Support variations of fonts in WOFF and WOFF2 containers

[sidorares]

font.getVariation() works on a plain sfnt and throws, or returns a font with no tables, on the same font in a WOFF or a WOFF2. Nearly every variable web font is served as WOFF2, so this is the case a web page hits: Geist, Inter, anything from next/font or Google Fonts.

fontkit.openSync('Geist-Variable.woff2').getVariation({ wght: 700 });
// 2.0.4: a TTFFont whose directory was parsed out of Brotli-compressed
// bytes. No cmap; "Cannot read properties of undefined (reading 'tables')"
// at the first glyph. On a small file, "Offset is outside the bounds of the
// DataView" from getVariation itself.

Cause

getVariation rebuilds the font as new TTFFont(stream, coords), from this.stream.buffer at the directory's offset.

• WOFF: that reads a WOFF directory as an sfnt directory.
• WOFF2: the stream is the compressed file, or, once anything was decoded, the decompressed tables with no directory in front of them.

Two more things were in the way once the font was built by the right class:

• GlyphVariationProcessor read the glyph variation data from font.stream at offsets that are positions in the stream the gvar table was decoded from. For a WOFF that is the inflated table, not the file.
• WOFF2Glyph._decode returns the glyph decoded in advance from the transformed glyf table, and the variation is applied in TTFGlyph._decodeSimple / _decodeComposite, which a WOFF2 glyph never goes through.

Fix

• TTFFont#_createVariation(coords) makes the font getVariation returns, as new this.constructor(...). A container reads its directory and its tables the way it holds them.
• WOFF: the processor reads gvar through font._getTableStream('gvar'). WOFFFont inflates a table once and keeps it (shared with its variations) — it inflated the whole table again for every glyph that asked, glyf included, which is a speed-up for a WOFF font with no variation at all.
• WOFF2: a variation reads the directory from the file again and shares the font's decompressed tables and its decoded glyphs, so Brotli runs once per font however many variations are made. The transformed glyf table is decoded without a variation applied; WOFF2Glyph moves a copy of a glyph's points, or of its components' offsets, through the same code TTFGlyph uses, now _applyVariation. The phantom points need the bounding box a glyf header carries: the transformed table stores it for some glyphs and leaves it to be computed from the points for the rest (WOFF2, 5.1), so the bounding box stream is read, where it was skipped.

Nothing changes for an sfnt: new this.constructor is new TTFFont, and _getTableStream('gvar') is the font's own stream.

Tests

test/variations.js, "variations of a font in a container": every glyph of a variation — path, bounding box, advance width and height — is compared with the same variation of the sfnt the container was made from.

font	containers	what it covers
TestGVAROne	WOFF, WOFF2, WOFF2 with an untransformed glyf	simple glyphs, advances from the phantom points
TestGVARFour	WOFF, WOFF2	two axes, advances from HVAR
Mada-VF	WOFF2	292 composite glyphs of 529
AdobeVFPrototype-Subset	WOFF, WOFF2	CFF2 outlines

Plus: the font and its other variations are unchanged after a variation is read (the decoded glyphs are shared), a variation of a variation, a named variation, and GPOS mark anchors in a layout.

The containers were made with fontTools from the fonts already in test/data (TTFont(path); font.flavor = 'woff2'; font.save(...)), 87 KB in all.

556 passing, 7 pending, 1 failing — the failure is i18n › can set global default language to "ar", which fails the same way on master. The suite needs Node ≤ 24 to start: mocha 10's yargs does not load on Node 26.

Checked outside the suite: the 975 glyphs of Geist's variable WOFF2 at wght 600 and 900 have the same paths as fontTools' instancer makes from it, and advances within a unit.

Geist, 975 glyphs	open	first variation	every glyph path	next variation
.ttf	1.7 ms	1.5 ms	46 ms	0.2 ms
.woff2	0.9 ms	27 ms	30 ms	0.2 ms

The first variation of a WOFF2 is where it is decompressed and its glyf table decoded.