Use glyph indices for font tracking in vector formats #30335
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Previously, these were both typed as `int`, which means you could mix and match them erroneously. While the character code can't be made a distinct type (because it's used for `chr`/`ord`), typing glyph indices as a distinct type means these can't be fully swapped. Unfortunately, you can still go back to the base type, so glyph indices still work as character codes. But this is still sufficient to catch errors such as the wrong call to `FT2Font.get_kerning` in `_mathtext.py`.
The `FontInfo.num` value returned by `TruetypeFonts._get_info` is a character code, but `FT2Font.get_kerning` takes *glyph indices*, meaning that kerning was likely off in most cases.
With libraqm, string layout produces glyph indices, not character codes, and font features may even produce different glyphs for the same character code (e.g., by picking a different Stylistic Set). Thus we cannot rely on character codes as unique items within a font, and must move toward glyph indices everywhere.
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PR summary
With libraqm, string layout produces glyph indices, not character codes, and font features may even produce different glyphs for the same character code (e.g., by picking a different Stylistic Set). Thus we cannot rely on character codes as unique items within a font, and must move toward glyph indices everywhere.
The only thing I don't quite like is that PDF uses character codes for its lookup, and I have to map glyph indices back through an inverse charmap. I think I may have to send everything through
CharacterTrackerand produce my own limited charmap, but still need to test out what's required.This is based on #30143.
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