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

1const subsetFont = require('subset-font');
2
3const mySfntFontBuffer = Buffer.from(/*...*/);
4
5// Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
6const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
7  targetFormat: 'woff2',
8});

Reducing the variation space

1const subsetFont = require('subset-font');
2
3const mySfntFontBuffer = Buffer.from(/*...*/);
4
5// Create a new font with only the characters required to render "Hello, world!" in WOFF2 format:
6const subsetBuffer = await subsetFont(mySfntFontBuffer, 'Hello, world!', {
7  targetFormat: 'woff2',
8  variationAxes: {
9    // Pin the axis to 200:
10    wght: 200,
11    // Reduce the variation space, explicitly setting a new default value:
12    GRAD: { min: -50, max: 50, default: 25 },
13    // Reduce the variation space. A new default value will be inferred by clamping the old default to the new range:
14    slnt: { min: -9, max: 0 },
15    // The remaining axes will be kept as-is
16  },
17});

API

subsetFont(buffer, text, options): Promise<Buffer>

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.