feat: add support for Ideographic Variation Sequences (IVS) in TrueType fonts #1455
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Description
This PR implements support for Ideographic Variation Sequences (IVS) in TrueType font rendering. IVS allows proper display of variant glyphs for CJK (Chinese, Japanese, Korean) characters, which is essential for correct typography in East Asian languages.
Key Changes
Added IVS lookup tables parsing in TrueType font processor
Implemented Unicode Variation Selector matching logic
Enhanced glyph substitution mechanism to handle IVS variants
Added fallback mechanisms for fonts without IVS support
Updated font metrics calculation for variant glyphs
Technical Details
Supports Unicode Variation Selectors
Implements gsub table lookup for uvs and non-default UVS features
Maintains backward compatibility with existing font rendering
Testing
✅ use template text and font export for IVS sequence detection
✅ Tested with known IVS-enabled fonts (e.g., Source Hei_MSCS)
✅ Verified rendering of CJK variant characters
✅ Confirmed backward compatibility with standard TrueType fonts
Impact
Enables correct display of CJK typographic variants
Improves PDF generation quality for East Asian documents
No breaking changes to existing API
Minimal performance impact (only activates when IVS sequences are present)
Related Issues
Closes #[issue-number-here]
Compatibility
✅ Fully backward compatible
✅ No changes to public API signatures
✅ Works with existing TrueType/OpenType fonts
✅ Proper fallback for fonts without IVS support
Additional Notes
This implementation follows the Unicode Standard Annex #44 and OpenType specification for IVS handling. The feature only activates when both the font supports IVS and the text contains variation selector characters, ensuring minimal overhead for regular documents.
Your Name: oBo
Testing Instructions:
Use a font with IVS support (e.g., Source Han Sans)
Create PDF with CJK text containing variation selectors
Verify variant glyphs render correctly
Test with fonts without IVS support to confirm fallback behavior