opentype_shaper 0.1.1

OpenType text shaping for Dart and Flutter: GSUB substitutions and GPOS mark positioning, returning glyph ids with per-glyph offsets.

opentype_shaper

OpenType text shaping for Dart and Flutter.

Dart's PDF and text libraries do not run a font's OpenType layout tables. For a script that positions marks through GPOS, such as Hebrew with nikud and te'amim, that leaves every mark drawn at the pen instead of on its letter, and a glyph that only a GSUB rule can produce is never reached at all.

This package runs the shaping and hands back what a renderer or a PDF writer needs to place text itself: a glyph id per position, with the offset and advance the font's rules produced, in font units.

The engine is HarfRust, the Rust port of HarfBuzz, compiled into a small native library with no C or C++ dependencies. The Dart side talks to it over a plain C ABI, so there is no bridge runtime and nothing to initialise at startup.

Usage

import 'package:opentype_shaper/opentype_shaper.dart';

final font = ShaperFont.register(fontBytes);

final run = font.shape('שָׁלוֹם', rtl: true, script: 'hebr');
for (var i = 0; i < run.glyphCount; i++) {
  // run.glyphId(i), run.xAdvance(i), run.xOffset(i), run.yOffset(i)
}

font.dispose();

Results are in font units; divide by font.metrics.unitsPerEm and multiply by the point size. Glyphs of a right-to-left run come back in visual order, so a consumer advances the pen positively while emitting them in sequence.

ShaperFont.attach(handle) lets a background isolate use a font the main isolate registered, by passing the integer handle rather than the bytes.

Building

The Rust crate lives in rust/ and is built by cargokit as part of the Flutter build. Until precompiled binaries are published, a Rust toolchain is required. Adding rust/cargokit.yaml with a precompiled_binaries block (release URL prefix and the signing public key) switches consumers to downloading them instead; the file must then hold that block, because cargokit rejects an empty one.

Tests need the compiled library and a font:

cargo build --release --manifest-path rust/Cargo.toml
OPENTYPE_SHAPER_TEST_FONT=/path/to/NotoSerifHebrew.ttf flutter test

The repository vendors no fonts. Continuous integration fetches an OFL font.

License

MIT. See LICENSE.