Games Development 2Scripting for Games

CO3301

Week 5

ContentsContents

• Why Scripting

• Scripting Considerations

• Scripting Languages

• Python: Language Overview

Why Scripting?Why Scripting?

• Ease of Development– Higher-level – less prone to errors

• E.g. no pointers, memory allocation etc.

– Less intricate - can be easier to write• By less experienced developers and maybe technical designers

• Iteration Time– Much quicker to change and test– No recompilation, change at runtime

• Similar rationale to text-based data files

Why Scripting?Why Scripting?

• Scripts as Assets– Possible to regard scripts as a data resource to cut down on the

amount of game-specific embedded code• Use scripts to control entity behaviour in particular

– Treat like any other resource, e.g. meshes, animations• Script loaded with other entity assets• Script support built into the resource manager

– Makes entity behaviour editable outside the compilation process• For example, in a level editor

– Allows entity behaviour to be added/modified at a later stage• Post-release modding

• Scripting languages can be quite powerful– And customised for particular uses, e.g. AI

Scripting DrawbacksScripting Drawbacks

• Performance– Scripting languages are often interpreted– Some are part-compiled into bytecode (like Java)

• Instructions for a virtual machine

– However, still poor performance compared to C++, ten times slower or more

• No control of memory management can also cause issues– Dynamic allocation of new variables– Automatic garbage collection– No control over timing, or amount of memory used

• Tool support sometimes limited, but much better now• Errors can be hard to spot – usually occur at runtime• Need to write (clumsy) interface to our C++

Scripting ConsiderationsScripting Considerations

• Performance may not be such an issue– If scripting limited to high-level game logic– Tricky if 1000's of scripted entities though

• Don't necessarily need to use scripting languages– A simple, well defined game, with no need for modding

• Otherwise, consider language to use based on:– Performance needs, memory footprint, feature-set

• Developer can write a custom scripting language– E.g. UnrealScript– Developing a new language is tricky and usually unnecessary -

only recommended for experts

Popular Scripting LanguagesPopular Scripting Languages

• Python is a portable, interpreted, object-oriented programming language– Mature, well-used language with clear syntax– Notable for whitespace defining structure– Used on several games, especially MMOs (e.g. EVE)

• Also built into Autodesk Maya and Max for plugin development

– Large memory use makes it a choice for more heavyweight games

• Lua is a lightweight scripting language– Not natively object-oriented– Small and simple feature-set– Small memory footprint– Very widely used, on PC and consoles

Python: Language OverviewPython: Language Overview

• We will look at Python in this week’s lab– Well suited to PC development

• Python is a complete, high-level OO language– Reads a little like psuedo-code

• It is dynamically typed– No need to declare variable types

• It performs automatic garbage collection– No need to new / delete objects– No pointers

• Blocks are defined by indentation– No braces (curly brackets)

Python: Variables / BlocksPython: Variables / Blocks

• Variable use (no types, equate several at once, no semicolons):

x,y,z = 1,2,3

first, second = second, first

a = b = 123

• Indentation defines blocks (plus logical operators):if x < 5 or (x > 10 and x < 20):

print "The value is OK"

if x < 5 or 10 < x < 20:

print "The value is OK"

Python: Iteration / InputPython: Iteration / Input

• Loops (and comments): for i in [1,2,3,4,5]: print "Iteration number", i

# Print values from 0-99 inclusive for value in range(100): print value

x = 10while x >= 0: print "x is not negative" x = x – 1

• Simple input:x = input("Please enter a number: ")print "The square of that number is", x*x

Python: ListsPython: Lists

• Lists, indexing and slicingfruit = ["plum", "pear", "apple", "banana", "fig"]print fruit[2], fruit[0] # Prints "apple plum"print fruit[-2] # Prints "banana" (counts from end)print len(fruit) # Prints "5"

# Slicing is selecting sections of a list# Note: 1st example excludes last index in rangeprint fruit[1:3] # Prints "pear apple"print fruit[:3] # Prints "plum pear apple"print fruit[3:] # Prints "banana fig"

• Strings can be treated as read-only listsx = "apple"print x[3] # Prints "l" x[0] = "e" # Error

Python: Dictionaries / FunctionsPython: Dictionaries / Functions

• A dictionary is a maptel = {"Joe":123, "Jim":555, "Jane":999}

person = {'Job':'Builder', 'DOB':'12/12/66'} # '="

print tel["Jane"] # Prints 999

print person["Job"] # Prints "Builder"

• Functions (use keyword def):def Square(x):

return x*x

print Square(3)

