This document describes the development of 8 physics test scenes in GameSalad. Scene 1 demonstrates gravity and spawning balls. Scene 2 adds ball collision. Scene 3 introduces bounciness controlled by touch input. Scene 4 simulates elasticity. Scene 5 experiments with friction. Scene 6 attracts planets to a touch point "sun". Scene 7 tests drag. Scene 8 simulates a rope using "fake" joint dynamics between linked "rivets".

1. GameSalad
Physics
Young-min Kang
Tongmyong University

2. goal
• Physics Test Application

3. scene1 - gravity
• prepare some images
and actors

4. add some behaviors

5. create a ball actor
If it’s out of window, destroy it

6. add gravity and spawn balls
gravity
Behavior of the instance

7. Be careful
• These are not ‘moveable’
only moveable

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

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

10. ball collision
• add more
behaviors to the
ball actor

11. You will see colliding balls

12. 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

13. 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

14. table for bouncinesses
• Prepare a table “TB Bounciness”

15. place them in the scene
actors for some message display

16. bounciness input
Spawn balls whenever this actor is touched

17. change bounciness
• add more behaviors

18. Test
• balls bounce with different bounciness values

19. scene 4 - elasticity
• Prepare a scene which look like this
final result

20. add properties
• force and other variable for calculating the force

21. set values for each instance
moveable
not moveable not moveable

22. index setting
• behavior for circle_icon
1 2 3 4 5 6 7 8 9 10

23. prepare a table
• table for storing the locations of circles

24. behavior of moveable circles

25. Compute Difference
index-1
index+1
index
(dx1,dy1)
(dx2,dy2)

26. compute distances
index-1
index+1
index
l1
l2

27. normalize the directions

28. compute spring force
f =
nX
i=1
k(li l0
i )~di
k = 10
l0
i = 5

29. update velocity
vt+h
= vt
+
Z t+h
t
adt
m = 1 ) f = a vt+h
= vt
+
Z t+h
t
fdt
Euler integration
vt+h
= vt
+ hf
h=0.03

30. Test it

31. scene 5 - friction
• prepare a scene

32. 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

33. scene 6 - attraction
• prepare a scene like this
final result

34. 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

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

36. 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

37. “sun” actor
• prepare a “sun” actor and constrain its location to
the touched point

38. “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”

39. vector from planet to sun
• d = (dx, dy)
• sun is located at (Game.touchX, Game.touchY)

40. compute distance between
planet and sun
• simple computation

41. normalize the direction
• also simple

42. compute “attraction”
• attraction force
fattr = Cattr
mplanet · msun
l2
d
Cattr · mplanet · msun = 2, 000, 000

43. collision handling

44. test

45. scene 7- drag
• prepare a scene like this

46. set different drag values

47. test

48. 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

49. model
• rope model
• linked “line segments”
• line segments are linked with rivets
• we will display only the rivets

50. table for locations of rivets
• 16 rivets
• location (x,y)
• index

51. 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

52. 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

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

54. 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

55. rivet with index 1
constrain its location to the touched position
store its location to the table

56. other rivets
• update its location every 0.03 seconds
store its location to the table
state update

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

58. compute difference
• difference with the previous rivet

59. distance to the previous one
• simple…

60. normalize the direction
• simple…

61. 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

62. adjust location and velocity
• try to keep the given distance

63. test