hello in libass Changelog, it is said that they implement a faster stroker than freetype. I think that the code is done the 31st July 2017 :
https://github.com/libass/libass/commits/master?after=d149636f502f5774ae1a8fb4c554b122674393b2+209&branch=master what do you think of this ? thank you Vincent Torri
