LECTURE 10Announcements

Final 1 Feedback• Almost completely done with

your 2D game engine!– Congratulations!– Feel free to use it/improve after

the class is over (some of us have/still are)

• Time to start showing off your product– ~2.5 weeks of gameplay coding!– Content creation!– Tons of playtesting!

• More on public playtesting later…

QUESTIONS?Announcements

LECTURE 10Sound

SOUND APPLICATIONSSound

Sound in Games• In the real world,

computers have sound• Background music• Sound effects• Can be an important

part of gameplay– Listening for footsteps– Dramatic music

Sound File Formats• Many ways to encode

and store sound• Open standards

– Ogg Vorbis– FLAC

• Closed standards– mp3– m4a– wav

Sampled Audio• mp3, wav, and most other

familiar extensions• Usually recordings of live

sounds• Samples of sound wave at

regular intervals• Prevalent in modern games• Refers to data type, not

origin– Touchtone telephone is

generated but still sampled11001001101010110111010110010001

10101

Generated Audio• MIDI• File provides information on

instruments and notes– Similar to sheet music

• Sound cards translate from instruments/notes to sound

• Can instruct computer to play something even if you can’t play it

• Used to be popular to save space, not as common now

Compressed vs. UncompressedCompressed Sound Files• Lossy or Lossless?

– Lossy remove “least important” parts of sound wave

– Lossless just use smart compression on raw wave

• Smaller file size (esp. lossy)• Lossy is lower quality• Slower to decode and play• Often used for music

Uncompressed Sound Files

• Record as much as possible of sound wave

• Much larger file size• Usually high quality• Faster to decode and

play• Often used for sound

effects

Buffering• Decompressing and

decoding is slow• Read sound into buffer,

play back from buffer• Size of buffer depends

on speed of system• Playback delay while

buffer is filled

Buffer

Sound file

Sound device

Decoding

SOUND IMPLEMENTATIONSound

javax.sound.sampled• AudioSystem: Provides factory

methods for loading audio sources

• Clip: Any audio that can be loaded prior to playback

• Line: Any source of streaming audio

• DataLine: An implementation of Line with helpful media functionality (start, stop, drain, etc)

• Other classes for mixing, ports, and other utilities

File file = new File(“mysound.wav”);InputStream in =

new BufferedInputStream(new

FileInputStream(myFile));

AudioInputStream stream = AudioSystem .getAudioInputStream(in);

Clip clip = AudioSystem.getClip();clip.open(stream);clip.start();

javax.sound.midi• MidiSystem: The

AudioSystem for MIDI files• Sequencer: Plays MIDI

sounds• Other classes for

manipulation of instruments, notes, and soundbanks– So you can create MIDI sounds

in realtime– Much harder to manipulate

samples

Sequence song = MidiSystem.getSequence(new File(“mysong.mid”));Sequencer midiPlayer = MidiSystem.getSequencer();midiPlayer.open(); midiPlayer.setSequence(song);midiPlayer.setLoopCount(0);midiPlayer.start();

Alternatives?• Some drawbacks of the built-in

sound classes…– Imprecise control over exact

playback start/stop positions– Almost impossible to

manipulate or even examine samples in realtime

– While Java offers pan and reverb, other libraries offer more varied effects

• But it’s very effective for simple background music and sfx!

OpenAL• Cross-platform audio API

modeled after OpenGL• Pros:

– Built for positional sound (distance attenuation, Doppler shift, etc all built in)

– More fine-grain control available

• Cons:– Single listener model– Modeled on OpenGL

Others• Most other libraries are

platform-specific or wrappers for OpenAL

• …except for synthesis libraries!– Jsyn, Beads, etc– Useful for composer

programs and the like, not so much for sound playback

QUESTIONS?Sound

LECTURE 10Collision Detection III

Point of Collision• We want to find out

where shapes hit• In our simulation,

colliding shapes are intersecting– Generally the intersection

is small– So we choose a point that

approximately represents where the intersection is

Poly-Poly• When two polygons (AABs

are polygons too!) collide, at least one vertex of one shape is inside the other (almost always)– If there’s only one point,

use that as the point of collision

– If there’s more than one, average them!

