…flowing cloth (flags, tarps) that use PhysX to flow realistically are solid with PhysX off, and block bullets completely. With PhysX on, the cloth can be shot apart and thus shot through.
– from an answer to the question, "What do the levels of Nvidia PhysX do?"
A comment in this answer to this question by @BenBrocka also states:
…the hitboxes are actually different when PhysX are on. Bullets will be stopped by non-PhysX cloth because it's a wall. Bullets cut and then go through PhysX cloth because that's the point, and I do think they will do damage after passing through where the cloth was.
I can confirm that this is true, visually, in a single player game. (I haven't tested this in a co-op game, hence this question.) I've noticed that flowing cloths appear to be rock-solid and bulletproof if playing with an AMD card with PhysX disabled, and that projectiles appear to pass through flowing cloth if playing with an Nvidia card with PhysX enabled. However, if this is not true, i.e. even if projectiles appear to be tearing or passing through flowing cloth with PhysX enabled, they actually aren't, then please feel free to correct me.
If in a co-op game, what if the host has an Nvidia GPU and PhysX is enabled, while the client has an AMD GPU and PhysX is disabled (set to 'low'), and then vice-versa?
What determines whether a projectile can pass through a flowing cloth affected by PhysX (and hit an enemy behind it) – is it the host's or the client's computer? Or does each computer show a different effect for flowing cloths? (Which is weird if in a co-op game.)
Best Answer
Just tested it. There's a string of flags right above the Ammo Vendor in Southern Shelf, very convenient.
Indeed, PhysX setting changes the hitboxes of objects - some non-simulated cloth stops bullets, while the same simulated cloth is not in the same place at all, and stops some bullets elsewhere.
The answer is: each computer determines bullet collision on its own, then sends the bullet path to other players. That is:
Science!