Fix pedantic TrueType interpreter behaviour for out-of-bounds CVT and storage access #496
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Pull request overview
This PR hardens the TrueType interpreter against out-of-bounds CVT/storage accesses and adds regression coverage (fonts + golden images) for the Runic, Ogham, and MgOpenCanonic cases that previously triggered exceptions (issue #493).
Changes:
- Replaced direct array accesses in
TrueTypeInterpreterwith unsigned-bounds-checked reads/writes for storage and CVT, returning 0 or skipping writes when out of range, and removed theCheckIndexhelper. - Updated initialization style (collection expressions, explicit types) in
TrueTypeInterpreterandZone. - Added test font paths and new issue-regression tests for Ogham, Runic, and MgOpenCanonic, along with embedded TrueType font files and reference output PNGs.
Reviewed changes
Copilot reviewed 6 out of 9 changed files in this pull request and generated 1 comment.
Show a summary per file
| File | Description |
|---|---|
| src/SixLabors.Fonts/Tables/TrueType/Hinting/TrueTypeInterpreter.cs | Adds safe index handling for storage and CVT ops, updates CVT delta handling, and slightly modernizes local declarations. |
| tests/SixLabors.Fonts.Tests/TestFonts.cs | Exposes new test font paths for Noto Sans Ogham, Noto Sans Runic, and MgOpenCanonic. |
| tests/SixLabors.Fonts.Tests/Issues/Issues_493.cs | Introduces regression tests for issue #493 covering Ogham, Runic, and a MgOpenCanonic scenario (with a font selection bug noted below). |
| tests/SixLabors.Fonts.Tests/Fonts/NotoSansOgham-Regular.ttf | Adds Ogham test font used by the new regression test. |
| tests/SixLabors.Fonts.Tests/Fonts/NotoSansRunic-Regular.ttf | Adds Runic test font used by the new regression test. |
| tests/Images/ReferenceOutput/Test_Issue_493_Ogham-.png | Baseline reference rendering for the Ogham issue test. |
| tests/Images/ReferenceOutput/Test_Issue_493_Runic-.png | Baseline reference rendering for the Runic issue test. |
| tests/Images/ReferenceOutput/Test_Issue_493_MgOpenCanonic-.png | Baseline reference rendering intended for the MgOpenCanonic issue test. |
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Pull request overview
Copilot reviewed 12 out of 15 changed files in this pull request and generated 4 comments.
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src/SixLabors.Fonts/Tables/TrueType/Hinting/TrueTypeInterpreter.cs
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src/SixLabors.Fonts/Tables/TrueType/Hinting/TrueTypeInterpreter.cs
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src/SixLabors.Fonts/Tables/TrueType/TrueTypeGlyphMetrics.CompatibilityLists.cs
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Prerequisites
Description
Fixes #493
This pull request introduces significant improvements to the TrueType font hinting system, focusing on enhancing robustness, error handling, and font-specific compatibility. The main changes include making the interpreter more resilient to malformed instructions, introducing font-specific hinting overrides, and updating test reference images to reflect these improvements.
Robustness and Error Handling Improvements
TryHintGlyphmethod that returns a boolean indicating success or failure, ensuring that glyph outlines remain unmodified if hinting fails due to malformed instructions or disabled grid fitting. The interpreter now resets its state and avoids leaking changes between glyphs on errors. (GlyphVector.cs,TrueTypeInterpreter.cs) [1] [2] [3]TrueTypeInterpreter.cs) [1] [2]Font-Specific Hinting Compatibility
TrueTypeGlyphMetrics.CompatibilityLists.csthat enforces or disables hinting for specific font families based on known rendering issues, using logic derived from FreeType and community reports. The rendering pipeline now checks these lists to determine the effective hinting mode for each font. (TrueTypeGlyphMetrics.CompatibilityLists.cs,TrueTypeGlyphMetrics.cs) [1] [2] [3]Hinting Logic and Interpreter Behavior
TrueTypeInterpreter.cs) [1] [2] [3] [4]TrueTypeInterpreter.cs) [1] [2] [3] [4] [5]Test and Reference Output Updates
Test_Hinting_Robustness_-Arial-.png,Test_Hinting_Robustness_-OpenSansFile-.png,Test_Hinting_Robustness_-Tahoma-.png,Test_Issue_493_MgOpenCanonic-.png,Test_Issue_493_Ogham-.png) [1] [2] [3] [4] [5]