oni-fontkit

This is a fork of the awesome font engine fontkit which was optimized for usage within the Oni editor. The fork exists because we need to remove unrequired but security-critical code from the package while at the same time adding a few tweaks to make the logic fit our use-cases.

If you did not come here as an Oni contributor, the original repo is probably what you're looking for.

The most notable adaptations are:

• No more layout-related logic since it involves heavy logic and bundle size additions which are not required in Oni
• No more Glyph.render method since the implementation was considered a security risk in the context of our electron application
• Added Typescript definitions
• Additional return values and a new method Font.applySubstitutionFeatures for applying context-based substitutions and then extracting the metadata

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
• Supports TrueType (glyf) and PostScript (CFF) outlines
• Support for color glyphs (e.g. emoji), including Apple’s SBIX table, and Microsoft’s COLR table
• Support for AAT variation glyphs, allowing for nearly infinite design control over weight, width, and other axes.
• 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');

// map a string to the corresponding Glyphs.
// Does not perform advanced features based on context yet
var glyphs = font.glyphsForString('hello world!');

// apply contextual substitution on the glyphs
// the returned array contains a contextGroupId for each glyph which
// can be used to group the glyphs together based on the contexts they triggered
var contextGroupIds = font.applySubstitutionFeatures(glyphs, ['calt'])

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

// create a font subset
var subset = font.createSubset();
run.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.

• 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.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.

font.glyphsForString(string)

This method returns an array of Glyph objects for the given string. This is only a one-to-one mapping from characters to glyphs. For most uses, you should use font.layout (described below), which provides a much more advanced mapping supporting AAT and OpenType shaping.

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.applySubstitutionFeatures(string, features = [])

This method performs in-place substitutions on the given array of glyphs. The substitutions are acrivated by passing the corresponding feature tags which are mapped in the font.

Returns an array of context group IDs per glyph which can be used to backtrack which Glyphs were involved together in context-based substitutions.

The features parameter is an array of OpenType feature tags to be applied in addition to the default set. If this is an AAT font, the OpenType feature tags are mapped to AAT features.

Variation fonts

Fontkit has support for AAT variation fonts, where glyphs can adjust their shape according to user defined settings along various axes including weight, width, and slant. Font designers specify the minimum, default, and maximum values for each axis they support, and allow the user fine grained control over the rendered text.

font.variationAxes

Returns an object describing the available variation axes. Keys are 4 letter axis tags, and values include name, min, default, and max properties for the axis.

font.namedVariations

The font designer may have picked out some variations that they think look particularly good, for example a light, regular, and bold weight which would traditionally be separate fonts. This property returns an object describing these named variation instances that the designer has specified. Keys are variation names, and values are objects with axis settings.

font.getVariation(variation)

Returns a new font object representing this variation, from which you can get glyphs and perform layout as normal. The variation parameter can either be a variation settings object or a string variation name. Variation settings objects have axis names as keys, and numbers as values (should be in the range specified by font.variationAxes).

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.

collection.fonts

This property is a lazily-loaded array of all of the fonts in the collection.

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.

glyph.render(ctx, size)

Renders the COLR or SBIX 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.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