Scripting Ian P. Warfield CSE 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. –

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

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

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

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

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

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

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

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. Definition of Script (computer programming). Game Engine. Niestadt, Jan. Implementing a Scripting Engine. Patel, Amit. Amit’s Game Programming Information. students.stanford.edu/~amitp/. students.stanford.edu/~amitp/ Rabin, Steve, et al. AI Game Programming Wisdom 2. Hingham: Charles River Media, Inc., Scripting. Simpson, Jake. Game Engine Anatomy Sweeny, Tim. UnrealScript Language Reference. White, Stephen. Postmortem: Naughty Dog's Jak and Daxter: the Precursor Legacy.