GameSalad PhysicsYoung-min Kang

Tongmyong University

goal• Physics Test Application

scene1 - gravity• prepare some images

and actors

add some behaviors

create a ball actor

If it’s out of window, destroy it

add gravity and spawn balls

gravity

Behavior of the instance

Be careful• These are not ‘moveable’

only moveable

Test it• When you touch the background, balls will appear and fall

scene 2 - collision• prepare more actors • place them in the scene

ball collision• add more

behaviors to the ball actor

You will see colliding balls

scene 3 - bounciness• add new actors of which bounciness will be changed

moveable

not moveable

will collide with our new floor actor

these guys will display their own bounciness

touch input for bounciness• prepare this image

bouncinesses of the ball and the floor will be determined by where you touch inside this actor !x direction: floor direction y direction: ball direction

new attributes

table for bouncinesses• Prepare a table “TB Bounciness”

place them in the scene

actors for some message display

bounciness input

Spawn balls whenever this actor is touched

change bounciness• add more behaviors

Test• balls bounce with different bounciness values

scene 4 - elasticity• Prepare a scene which look like this

final result

add properties• force and other variable for calculating the force

set values for each instancemoveable

not moveable not moveable

index setting• behavior for circle_icon

1 2 3 4 5 6 7 8 9 10

prepare a table• table for storing the locations of circles

behavior of moveable circles

Compute Differenceindex-1

index+1index

(dx1,dy1)

(dx2,dy2)

compute distancesindex-1

index+1

index

l1

l2

normalize the directions

compute spring forcef =

nX

i=1

k(li � l0i )~di

k = 10

l0i = 5

update velocityvt+h = vt +

Z t+h

tadt

m = 1 ) f = a vt+h = vt +

Z t+h

tfdt

Euler integration

vt+h = vt + hfh=0.03

Test it

scene 5 - friction• prepare a scene

different friction values• 30, 20, 10, 0 for floors which are not moveable

• 0 for the left box on each floor

• 3, 2, 1, 0 for the right boxes

scene 6 - attraction• prepare a scene like this

final result

we need more attributes• add an attribute to the background actor

• count:integer

• add two attributes to the game (global variables)

• touchX:real

• touchY:real

behaviors of background• spawn planets that will be attracted by sun

behaviors of background• prepare a sun actor and spawn it

• when the background it touched

• store the touch location to game.touchX and game.touchY

“sun” actor• prepare a “sun” actor and constrain its location to

the touched point

“planets” and its behaviors• we need some attributes for this actor

• these planets will be attracted by “sun”

• planet behavior

• compute attraction force and update its velocity

• “every 0.03 seconds (=h), update the state of the planet”

vector from planet to sun• d = (dx, dy)

• sun is located at (Game.touchX, Game.touchY)

compute distance between planet and sun

• simple computation

normalize the direction• also simple

compute “attraction”• attraction force

fattr = Cattrmplanet ·msun

l2d

Cattr ·mplanet ·msun = 2, 000, 000

collision handling

test

scene 7- drag• prepare a scene like this

set different drag values

test

scene 8 - rope physics

• rope is not elastic

• joint is required to express rope

• no joints are available in GameSalad

• we will use “fake” dynamics

model

• rope model

• linked “line segments”

• line segments are linked with rivets

• we will display only the rivets

table for locations of rivets

• 16 rivets

• location (x,y)

• index

behaviors of background• spawn 16 rivets and locate them in accordance with the table contents

• assign indices to the rivets Explained in the next slide

assigning unique index to each rivet

• add an attribute to Game

• Shared Integer Variable: integer

• every time you spawn a rivet, you set this value and the spawned rivet will use this for its own index

behaviors of background• if touched, adjust Game.touchX and Game.touchY

How rivets move• when it is spawned

• set its unique index

• rivet with index 1

• immediately moves to the touched position

• other rivets

• each rivet tries to keep the given distance to the previous rivet of which index is 1 less than its own

rivet with index 1constrain its location to the touched position

store its location to the table

other rivets• update its location every 0.03 seconds

store its location to the table

state update

apply gravity• we apply our own gravity instead of GameSalad gravity

compute difference• difference with the previous rivet

distance to the previous one• simple…

normalize the direction• simple…

adjust location and velocity• try to keep the given distance

Padjusted = P+ (l � l0)~d

• change of position

• velocity that incur the change should be computed

!

• original velocity: v

vadjusted = 0.5 · v + 0.5 · v+

Our model

v+

v+ =(l � l0)~d

h

adjust location and velocity• try to keep the given distance

test