Python: FunctionsPython: Functions

• Simple parameters are passed by valuedef square(x):

x = x*xreturn x

y = 3print square(y), y # Prints "9 3", y unchanged by fn

• But lists / dictionaries are affected in a functiondef change_list(l): l[0] = "apple"

fruit = ["pear", "orange", "fig"] change_list(fruit)print fruit # Prints "apple orange fig"

Python: StringsPython: Strings

• Wide range of string support:

# Basic operationsprint 'Hello ' + 'World' # Hello Worldprint "Spam "*4 # Spam Spam Spam Spam

# Strings can’t be written to – easy to get roundstr = "Hello World"newstr = 'M' + str[2:] # Mello World

# Example of other methodsprint len( "Piece of String" ) # Output 15print "london".capitalize() # Output Londonprint "too times too".replace( "too", "two" )# Output two times twofolderlist = "C:\Work\Python".split("\\")# folderlist = ["C:", "Work", "Python“]

Further ConditionsFurther Conditions

• More Conditions

# Set membershipprimes = [1,2,3,5,7,11,13,17,19,23,29]if x in primes: print "x is prime"

if year not in range(2000, 2100, 4): # 2000->2100 step 4 print "Not a leap year"

# String / List comparison"one" < "two" # true – lexicographical comparison

[1,2,3] < [1,2,4] # Compare elements[1,3] < [1,2,3] # Shorter list always less than

Working with ListsWorking with Lists

• List operations

# Working with this lista = [66.25, 333, 333, 1, 1234.5]

# Counting instances of a valueprint a.count(333), a.count(66.25), a.count('x')# Output: 2 1 0

# Insertion (anywhere), appending (at the end)a.insert(2, -1) # Location, valuea.append(333)print a# Output [66.25, 333, -1, 333, 1, 1234.5, 333]

Working with ListsWorking with Lists

• More list operations

# Working with this lista = [66.25, 333, -1, 333, 1, 1234.5, 333]

# Removal (by location)x = a.pop() # Remove last elementy = a.pop(2) # Remove specific elementprint a, x, y# Output [66.25, 333, 333, 1, 1234.5] 333 -1

# Removal (by value)a.remove(333) # Remove first instance of 333print a# Output[66.25, 333, 1, 1234.5]

Working with ListsWorking with Lists

• List reordering

# Working with this lista = [66.25, 333, 1, 1234.5]

# Reversala.reverse()print a# Output [1234.5, 1, 333, 66.25]

# Sortinga.sort()print a# Output[1, 66.25, 333, 1234.5]

Functional List CreationFunctional List Creation

• List creation using functions

# map – applies function to list elts to create new listdef Square(x): return x*xprint map(Square, range(1, 11)) # Output [1, 4, 9, 16, 25, 36, 49, 64, 81, 100]

# filter – uses boolean function to filter elts in a listdef NotDiv3(x): return x % 3 != 0 filter(NotDiv3, range(1, 15))# Output [1, 2, 4, 5, 7, 8, 10, 11, 13, 14]

# List comprehension – powerful list creationdef Square(x): return x*xprimes = [1,2,3,5,7,11,13,17,19,23,29]print [Square(x) for x in primes if x%10 < 5]# Output [1,4,9,121,169,529]

Function TechniquesFunction Techniques

• More function techniques

# Argument passing by namedef AddRecord( name, ID = 0, job = "Unemployed” ): ... AddRecord( "Bill", 5 ) # Normal styleAddRecord( job="Builder", name="Bob" ) # By name

# Argument unpackingdef VectorLength( x, y ): ...vector = (10, 15)print VectorLength( *vector ) # * unpacks 1 argument to 2

# Documentation – first line can be string literaldef Square(x): "Returns the square of the argument" return x*x

LibrariesLibraries

• Can import support libraries:

# Mathematics – as in Cimport mathprint math.cos( math.pi / 3 ) # Output 0.5

# Random numbersimport randomprint random.choice(['apple', 'pear', 'banana'])print random.random() # random float 0-1 print random.randrange(6) # random integer from 0-5# 5 random samples from list, e.g. [30,83,16,4,8]print random.sample(range(100), 5)

Python: ClassesPython: Classes

• Class definition (each fn has extra 'self' argument):

class Person:

# __init__ is a constructor __init__(self, name, job="Unemployed"):# Default

self.name = name self.job = job

def output(self): print self.name, "the", self.job

guy1 = Person("Bob", "Builder")guy2 = Person("Bill")guy1.output() # Prints "Bob the Builder"guy2.output() # Prints "Bill the Unemployed"