Scripting

Ian P. WarfieldCSE 497 - Topics on AI and

Computer Game Programming

October 25, 2004

Definition of Scripting “Scripting is the technique of specifying

a game’s data or logic outside of the game’s source language.” – AI Game Programming Wisdom 2

“A script is a program that automates a sequence of tasks… Early script languages were often called batch languages or job control languages. – http://www.wordIQ.com/

Advantages of Scripting Game designers can write a lot of

code for comparatively little effort The scripting language can be

designed specifically for the game under development, minimizing programming overhead

Scripts can be changed more easily than code

Advantages of Scripting The more “moddable” your game is, the

more people will buy it and the longer it will be marketable

If your game is popular enough, you can license your game engine to other developers This business practice was spurred by the

popularity of DOOM and Quake Later games, such as Quake 3, Unreal, and DOOM

3 were designed specifically with this in mind

Disadvantages of Scripting Nonprogrammers are required to

program Creating a custom scripting

language is an extra step in the game development

The new language requires technical support and tools for debugging

Levels of Scripting Level 0: Everything hard-coded in the

source language Level 1: Stats and locations specified in files Level 2: Scripted non-interactive cutscene

sequences Level 3: Lightweight logic specified by tools

or scripts, as in a trigger system Level 4: Heavy logic in scripts that rely on

core functions in the game engine Level 5: Everything coded in scripts

Level 0: Hard-coded AI

Level 0: Hard-coded AI Quickest to code, especially for small

development teams or small projects Scales very poorly; difficult to debug

complex programs Best suited for simple projects Used in the earliest games such as

Pong, Space Invaders, and Pac-Man

Level 1: Data Specified in Files

Level 1: Data Specified in Files The player’s character, his enemies, the

surrounding objects, and the goals all remain the same – only the arrangement is different

Scales extremely well in terms of game size but poorly in terms of game extensibility

Suitable for repetitive games with multiple levels

Used in early games such as Super Mario, Lode Runner, and Chip’s Challenge

Level 2: Scripted Cutscenes

Level 2: Scripted Cutscenes Scripted cutscenes were actually

quite rare until recently Game designers traditionally prefer to

pre-render cutscenes Game engines did not have the power

to render cutscene-quality graphics on-the-fly

So this is a little out-of-order

Level 2: Scripted Cutscenes Now that game engines can render high

quality graphics, scripted cutscenes are preferred

Cost and file size are both dramatically improved

Scripting also allows for adaptive cutscenes depending on story developments

Used in games such as Half-Life and Star Trek Voyager: Elite Force

Level 3: Trigger System

Level 3: Trigger System Functions are coded in C/C++ and

linked by scripts Scripts consist of “events” which

trigger actions or possibly other events

Offers best balance of flexibility, ease of use, power, speed, and extensibility

Used in Freespace 2

Level 4: Heavy Script Logic

Level 4: Heavy Script Logic Only the core functions are coded in

C/C++; most of the logic is in the script Handle interesting events and

character AI, but let the game handle physics, etc.

Offers large degree of flexibility while still providing a solid code foundation

Used in Unreal and Half-Life

Level 5: Everything in Scripts

Level 5: Everything in Scripts The bare minimum of functions are

coded in C/C++; everything else is scripted

Scripts resemble full-fledged programming languages

Offers maximum degree of flexibility but carries most of the same risks as conventional programming

Used in Jax and Daxter

Level 5: Everything in Scripts Nearly all the run-time code written in GOAL

(similar to LISP) Code could be modified on-the-fly

No need to recompile the engine Debugging took much less time

There were setbacks, however Compiler took a long time to develop Developers had to create their own debugging

and support utilities Programming in GOAL required adjustment from

C/C++

Achieving a Balance Most games are designed for

scripting at Level 3 or Level 4 Flexibility of scripting combined

with power of C/C++ Libraries, tools, profilers,

debuggers are still available

Achieving a Balance Level 3 and Level 4 can be thought

of as “declarative” and “imperative” models, respectively

Declarative – program in terms of “what” needs to be accomplished

Imperative – program in terms of “how” to accomplish it

Declarative Model No implicit state, no assignments Expression evaluation instead of

instruction sequencing Recursion instead of loops Tends toward “functional”

programming paradigm, used in LISP

Declarative Model Sample code:

( when( is-destroyed “NTD Repulse” )( send-message “Well done” )

)

Imperative Model States defined in variables Variable modification through

explicit assignment Loops and iterative sequences of

statements

Imperative Model Sample code:

ship *ship_ctr;for (ship_ctr = GET_FIRST(ship_list); ship_ctr != GET_LAST(ship_list); ship_ctr =

GET_NEXT(ship_ctr)){

if (strcmp(ship_ctr->name, “NTD Repulse”) == 0){if (ship_ctr->flags & SF_DESTROYED){message_queue_add(“Well done”);break;}}

}

A Level 3 Demonstration FRED – the FReespace EDitor Visit the Freespace 2 community

forums at http://www.3dap.com/hlp/

Summary Core C/C++ functions do the hard work and

the script provides the creative element Scripting provides the flexibility and

moddability your game needs to be successful The write-time advantage far outweighs the

run-time and development-time disadvantage Choose an appropriate level of scripting that

balances flexibility and user-friendliness with the power of your underlying game engine

Sources Combs, Nathan, and Jean-Louis Ardoint. Declarative versus Imperative

Paradigms in Games AI. http://www.roaringshrimp.com/. Definition of Script (computer programming). http://www.wordIQ.com/. Game Engine. http://www.wikipedia.com/. Niestadt, Jan. Implementing a Scripting Engine. http://www.flipcode.com/. Patel, Amit. Amit’s Game Programming Information. http://www-cs

-students.stanford.edu/~amitp/. Rabin, Steve, et al. AI Game Programming Wisdom 2. Hingham: Charles

River Media, Inc., 2004. Scripting. http://www.wikipedia.com/. Simpson, Jake. Game Engine Anatomy 101. http://www.extremetech.com/. Sweeny, Tim. UnrealScript Language Reference. http://unreal.epicgames

.com/. White, Stephen. Postmortem: Naughty Dog's Jak and Daxter: the Precursor

Legacy. http://www.gamasutra.com/.