Use correct font metrics for fallback glyph substitutions #485
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Pull request overview
This PR fixes a bug where fallback font glyph substitutions were incorrectly using font metrics from the main font instead of the fallback font. This affected complex scripts that require advanced shaping (like Kannada, Indic, Hangul) when rendered with fallback fonts.
- Threaded
FontMetricsparameter through shaper factory and into shapers that perform font-specific lookups - Converted static methods to instance methods in shapers to access correct font metrics
- Optimized by hoisting font metric lookups outside loops where possible
Reviewed changes
Copilot reviewed 9 out of 9 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| tests/SixLabors.Fonts.Tests/Issues/Issues_483.cs | Adds regression test for Kannada text rendering with fallback fonts using snapshot-based validation |
| tests/Images/ReferenceOutput/Test_Issue_483-.png | Reference snapshot image for the regression test |
| src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/UniversalShaper.cs | Adds fontMetrics field and converts methods to instance methods to use correct font metrics |
| src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/ShaperFactory.cs | Updates factory to accept and pass fontMetrics parameter to shapers that need it |
| src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/IndicShaper.cs | Adds fontMetrics field, optimizes by hoisting lookups outside loops, and uses correct font metrics |
| src/SixLabors.Fonts/Tables/AdvancedTypographic/Shapers/HangulShaper.cs | Adds fontMetrics field, converts methods to instance methods, and simplifies nested conditionals |
| src/SixLabors.Fonts/Tables/AdvancedTypographic/GSubTable.cs | Passes fontMetrics to shaper factory during glyph substitution |
| src/SixLabors.Fonts/Tables/AdvancedTypographic/GPosTable.cs | Passes fontMetrics to shaper factory during glyph positioning |
| src/SixLabors.Fonts/GlyphSubstitutionCollection.cs | Already uses collection expression syntax (no functional change in diff) |
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Prerequisites
Description
Fixes #483
This PR fixes a bug where fallback font glyph substitutions were incorrectly using font metrics from the main font instead of the fallback font. This affected complex scripts that require advanced shaping (like Kannada, Indic, Hangul) when rendered with fallback fonts.
FontMetricsparameter through shaper factory and into shapers that perform font-specific lookups