Circle-Circle• Circle-Circle is easy:

– It’s on the line connecting the centers, with ratio of the radii

• +()• Remember this is in

world (absolute) coordinates

�⃗�1

�⃗�2

Circle-Poly• If vertices of the poly

are within the circle, then average them

• If not, then take the point along the MTV:

• (Depends on MTV direction)

QUESTIONS?Collision Detection III

LECTURE 9Physics III

ROTATIONPhysics III

Rotation• We currently have

shapes that don’t rotate

• First step is to be able to rotate shapes

• Next step is to provide collision response for rotating entities

Terminology• Let’s define some

things:• Angle, θ (CCW)• Angular velocity, ω• Angular acceleration, α• Moment of Inertia, – Analogous to mass

(inertia) for rotation

θω

Basics• Your physical entities

should have an angle, angular velocity, and angular acceleration

• You should integrate these as before

• But whenever you do this you have to physically rotate the shape

public class PhysicalEntity{ float angle, aVel, aAcc;

void move(float time) { //integrate position aVel += aAcc*time; angle += aVel*time; aAcc = 0; rotate(aVel*time); }}

Rotating Shapes• What shapes do we need to rotate?• AAB doesn’t rotate, by definition• Circles are circles– You still need angular values for the circle

though, what if a hitbox is a circle?

• Therefore only polygons need to rotate• Rotate polygons by rotating their vertices

Centroid of Polygon• Every shape rotates around its centroid• The centroid of a polygon with vertices is:

• Where • and are coordinates of vertices in CCW

order

Rotating Polygons• To rotate a polygon, rotate

each vertex by the angle

• These vectors are the vertices relative to the centroid!

• Remember to update edges as well

θ𝑣0′

𝑣0❑

Inertia• We also need the moment of inertia of an

object• You can define or calculate it• Circle: • Polygon:

QUESTIONS?Rotation

ROTATIONAL PHYSICSPhysics III

Impulse and Forces• How can we cause

shapes to rotate in the world?

• Currently we are applying impulse/forces the centroids of entities

• Apply impulse/force to object, but not at centroid

Impulse and Forces• Now your impulses and

forces have a magnitude and a point of application

• is relative to the centroid

• The magnitude is actually a vector

• for impulse and for force from now on

𝑟 or

Angular Impulse and Forces

• In relation with angular velocity and acceleration:

or

𝑟

Collision Response• We need to change

the impulse we calculated in Physics II

• It’s now a different value that is applied at some specific point– It’s applied to the point

of collision!

Some Definitions• More definitions:• , are the vectors from

the centroids of the shapes to the collision point

• , are the perpendiculars to ,

• is the normalized MTV

𝑟𝑎

𝑟𝑏

𝑟𝑏⊥

𝑟𝑎⊥

�̂�

Collision Response• Magnitude of the impulse

• , are projections of velocities onto the • The impulse is in the direction of ,

determine the sign based on your MTV direction

Fixed Rotation• Just like with static shapes, there should also

be shapes that don’t rotate• Just like with the previous impulse equation,

have a special case for non-rotating objects• Replace with if the entity doesn’t rotate• Note that if both objects don’t rotate, the

equation reduces to the old equation

QUESTIONS?Rotational Physics

FRICTIONPhysics III

Friction• We don’t want

everything to be slippery– Friction slows things

down

• Give every physical entity a friction value greater than 0

Frictional Force• The frictional force is

parallel to the surface of contact– i.e. perpendicular to

MTV

• The direction is determined by the direction of the relative velocity (1D):

a

b

a

b

�⃗�𝑡𝑣

𝐹 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛

−𝐹 𝑓𝑟𝑖𝑐𝑡𝑖𝑜𝑛

Relative Velocity• Only velocity

perpendicular to the MTV is relevant

• Direction of the perpendicular () doesn’t matter– Consistency matters

�⃗�𝑎

�⃗�𝑏

�̂�⊥

How Much Force?• From physics, the

friction force on object A due to object B is proportional to the force exerted on object A by object B

• We don’t really have that force…– But we did apply impulse

to the objects!

�⃗� 𝑛𝑜𝑟𝑚𝑎𝑙

𝑣

𝑘 �⃗�𝑛𝑜𝑟𝑚𝑎𝑙

The Force• So we have

• is the impulse applied in collision response

• is a constant

�⃗�𝑎

�⃗�𝑏

�̂�⊥

�⃗�

− �⃗�

Disclaimer• This friction works for

the case when the relative velocity is linear

• With rotation, things become much more difficult

• If you want to combine these, good luck!

