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 SelectIdleSchedule(). 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_WORLD  ) ||
		 HasCondition ( COND_HEAR_PLAYER ) )
	{
		return SCHED_ALERT_FACE_BESTSOUND;
	}

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_BULLET_IMPACT ) ||	
	     HasCondition ( COND_HEAR_DANGER ) ||		  
		 HasCondition ( COND_HEAR_COMBAT ) ) 	
	{
		return SCHED_INVESTIGATE_SOUND	;
	}

Open the ai_default.cpp file and look for the schedule SCHED_ALERT_FACE_BESTSOUND. Look for the following code:

AI_DEFINE_SCHEDULE
(
	SCHED_ALERT_FACE_BESTSOUND,

	"	Tasks"
	"		TASK_STORE_BESTSOUND_REACTORIGIN_IN_SAVEPOSITION		0"
	"		TASK_STOP_MOVING			0"
	"		TASK_FACE_SAVEPOSITION		0"
	"		TASK_SET_ACTIVITY			ACTIVITY:ACT_IDLE"
	"		TASK_WAIT					1.5"
	"		TASK_FACE_REASONABLE		0"
	""
	"	Interrupts"
	"		COND_NEW_ENEMY"
	"		COND_SEE_FEAR"
	"		COND_LIGHT_DAMAGE"
	"		COND_HEAVY_DAMAGE"
	"		COND_PROVOKED"
);

Replace this code with:

AI_DEFINE_SCHEDULE
(
    SCHED_ALERT_FACE_BESTSOUND,

    "    Tasks"
    "        TASK_STOP_MOVING                0"
    "        TASK_STORE_LASTPOSITION            0"
//    "        TASK_SET_TOLERANCE_DISTANCE        32"
    "        TASK_GET_PATH_TO_BESTSOUND        0"
    "        TASK_FACE_IDEAL                    0"
    "        TASK_RUN_PATH                    0"
    "        TASK_WAIT_FOR_MOVEMENT            0"
    "        TASK_STOP_MOVING                0"
    "        TASK_WAIT                        5"
    "        TASK_GET_PATH_TO_LASTPOSITION    0"
    "        TASK_WALK_PATH                    0"
    "        TASK_WAIT_FOR_MOVEMENT            0"
    "        TASK_STOP_MOVING                0"
    "        TASK_CLEAR_LASTPOSITION            0"
    "        TASK_FACE_REASONABLE            0"

    ""
    "    Interrupts"
    "        COND_NEW_ENEMY"
    "        COND_SEE_FEAR"
    "        COND_LIGHT_DAMAGE"
    "        COND_HEAVY_DAMAGE"
    "        COND_PROVOKED"
    "        COND_HEAR_BULLET_IMPACT"
    "        COND_HEAR_DANGER"
    "        COND_HEAR_COMBAT"
);

Look for the schedule SCHED_INVESTIGATE_SOUND. Look for the following code:

AI_DEFINE_SCHEDULE
(
	SCHED_INVESTIGATE_SOUND,

	"	Tasks"
	"		TASK_STOP_MOVING				0"
	"		TASK_STORE_LASTPOSITION			0"
//	"		TASK_SET_TOLERANCE_DISTANCE		32"
	"		TASK_GET_PATH_TO_BESTSOUND		0"
	"		TASK_FACE_IDEAL					0"
	"		TASK_RUN_PATH					0"
	"		TASK_WAIT_FOR_MOVEMENT			0"
	"		TASK_STOP_MOVING				0"
	"		TASK_WAIT						5"
	"		TASK_GET_PATH_TO_LASTPOSITION	0"
	"		TASK_WALK_PATH					0"
	"		TASK_WAIT_FOR_MOVEMENT			0"
	"		TASK_STOP_MOVING				0"
	"		TASK_CLEAR_LASTPOSITION			0"
	"		TASK_FACE_REASONABLE			0"
	""
	"	Interrupts"
	"		COND_NEW_ENEMY"
	"		COND_SEE_FEAR"
	"		COND_SEE_ENEMY"
	"		COND_LIGHT_DAMAGE"
	"		COND_HEAVY_DAMAGE"
	"		COND_HEAR_DANGER"
);

Replace this code with:

AI_DEFINE_SCHEDULE
(
    SCHED_INVESTIGATE_SOUND,

    "    Tasks"
    "        TASK_STOP_MOVING                0"
//    "        TASK_SET_TOLERANCE_DISTANCE        32"
    "        TASK_GET_PATH_TO_BESTSOUND        0"
    "        TASK_FACE_IDEAL                    0"
    "        TASK_RUN_PATH                    0"
    "        TASK_SET_ROUTE_SEARCH_TIME        25"    // Spend 25 seconds trying to build a path if stuck
    "        TASK_GET_PATH_TO_RANDOM_NODE    200"
    "        TASK_RUN_PATH                    0"
    "        TASK_TURN_LEFT        45"
    "        TASK_WAIT            2"
    "        TASK_TURN_RIGHT        45"
    "        TASK_WAIT            2"

    ""
    "    Interrupts"
    "        COND_NEW_ENEMY"
    "        COND_SEE_FEAR"
    "        COND_SEE_ENEMY"
    "        COND_LIGHT_DAMAGE"
    "        COND_HEAVY_DAMAGE"
    "        COND_HEAR_DANGER"
    "        COND_HEAR_COMBAT"
    "        COND_HEAR_PLAYER"
    "        COND_HEAR_BULLET_IMPACT"
);

Alternatively you could just replace these lines in the file ai_basenpc_schedule.cpp (this code will only have the AI walk to the source of the sound and have them walk back without looking for the player or the source of the sound afterwards).

Open the ai_basenpc_schedule.cpp file and look for the function SelectIdleSchedule(). 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;
	}

Conclusion

With this code, NPC will hear sounds and react to them according to their loudness and intensity

Warning:This code does not have the changes required to allow AI to hear sounds from prop_physics.

Warning:This code (the one with changes to "ai_basenpc_schedule.cpp" and to "ai_default.cpp") will break animations when multiple gunshots occur ! Use the simpler implementation (the one without changes to "ai_default.cpp") if your mod will have loud gun battles.