Re: [ft-devel] Turning off stem darkening by default until all drivers support it?

[Nikolaus Waxweiler]

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!