Scripting and AI Damián Isla, Moonshot Games

Scripting vs. Scripting Distinguish between scripting as technology and scripting as filthy hackery.

Scripting as Filthy Hackery “Lazy devs faking it in script instead of doing it for real in code.” Obviously absurd.  Code doesn’t make it more real.  Script is code.  It’s all fakery if you go down far enough.

Faking it  Robust vs. brittle systems  Orthogonal to questions of implementation language  Robust is always better  A Matter of Economics  we don’t have time to do everything well  it works most of the time

The World is Lies... so what we should be asking ourselves is not “how do we avoid faking it” but “what’s the best way to fake it?”

The Most Important AI Technology Ever if then else

But this sucks if (ai.health < health_cover_threshold) { } else { }

Scripting as Architecture LUA  Compiled to bytecode  Hot swapping  Highly embeddable  Highly customizable  Dynamic typing Months of dev time saved Kill me now

Pros and Cons We like  Iteration times  Expressivity  Designer-accessibility We don’t like  Performance  Designer-accessibility

El Compromiso Systems that are performance-sensitive are generally NOT the systems where designer- expressivity is important. And vice versa.

Our Strategy  Script as a pillar of the the Engine  Component object model  Components can be implemented in  C++  Lua  Script is also the special sauce  Script is the glue

The [Production] Goal Lua can be used to create first-class systems from the ground up. 3 parts:  component scripts  system scripts  data

Components and Script ABC D

AB

A Path This is where the fun lives B Animation

Component Soup A* Perception/ Ray-casting Target selection Behavior Physics Animation playback Animation control Squad behavior Random Script

Component Soup A* Perception/ Ray-casting Target selection Behavior Physics Animation playback Animation control Squad behavior Random Script Emit balloons from ears whenever player hits the elephant with the frying pan

Component Soup A* Perception/ Ray-casting Target selection Behavior Physics Animation playback Animation control Squad behavior Random Script

Component Soup A* Perception/ Ray-casting Target selection Behavior Physics Animation playback Animation control Squad behavior Random Script

Component Soup A* Perception/ Ray-casting Target selection Behavior Physics Animation playback Animation script Squad behavior Random Script Locomotion control

Component Soup emit_balloons initialize() attach() update() detach()

System Soup System script

System Soup Boids

System Soup

Data  Need to be able to:  save out the state of the scripts  drive the system with data  define the structure for both  Lua can define structures  define structures for engine-allocated data blocks  engine controls the lifetime of these blocks  Lua  reads and writes to the blocks  is NEVER allowed to allocate it’s own dynamic memory  we lock the global scope after creation

System Soup ParametersState ParametersState System script

Script-based Systems What it gets us  A fluid design environment  avoid the “data-driven” code trap  A place to put our hacks  (Embrace the hackery)  Continuity between prototyping and production

Initial Framework (Engineer) Implementation (Design) Final implementation (Engineer) Final buy-off (Design)

Engineer makes it work Design makes it work well Engineer makes it fast Design makes sure it still works well

Not so fast... Doesn’t mean you don’t need to  Provide perf tools  Think systemically about script  Think seriously about interfaces between components  Design all pieces as an architecture  Help designers write good systems, not just good code But does mean that the architecture can evolve more fluidly and in a more design-driven way.

Concepts that don’t Exist (In code)  Health / Death / Damage  Weapons / Items / Equipment / Powers  AI  Behavior  HUD / UI / Menus  Missions / Levels / Objectives

One more distinction C++  high performance  low (designer) expressivity  game-agnostic Script  low performance  high (designer) expressivity  game-specific The technology you keep around for next time The stuff that makes this game unique and awesome

Thanks!