feat: Add font hinting support via skrifa #1000
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I appreciate the effort, but unfortunately I don't think we can pull in skrifa as a dependency right now, otherwise we have ttf-parser AND skrifa in the dependency tree. We have plans for replacing ttf-parser, but it requires quite a few changes and I don't know yet when that will happen. |
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The transition to Skrifa/Fontations is implemented in #922 (except for fontdb). I suggest we merge that after the next release at which point we could look at this PR again. |
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I agree. #922 should be merged first. |
Supersedes linebender#922 (nicoburns' "Port resvg to harfrust and fontations") This patch builds upon the excellent groundwork laid out in PR linebender#922, which identified harfrust and skrifa as the path forward for long-term maintainability. As noted in that discussion, these actively-maintained libraries offer significant advantages: harfrust passes substantially more shaping tests than rustybuzz, and skrifa provides a modern, well-designed API for font parsing. Core migration (shared with linebender#922): - Replace rustybuzz with harfrust for OpenType text shaping - Replace ttf-parser with skrifa for font parsing and metrics - Update font metrics extraction to use skrifa's MetadataProvider Where this implementation extends linebender#922: 1. Font Hinting Support - Add optional `hinting` feature and HintingOptions in parser options - Implement hinted outline extraction via skrifa's HintingInstance - Honor CSS text-rendering property: geometricPrecision disables hinting, optimizeLegibility/optimizeSpeed enable full hinting - Configurable DPI for ppem calculation 2. Full Variable Font Support - Parse font-variation-settings CSS property - Parse font-optical-sizing CSS property (auto/none) - Auto-map standard properties to variation axes: * font-weight → wght axis * font-stretch → wdth axis * font-style: italic → ital axis * font-style: oblique → slnt axis - Auto-set opsz axis based on font size when font-optical-sizing: auto - Store per-glyph variation context in PositionedGlyph for correct outline extraction with variations applied 3. Modular Skrifa Integration - New skrifa_metrics.rs: Font metrics via skrifa's MetadataProvider - New skrifa_colr.rs: COLR rendering via skrifa's ColorPainter trait - Delete legacy colr.rs (ttf-parser based implementation) 4. Enhanced COLR Support - Implement skrifa's ColorPainter trait for SVG output - Support COLRv1 features: transforms, clips, composite modes - Handle linear, radial gradients with proper spread methods 5. Test Infrastructure - Add tests-hinted/ directory with hinted reference images - Add font-variation-settings test suite - Include RobotoFlex variable font for variation testing We appreciate the careful analysis in linebender#922 regarding the ecosystem trade-offs, particularly around compile times and binary size. This implementation accepts those trade-offs in exchange for the additional capabilities (hinting, variable fonts) that skrifa enables. Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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closing and opening fresh |
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Implement optional font hinting using Google's skrifa library for better
text rendering at small sizes. Hinting grid-fits glyph outlines to pixel
boundaries, improving legibility.
Features:
hintingcargo feature flag (requirestextfeature)Implementation:
to work with existing transform pipeline
Tests: