AI Perception Behavior Enhancement

In Half-Life 2, if an idle NPC hears a sound but doesn't see its emitter, it will only turn in the direction of the sound for a bit before returning to its previous action.

With the modification below the NPC will investigate the source of the sound instead.

Open the ai_basenpc_schedule.cpp file and look for the function SelectAlertSchedule(). Look for the following code:

	if ( HasCondition ( COND_HEAR_DANGER ) ||
			  HasCondition ( COND_HEAR_PLAYER ) ||
			  HasCondition ( COND_HEAR_WORLD  ) ||
			  HasCondition ( COND_HEAR_BULLET_IMPACT ) ||
			  HasCondition ( COND_HEAR_COMBAT ) )
	{
		return SCHED_ALERT_FACE_BESTSOUND;
	}

Replace this code with:

	if ( HasCondition( COND_HEAR_DANGER ) ||
		HasCondition( COND_HEAR_PLAYER ) ||
		HasCondition( COND_HEAR_WORLD  ) ||
		HasCondition( COND_HEAR_BULLET_IMPACT ) ||
		HasCondition( COND_HEAR_COMBAT ) )
	{
		return SCHED_INVESTIGATE_SOUND;
	}

Alternate Code:

	if ( HasCondition( COND_HEAR_DANGER | COND_HEAR_PLAYER | COND_HEAR_WORLD | COND_HEAR_BULLET_IMPACT | COND_HEAR_COMBAT ) )
	{
		return SCHED_INVESTIGATE_SOUND;
	}

Conclusion

With this code, as soon as NPC hears sounds that meet conditions, they will investigate it.