Source is pretty expensive due to the mandatory Havok fee.
I heard they used to use an decade old physics library that just got bought by Havok company in the meantime and has nothing to do with the Havok physic engines everyone knows.
Is Havok really mandatory? Wouldn't it be possible to replace Havok with Bullet or some other related system?
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[1] https://www.youtube.com/watch?v=YTC0V0LWeGI
[1] http://media.steampowered.com/apps/valve/2014/Sergiy_Migdals...
[1] http://media.steampowered.com/apps/valve/2015/Migdalskiy_Ser...