subset-font

Create a subset font from an existing font in SFNT (TrueType/OpenType), WOFF, or WOFF2 format. When subsetting a variable font, you can also reduce the variation space at the individual axis level.

These operations are implemented using harfbuzzjs, which is a WebAssembly build of HarfBuzz.

Basic example

``js
const subsetFont = require('subset-font');

const mySfntFontBuffer = Buffer.from(/.../);

// Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
targetFormat: 'woff2',
});
`

Reducing the variation space

`js
const subsetFont = require('subset-font');

const mySfntFontBuffer = Buffer.from(/.../);

// Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
targetFormat: 'woff2',
variationAxes: {
// Pin the axis to 200:
wght: 200,
// Reduce the variation space, explicitly setting a new default value:
GRAD: { min: -50, max: 50, default: 25 },
// Reduce the variation space. A new default value will be inferred by clamping the old default to the new range:
slnt: { min: -9, max: 0 },
// The remaining axes will be kept as-is
},
});
`

API

#### subsetFont(buffer, text, options): Promise

Asynchronously create a subset font as a Buffer instance, optionally converting it to another format.

Returns a promise that gets fulfilled with the subset font as a Buffer instance, or rejected with an error.

Options:

- targetFormat - the format to output, can be either 'sfnt', 'woff', or 'woff2'.
- preserveNameIds - an array of numbers specifying the extra name ids to preserve in the name table. By default the harfbuzz subsetter drops most of these. Use case described here.
- variationAxes - an object specifying a full or partial instancing of variation axes in the font. Only works with variable fonts. See the example above.
- noLayoutClosure - don't perform glyph closure for layout substitution (GSUB). Equivalent to hb-subset --no-layout-closure and pyftsubset --no-layout-closure.

For backwards compatibility reasons, 'truetype' is supported as an alias for 'sfnt'.

Why not use harfbuzzjs directly?

This middle-man module only really exists for convenience.

- harfbuzzjs is deliberately low-level bindings for HarfBuzz. While very flexible, it means that you need a series of hard-to-get-right incantations to move data in and out of the WebAssembly heap and carry out a subsetting operation. See harfbuzz/harfbuzzjs#9.
- The subsetting routines in HarfBuzz only support the SFNT (TrueType/OpenType) format. subset-font adds support for reading and writing WOFF and WOFF2 via the fontverter library.

Releases

Changelog

License

3-clause BSD license -- see the LICENSE` file for details.