I'm also running Jan's patch and have disabled stem darkening. And I
think this approach is actually a good idea.
1. Fontconfig's hintslight is translated to FT_LOAD_TARGET_LIGHT by
cairo and probably also Qt. And so far, hintslight meant the autohinter
regardless of the "autohint" property. Changing the meaning of
FT_LOAD_TARGET_LIGHT to "Use native Y-only-snapping with linear advances
if driver and font supports it and Y-only autohinter with linear
advances otherwise" would restore orthogonality to the hintstyle
property. This also means that no code has to be changed or ripped out
to support the above meaning while still delivering consistent rendering
across fonts and font formats. Medium/Full then wouldn't change anything
for CFF fonts but would trigger ClearType rendering without linear
advances or with backwards-compatible X-and-Y-snapping.
I noticed that Liberation Sans/Serif/Mono and Open Sans looked pretty
ClearType-y with hintfull but I wasn't sure about the advance widths. Is
there any way to detect this from within FT_Load_Glyph or something so
that native ClearType fonts with linear advance widths can be send to
the native hinter?
2. I think it might be best to disable stem darkening for now when
including Jan's patch (my suggestion) for the sake of consistent
rendering. My planned blog post would still stand, I'd just call them
experimental features and work on refactoring stem darkening support in
the future.
Ideally, could this be controlled by an optional FT_LOAD_STEM_DARKENING
flag to FT_Load_Glyph? So when Qt5/Skia/etc. decide to ship with LAB+GC,
they can just look for the flag in FreeType and enable it, otherwise
proceed like before. This would lessen the need to coordinate a FreeType
release, a toolkit/rendering library release and the packaging of them...
Conclusion: Let's change FT_LOAD_TARGET_LIGHT and disable stem darkening
for now!
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