ω

𝑢𝑟𝑒𝑙=?�⃗�

QUESTIONS?Friction

LECTURE 10Spatial Acceleration

Structures

Collisions aren’t cheap• An individual collision

calculation is cheap…• But number of collisions

calculated is O(n^2)– 2 objects = 1 calc– 3 objects = 3 calcs– …– n objects = calcs

• What if your world has 1,000 entities? 10,000? 1,000,000?

Can we do better?• We can tell that some

collisions don’t even need to be checked

• Spatial acceleration data structures reduce # of collisions by taking advantage of spatial locality – Use as a replacement for

your Set/List<PhysicsEntity>– Can even implement Java

Collection<T>!

Bounding boxes• Every shape has a

bounding box – min/max x/y values of any point in the shape

• Function of the shape:– AAB – itself– Circle – center ± (r,r)– Poly – min/max over all

vertices– Compound – min/max over

all subshapes

Quadtrees• Like a binary tree, but 4

children• Divide world into

quadrants– Recursively subdivide

quadrants based on # of entities in a quadrant

– Only try colliding with entities in your quadrant

• Demo: http://www.mikechambers.com/blog/2011/03/21/javascript-quadtree-implementation/

Quadtrees - insert• Starting at the root node:– If this node isn’t split yet, add the entity here

• If adding the entity results in X entities, split and re-insert everything in the split quadrant

– Else, find the quadrant that should bound the entity’s bounding box• If one does, recur down that node• If none do, add the entity to this node

Quadtrees - retrieve• Gets all the entities for possible collisions• Starting at the root node: with an

accumulator– Add all entities in this node to accumulator– Again, determine which quadrant this entity

should reside in• If none, recur down all children – any entity could be

valid!• Else, recur down that child quadrant

Quadtrees – pros/cons• Pros– Fairly easy to implement– Good at reducing # of collisions

• Cons– Only works with bounded worlds– Assumes objects are uniformly distributed

Can we do even better?• What if objects are

highly concentrated (non-uniformly distributed)?

• What if the world is unbounded? Where to mark quadrants?

KD-trees• K-dimensional trees (in

our case, k = 2)• Traversing to children is

subdividing by either x or y– At each subdivision,

choose the optimal x or y value to subdivide by

– Efficiently splits up entities into buckets

Choosing the split axis

Choosing the split axis

Choosing the split axis

KD-trees – add/retrieve• Implementation details are the same as

quadtrees, except for splitting/traversal– Splitting – each node is an x or y node, subdivide

based on coordinate at mean/median/etc..– Traversal – traverse based on x or y value – left

is less than, right is greater than

• Subdivision may involve sorting – how to optimize # of sorts required?

KD-trees – pros/cons• Pros– Takes entity distribution into account– Great at reducing # of collisions

• Cons– Trickier to implement– Slightly higher construction time

Using KD/quad trees• On each tick:– Rebuild the tree (clear and then re-insert all

entities)– Use retrieve(PhysicsEntity) to get entities to

attempt collision with for each entity– Run standard collision detection algorithm– ???– Profit!

QUESTIONS?Spatial Acceleration Structures

LECTURE 10Tips for Final 2

EFFECTIVE PLAYTESTINGTips for Final 2

Finding Playtesters

• CS students are easy targets, try the sun lab or MS lab– Ask nicely– Don’t ask busy people

• Keep your audience in mind, however– It probably isn’t all CS

students

Don’t Interrupt!• Be a fly on the wall

– Say as little as possible

– Don’t offer hints or instructions

• Your game should speak for itself– You won’t be there to

offer instructions when your game is released

When to Interrupt• Player is frustrated or

taking too long in a single area

• You’re no longer getting good feedback

• If the player moves on, you will resume getting good feedback

Keep a Log• What is the player getting

stuck on?– Make it smoother

• What is the player enjoying?– Emphasize it

• What is the player ignoring?– Take it out if it’s unnecessary

• Consider having the game keep an automated log– Analytics is wonderful!

QUESTIONS?Tips for Final 2

JAVA TIP OF THE WEEKTips for Final 2

Default Visibility

• You already know public, private, protected

• What happens when there’s no visibility modifier?

• Then the field/method is visible to anything in the same package

A Helpful Table

Own Class Same Package

Subclass All Others

public Visible Visible Visible Visible

protected Visible Visible Visible No

(default) Visible Visible No No

private Visible No No No

Some General Visibility Tips

• Use private wherever possible– “Encapsulation”

• If you have a large amount of protected methods you want truly protected, consider separating them into an interface

QUESTIONS?Tips for Final 2

NO PLAYTESTING?

Playtest with random people this week!