NPC movement

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January 2024

This is a list of important components of an NPC movement schedule and task. One day it may end up as a full tutorial.

Schedule tasks

All of these tasks can be found in CAI_BaseNPC::StartTask(). Their TASK_ prefixes have been removed for readability.

GET_PATH_TO_PLAYER: Sets the NPC's goal to the player's location at the time of calculation.
WANDER <int|min_dist * 10000 + int|max_dist>: Pick a random location for a goal. The parameter is complex because it is actually two values forced into one.
GET_PATH_TO_ENEMY_LOS
GET_FLANK_RADIUS_PATH_TO_ENEMY_LOS
GET_FLANK_ARC_PATH_TO_ENEMY_LOS
GET_PATH_TO_ENEMY_LKP_LOS: Various hostile-related goal selection tasks.
FIND_COVER_FROM_BEST_SOUND
FIND_HINTNODE
FIND_LOCK_HINTNODE

WALK_PATH
RUN_PATH
WALK_PATH_FOR_UNITS <units>
RUN_PATH_FOR_UNITS <units>
WALK_PATH_WITHIN_DIST <units>
RUN_PATH_WITHIN_DIST <units>: Basic movement tasks. WITHIN_DIST are useful when the target itself isn't accessible, e.g. is an entity's origin.
RUN_PATH_FLEE <units>: Run this many units. Won't set a goal though, so use it in conjunction with another task.
WALK_PATH_TIMED <float|seconds>
RUN_PATH_TIMED <float|seconds>: Wait this many seconds, then move.; Bug:Seems to be broken. [todo tested in?]
WEAPON_RUN_PATH
ITEM_RUN_PATH: Move to pick up a weapon or item; fail if it is picked up or otherwise removed.
SCRIPT_RUN_TO_TARGET
SCRIPT_WALK_TO_TARGET
SCRIPT_CUSTOM_MOVE_TO_TARGET: For scripted movement.
MOVE_TO_TARGET_RANGE
MOVE_TO_GOAL_RANGE: Move within weapons range. Identical, except that one uses a goal while the other uses a target entity.
GET_PATH_AWAY_FROM_BEST_SOUND
MOVE_AWAY_PATH

WAIT_FOR_MOVEMENT <float|seconds>: Wait this long for any ongoing movement to complete (pass 0 to wait forever if necessary). A very common task.
WAIT_FOR_MOVEMENT_STEP <float|seconds>: Same as above, but fails if the goal is a cover location which a hostile has LOS to.; Tip:Given the name, Valve presumably intended this task to handle all sorts of "mid-stride" fails that didn't make good interrupts. This makes it a good place to add your own!
SET_ROUTE_SEARCH_TIME <seconds>: Maximum time to spend building a route. The schedule will fail if it is overrun. Remember that different CPU speeds will give different results - don't use it except as a fail-safe.
STOP_MOVING: Like, duh.

SetTarget( CBaseEntity ): The NPC's target is the entity it is acting upon - moving towards, looking at, shooting at, etc. Theoretically, multiple targets can be acted upon in the same frame by changing this value over the course of execution.

The NPC's CAI_Navigator instance, which defines the core movement functions.

SetGoal(const AI_NavGoal_t &goal, unsigned int flags = 0): The standard function for telling an NPC where it should head to when next given the command to move. Can be called with only a vector thanks to an AI_NavGoal_t overload.
SetDirectGoal( vector &goalPos, Navigation_t navType = <npc default> ): Bypasses much of SetGoal's code. Purpose is unclear.
SetArrivalDirection( vector || qangle || CBaseEntity ): The direction to face on arrival at the task goal. The NPC will probably begin turning shortly before arrival.; Tip:If a CBaseEntity is passed the NPC will turn to look at it. To use an entity's angles themselves, call GetAbsAngles().
StopMoving( bool bImmediate ): bImmediate doesn't seem to have any effect on walking NPCs - they stop immediately either way.

Combat movement functions.