[Development] Font rendering on X11/Wayland: New FreeType API to toggle LCD filters and stem darkening on a per-font basis, would love a review

[Nikolaus Waxweiler]

>> I'm working on FreeType and want to merge a new API (FT_Face_Option)
>> that would allow setting LCD filter weights and stem darkening
>> properties on a per-face basis.
> 
> I'm fascinated but clueless !

https://www.freetype.org/freetype2/docs/text-rendering-general.html#experimental-stem-darkening-for-the-auto-hinter

Correct font rendering with linear alpha blending and gamma correction
can make text look pale like reported in
https://bugreports.qt.io/browse/QTBUG-41590 due to the way the math
behind it works out. Stem darkening compensates for the paleness. The
new API is trying to make it practical for toolkits to opt into correct
rendering of fonts by making it easy to toggle stem darkening (older
toolkits without opt-in will get the same glyphs as before to keep
backwards compatibility). No more bug reports about pale text!

(Windows, macOS and Adobe's own font software render fonts correctly
(read: compose glyphs correctly onto surfaces). macOS and I think Adobe
software also use stem darkening to counteract paleness. The effect is
most obvious on macOS. No toolkit on X11/Wayland does any of this
currently. I want to change that. Stem darkening was introduced to
FreeType by Adobe, by the way.)

>> A side issue this is trying to solve is making the API thread-safer,
> 
> The concept "thread-safer" amuses me.

As far as I understand, FT is not by itself thread-safe, the developer
has to use the API in a certain way to not blow things up. This API is
supposed to be another stepping stone towards
usefulness/pain-lesser-ness in a multithreading context.

> I've (as ediosyncratic) reviewed; nothing struck me as obviously
> problematic.  I am not, however, a graphics expert.

Thanks! :)