.createSubset()

Returns a Subset object for this font, described below.

Font Collection objects

For font collection files that contain multiple fonts in a single file, such as TrueType Collection (.ttc) and Datafork TrueType (.dfont) files, a font collection object can be returned by Fontkit.

collection.getFont(postscriptName)

Gets a font from the collection by its postscript name. Returns a Font object, described above.

Glyph objects

Glyph objects represent a glyph in the font. They have various properties for accessing metrics and the actual vector path the glyph represents, and methods for rendering the glyph to a graphics context.

You do not create glyph objects directly. They are created by various methods on the font object, described above. There are several subclasses of the base Glyph class internally that may be returned depending on the font format, but they all include the following API.

Properties

• id - the glyph id in the font
• codePoints - an array of unicode code points that are represented by this glyph. There can be multiple code points in the case of ligatures and other glyphs that represent multiple visual characters.
• path - a vector Path object representing the glyph
• bbox - the glyph’s bounding box, i.e. the rectangle that encloses the glyph outline as tightly as possible.
• cbox - the glyph’s control box. This is often the same as the bounding box, but is faster to compute. Because of the way bezier curves are defined, some of the control points can be outside of the bounding box. Where bbox takes this into account, cbox does not. Thus, cbox is less accurate, but faster to compute. See here for a more detailed description.
• advanceWidth - the glyph’s advance width. Equivalent to calling font.widthOfGlyph(glyph.id)

glyph.render(ctx, size)

Renders the glyph to the given graphics context, at the specified font size.

Color glyphs (e.g. emoji)

Fontkit has support for several different color emoji font formats. Currently, these include Apple’s SBIX table (as used by the “Apple Color Emoji” font), and Microsoft’s COLR table (supported by Windows 8.1). Here is an overview of the various color font formats out there.

glyph.getImageForSize(size)

For SBIX glyphs, which are bitmap based, this returns an object containing some properties about the image, along with the image data itself (usually PNG).

glyph.layers

For COLR glyphs, which are vector based, this returns an array of objects representing the glyphs and colors for each layer in render order.

Path objects

Path objects are returned by glyphs and represent the actual vector outlines for each glyph in the font. Paths can be converted to SVG path data strings, or to functions that can be applied to render the path to a graphics context.

path.moveTo(x, y)

Moves the virtual pen to the given x, y coordinates.

path.lineTo(x, y)

Adds a line to the path from the current point to the given x, y coordinates.

path.quadraticCurveTo(cpx, cpy, x, y)

Adds a quadratic curve to the path from the current point to the given x, y coordinates using cpx, cpy as a control point.

path.bezierCurveTo(cp1x, cp1y, cp2x, cp2y, x, y)

Adds a bezier curve to the path from the current point to the given x, y coordinates using cp1x, cp1y and cp2x, cp2y as control points.

path.closePath()

Closes the current sub-path by drawing a straight line back to the starting point.

path.toFunction()

Compiles the path to a JavaScript function that can be applied with a graphics context in order to render the path.

path.toSVG()

Converts the path to an SVG path data string.

path.bbox

This property represents the path’s bounding box, i.e. the smallest rectangle that contains the entire path shape. This is the exact bounding box, taking into account control points that may be outside the visible shape.

path.cbox

This property represents the path’s control box. It is like the bounding box, but it includes all points of the path, including control points of bezier segments. It is much faster to compute than the real bounding box, but less accurate if there are control points outside of the visible shape.

Subsets

Fontkit can perform font subsetting, i.e. the process of creating a new font from an existing font where only the specified glyphs are included. This is useful to reduce the size of large fonts, such as in PDF generation or for web use.

Currently, subsets produce minimal fonts designed for PDF embedding that may not work as standalone files. They have no cmap tables and other essential tables for standalone use. This limitation will be removed in the future.

You create a Subset object by calling font.createSubset(), described above. The API on Subset objects is as follows.

subset.includeGlyph(glyph)

Includes the given glyph object or glyph ID in the subset.

subset.encodeStream()

Returns a stream containing the encoded font file that can be piped to a destination, such as a file.

License

MIT