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Fonts

Font Registration​

lui does not ship fonts. You must register them at startup:

tree.register_font(FontFace {
    family: "sans-serif".into(),
    file: "fonts/Roboto-Regular.ttf".into(),
    weight: 400,
    style: FontStyle::Normal,
});

Supported formats: .ttf, .otf, .ttc (TrueType Collection).

Font Matching​

Fonts are matched by family → weight → style:

  • Family: exact match on the registered family string
  • Weight: closest available weight (100–900 range)
  • Style: Normal or Italic/Oblique

If no exact family match is found, falls back to the first registered font with a matching generic family (sans-serif, serif, monospace).

Multiple Weights and Styles​

Register multiple font files for the same family to enable weight and style variations:

tree.register_font(FontFace {
    family: "sans-serif".into(),
    file: "fonts/Roboto-Regular.ttf".into(),
    weight: 400,
    ..Default::default()
});
tree.register_font(FontFace {
    family: "sans-serif".into(),
    file: "fonts/Roboto-Bold.ttf".into(),
    weight: 700,
    ..Default::default()
});
tree.register_font(FontFace {
    family: "sans-serif".into(),
    file: "fonts/Roboto-Italic.ttf".into(),
    weight: 400,
    style: FontStyle::Italic,
});

System Font Discovery​

The winit driver includes a helper for automatic system font registration:

tree.register_system_fonts("sans-serif");

This scans standard OS font directories:

  • Windows: C:\Windows\Fonts\
  • Linux: /usr/share/fonts/, ~/.local/share/fonts/
  • macOS: /System/Library/Fonts/, /Library/Fonts/, ~/Library/Fonts/
// Discover available font variants without registering
let variants = system_font_variants("sans-serif");

Glyph Atlas​

Fonts are rasterized on the CPU and stored in a shared 2048×2048 RGBA glyph atlas texture on the GPU. The atlas uses a shelf-packing allocator to place glyph bitmaps efficiently.

Current Limitations​

  • No @font-face support for web fonts
  • No variable font axis support (font-variation-settings)
  • No font-kerning, font-language-override, font-optical-sizing
  • em/rem use a hard-coded 16px when no font-size is inherited