Ai Overview
Ai Overview
This article is about helping to understand how the Ai in Shards Online works.
Some Basics:
The Artificial Intelligence for NPCs follows a general Ai Principle which is the cycle of
- Sense
- Think
- Act
The model of implementation is a Finite State Machine(FSM). [Wikipedia Article Finite State Machine]
To understand how an FSM would transition from one state (e.g. "Idle") to another state (e.g. "Combat") in Shards Online (SO) it is important to understand the first step: "Sense".
How does an NPC sense the world?
There are several ways:
- Messages
- Messages using the
Sendmessage
function tell the NPC someone or something sent it a message. To understand Messages the NPC needs to have the appropriate EventHandlers. - Views
- Views are added to objects by using the
AddView
function. A view is basically a search function performed on a configurably regular basis which triggers an event if something matching that search function enters its configured range. Mobiles can carry around views with them. - FindObjects
- The
FindObjects
function can be executed whenever an NPC needs to find something.
Each of these sensing mechanisms can call functions provided by the core Ai which lives in base_ai_state_machine.lua
.
SchedulePulse() AI.StateMachine.Init(initialState) AI.StateMachine.Shutdown() AI.StateMachine.ChangeState(newState) AI.StateMachine.ChangeSubState(newState) AI.StateMachine.EndSubStates(success,reason) HandleAiPulse()
Especially important is function AI.StateMachine.ChangeState(newState) which could be called when something happens our NPC needs to react to, e.g. a wolf entering the perception range of a turkey.
In base_ai_mob.lua
we find the following line of code in the initialization part:
AddView("chaseRange",SearchMobileInRange(chaseViewRange,false,false,true))
And at the end of the file we find the Event Handler in case some Mobile enters that range and triggered the view:
--On the target coming into range
RegisterEventHandler(EventType.EnterView, "chaseRange",
function (objRef)
HandleMobEnterView(objRef,false)
end)
So it seems that HandleMobEnterView
is our canditate doing the real stuff.
Below is a shortened version cleared from comments and DebugMessages, with added comments to better understand it.
Nothing of the core logic of the function is changed
function HandleMobEnterView(mobileObj,ignoreVision,forceAttack)
-- Sanity Checks
if (ignoreVision == nil) then ignoreVision = false end
if not(AI.IsActive()) then return end
-- This check makes mobs ignore admins as possible targets
if (not AI.IsValidTarget(mobileObj)) then return end
-- Is what we see frined of foe?
local isFriend = IsFriend(mobileObj)
if (forceAttack) then isFriend = false end
-- if it is foe either attack or be alert
if (not isFriend) then
AI.AddToAggroList(mobileObj,2)
if (forceAttack) then
this:SendMessage("AttackEnemy",mobileObj,true)
else
AI.StateMachine.ChangeState("Alert")
end
end
end
In the last part there are two ways to react, either by a manual state change to the "Alert" state of the FSM or an indirect way by sending a message to attack, which in turn will find the appropriate combat state.
So - it seems we have at least half-way understood the sensing, and the function above is already "thinking", but what about the acting and what are these dreaded "states" which seem to be so important?
A state in the SO statemachine is implemented as a collection of stuff inside a table. So - lets have a look at that table: