Lag Compensation

You may be looking for Yahn Bernier's 2001 paper on game engine networking.

Historic client hitboxes (red) versus rewound server hitboxes (blue).

Lag compensation is the notion of the server calculating as best it can when a usercmd was sent, and rewinding time to that point when processing it. In combination with prediction, lag compensation can help to combat network latency to the point of almost eliminating it.

By default only players are rewound. For a more detailed explanation, see Source Multiplayer Networking.

Note:CBasePlayer does not use lag compensation; it must be implemented by a derived class. Valve's networked player classes do this already.

Tip:In Left 4 Dead 2, prop_physics can lag compensated. Do this sparingly!

Invocation

Simply wrap whatever code you want lag compensated within two calls to the lagcompensation object:

#ifdef GAME_DLL

#include "..\server\ilagcompensationmanager.h"

void CMyPlayer::FireBullets ( const FireBulletsInfo_t &info )
{
	// Source 2007
	lagcompensation->StartLagCompensation( this, this->GetCurrentCommand() );

	// Alien Swarm and later (see also sub-section below)
	lagcompensation->StartLagCompensation( this, LAG_COMPENSATE_HITBOXES );

	BaseClass::FireBullets(info);

	lagcompensation->FinishLagCompensation( this );
}
#endif

Time will be wound back on the call to StartLagCompensation(), and wound forward again on the call to FinishLagCompensation().

Lag Compensation Type

Starting with Alien Swarm, you can optimise lag compensation with these settings:

LAG_COMPENSATE_BOUNDS

Rewind entities' locations only, skipping animations.

LAG_COMPENSATE_HITBOXES

Rewind both locations and hitboxes. This is the standard setting.

LAG_COMPENSATE_HITBOXES_ALONG_RAY

Rewind hitboxes if an entity is hit by a ray defined by the "weaponPos" StartLagCompensation() overload. Otherwise rewind bounds only.

These setting chosen applies to all entities affected by the rewind.

Implementation

^[Todo]

See also

• Source Multiplayer Networking#Lag compensation
• NPC Lag Compensation
• Prediction
• Interpolation