I've come to the conclusion, finally, that the Source engine in its present form does not do multiplayer vehicles well.
The issue is this: for a smooth experience, we need prediction. To do prediction, we need to be able to simulate a vehicle forward multiple timesteps relative to the environment as a whole. For example, when the client copies an authoritative position for some time in the past, the client must be able to simulate the object forward from that position into the present time, while not affecting the world at large. Similarly, when the server receives a collection of user commands, the server must be able to simulate the object forward as many timesteps as there are commands. The player code is able to do this, as the extent of the simulation in the player code consists of some velocity integration combined with BBOX traces nand a whole set of heuristics for game-world permissibility; as the engine exposes facilities to do BBOX traces quite well, simulating multiple commands in a client/server frame is no problem. The shadow controller tied to the player, which interacts with the VPhysics world, does not completely march in lockstep with the player, but lags (at most) close behind. For vehicles, there is no analogous procedure. To simulate vehicles in game code, one would be required to use traces of collision hulls at arbitrary angles (as collision models for vehicles generally cannot be simplified to the BBOX that serves the player rather well). Unfortunately, no such facility exists. If one were to attempt to do this in VPhysics, one could use a motion controller, but the motion controller can only integrate a single command per world timestep; this does not fit the requirements for client prediction, or multi-command server simulation. So the solution that I see is that a facility be opened in the VPhysics interface that allows the simulation of any one arbitrary IPhysicsObject (and any associated shadow controllers/motion controllers) by one timestep, leaving the rest of the world that does not interact with this object alone. In essence, putting all objects except one to sleep, then simulating the world by one timestep, but done more efficiently. As fundamentally, a physics engine must simulate its objects serially, I believe that this is possible. I don't know Havok, though, so I won't know if that's actually true. I would, however, respectfully ask that VALVe look into the possibility. As far as I know, in the land of predicted multiplayer Source vehicles, Krenzo went with client-side authoritative vehicle physics for Empiresmod (with some server-side checking). HLRally gave up. I'll see what Eternal-Silence can come up with. -John Sheu _______________________________________________ To unsubscribe, edit your list preferences, or view the list archives, please visit: http://list.valvesoftware.com/mailman/listinfo/hlcoders
