KIPA Game Engine Seminars

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KIPA Game Engine Seminars
Day 18

• Jonathan Blow
• Ajou University
• December 16, 2002

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AI in Games

• Traditionally has been pathfinding and state
  machines
• State machine example Guards in a game like
  Thief or Metal Gear Solid
• Pathfinding example Guards calling for support
• How to come up with the state machine to use

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State machinesare not very good, really

• Hard to code sophisticated behavior fluid
  concepts, emotions dont state-ify very well.
• Thus gameplay ends up feeling mechanical
• AIs will only do things the designers explicitly
  thought of
• They had to put the transitions in!

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But most AI in games is still just state machines
pathfinding

• AI programmers are generally not as good at AI as
  graphics programmers are at graphics
• Most technical emphasis has been on graphics
• Graphics is visual, so its easier to judge the
  goodness of results
• So AI systems are generally unimpressive

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The alternative to state machinesis some kind of
planner (?)

• The simplest planners are about finding a path
  through a first-order predicate logic database
• Preconditions, effects
• Example in text file
• Essentially a theorem prover for first-order
  predicate logic

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Planning example

• Maybe you want to shoot some guy
• You need a gun, and that gun has to be loaded
• Type up quick example in a text file

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Problems with First-Order Predicate Logic

• Combinatorial explosion
• Inability of the planner to properly prune out
  potential search trees
• This is a very difficult problem!
• Inability to store subtle / nuanced data
• Hofstatders paper clip example
• We know huge amounts of things about a simple
  paper clip, that arent well-represented by
  textual statements in a database
• Higher-order predicate logic does not help

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Planning became more effectivewhen joined with
proceduralism

• Procedures that allow the AI to look at the state
  of the world and make observations about it
• Sounds natural in games but was uncommon in AI
  when it was first done
• SHRDLU (Winograds planner natural language
  processing system) did this and was very
  effective
• Some looking at SHRDLU web page

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Pathfinding

• A algorithm is usually used
• Actually because a lot of game programmers are
  lame, they dont even do something as good as A
• Basic description of algorithm

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A is a graph traversal algorithm

• How do you apply it to a continuous space?
• Build a grid out of the space?
• Example on whiteboard
• Difficult to extend to 3D
• Ramps? Ladders? Things you could maybe jump up
  and grab onto?
• Again, subtleties get lost in the gridding
  process

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Better than a grid

• Create an explicit graph of path points that
  visit the major areas of your map
• Maybe created by level-designer?
• Maybe auto-generated?
• To go to an arbitrary point on the map, find the
  path point closest to it (within view), navigate
  to that, then to the destination
• Example of guard responding to help call

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Creation of Path Networksin an editor

• Use point or line primitives in Max or Maya
• Line primitives if you want to always dictate
  connectivity
• Point primitives may be easier, if you want ease
  of mapmaking, or maybe not
• Augment these with (invisible) planes describing
  where the AI should not walk
• example off the side of a bridge

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Auto-generation ofpath networks?

• Scan the geometry of the level looking for
  walkable spaces
• If you have a BSP tree maybe?
• Combine them hierarchically (because there will
  be far too many at start)
• Paul Tozours system for Deus Ex 2 / Thief 3 does
  this
• but I think its a very complicated, heavyweight
  system
• Though to be fair, not many people manage to do
  even this well, so dont take this as a big
  criticism

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Auto-generation of path networks?

• Want some kind of more general system not
  susceptible to noise, which can work quickly
• Guided by statistical primitives
• Much like the autoportalization task I discussed
  on the first day
• Havent implemented such a thing yet, but I want
  to

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The Big ProblemCombining high-level
pathfinding/planning with low-level events

• If I am just patrolling, I dont want to walk
  through the corrosive sewage
• If I am chasing an enemy, I might try to jump
  over the sewage if I think it is safe to do so
• If the enemy is about to melt down the nuclear
  reactor, I am going to crawl through the
  corrosive sewage to stop him even if I am shot.

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Other examples

• I am sort of hungry
• If the path from here to the snack bar is not
  too long and inconvenient, I will get a snack,
  otherwise I will wait until later
• Unless I remember that later I need to go, so I
  can drive home before rush hour, so I really
  should get that snack now
• Because otherwise I will be very hungry on the
  long drive home and that will be bad!

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Example on Thief

• Circular zone around player parameterized by
  direction, showing appealingness of each
  direction
• Used to modify the fine level of the pathfind
  (but not necessarily the coarse level)

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Overall Goal of AI

• is to be interesting and fun
• Not to beat the player
• We can already beat the player easily in
  shooter-type games without AI just aim perfectly.

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Scripting Languages
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What is the goal of a scripting language?

• In other words, why do we want to make one?
• We dont know exactly!

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What is the goal of a scripting language?

• Data description language
• Dont need to write C code to make new objects
• Designers can make objects
• Get programmer out of the loop

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What is the goal of a scripting language?

• High-Level programming language
• Easier to write game logic in than C
• Maybe designers can write simple programs with
  this?
• System safety
• Buggy game code does not crash the game
• Again, important because of whos writing the code

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What is the goal of a scripting language?

• Expandability
• Mod authors can write things that change the game
  around
• Easy to integrate new modules into the game (just
  drop some files into the right place)

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Things we are learning

• Non-programmers cant program
• Even if you give them a high-level language
• All existing programming languages are too
  low-level for a normal person to use

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Things we are learning

• Maybe its best to integrate data description
  into the editor
• If were going to have visual tools, they should
  be as unified as possible, right?

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Scripting languages introduce problems

• Bugs in parser/execution system
• Actually less expressive than languages like C
• Absence of a debugger / profiler
• Creating a new language is a lot of work

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So you shouldnt make a new language if you dont
have to

• Use an existing system
• Tcl?
• Lua?
• Objective CAML?

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For a good game, none of these systems will
probably help

• Tcl
• Way too slow!
• Lua
• Bad garbage collector, unacceptable performance
• OCAML
• Hard to learn, hard to integrate with the rest of
  your engine which is probably written in C
• All of them lacking some primitives and models
  of interaction that we want
• Explicit yield, etc

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Brian Hooks RantDo we really want a scripting
language?

• He thinks in many cases programmers make
  scripting languages without clear ideas of their
  goals
• And often these goals could be accomplished more
  directly just with C and a simple data
  description system

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But if we do want a language

• It helps to realize
• There seem to be two levels of tasks, script
  writing and component programming
• Explanation on white board of the difference
• Designers can definitely do script writing, but
  not component programming. Maybe there should
  be 2 different languages?

31
Charles Blooms pointThread-like model,
explicit yield

• Want to write AI / series-of-events like a
  blocking thread
• Go to the door
• Open the door
• Step through
• Close the door
• Lock the door
• Sit down

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Example of a scripting / data system Scott
Bilas presentation on Dungeon Siege
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Example of a scripting / data system Doug
Churchs presentation on Thief