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FAQ
Why bind to platform-specific font libraries instead of replicating the native font rendering in Rust?
It's impractical to replicate the hinting behavior of Windows DirectWrite and the FreeType autohinter. See the following questions and answers.
The Windows font rasterizer heavily uses the TrueType hinting (more precisely, grid-fitting) feature. A TrueType hinting specification exists, and relatively-recent third-party implementations of it likewise exist, but the interaction between ClearType (Microsoft's implementation of subpixel antialiasing) and TrueType hinting is complex. Beat Stamm documents the basic problem: that a transform of 3x in the horizontal direction causes overflows in common TrueType fonts due to the pervasive use of 16-bit fixed point coordinates. Microsoft's solution is to patch the TrueType instructions in various ways at runtime in order to work around these issues. For GDI, the solution is a complex series of hacks; for DirectWrite, the solution is to ignore all horizontal hinting instructions, which still amounts to a series of hacks, albeit a less complex one. These hacks are not documented or specified, other than informally in Beat Stamm's book and in the FreeType source, but they are necessary to match the system rendering. In the author's judgment, attempting to reverse engineer and replicate these hacks is not worth the effort and risk.
Like the Windows TrueType font hinters, the FreeType autohinter is also an undocumented and unstable series of heuristics and hacks. These are more involved than even those of DirectWrite; they infer metrics from the shapes of well-known glyphs (like the Latin letter 'o' and so forth). Replicating these would be difficult, and the code would easily become out of date.
For a long time, FreeType was not commonly packaged with TrueType hinting enabled on common distributions. Therefore, Linux fonts were designed with the autohinter in mind. Desktop Linux users (as well of users of BSD and so forth) have come to expect the results of FreeType autohinting.
Yes, it would probably be desirable to have a rusttype loader for this use case. Contributions
are welcome!