[bb] Billiards Style Collision Physics by Phish [ 1+ years ago ]

[BlitzBot]

Title : Billiards Style Collision Physics
Author : Phish
Posted : 1+ years ago

Description : This type of collision response isn't just useful for pool and billiard style games, and in fact, I didn't write it for that reason at all. I wrote it because I needed to have collision for the space ships in my game 'Unity'. When their shields are up, they use this relatively simple method. It can be used for almost anything - player character bouncing off enemies in a platform game, to space ship collision, to a game which does actually involve balls. The basic algorithm which I used would work for both 2d and 3d so if you would like to see it, email me at phish@.... I think that realistic physics, even if pretty simple, can make a game feel a lot more immersive and interesting. The idea is that you should read my code, see how I use the collision response section, hack it up, and use it in your own game :-)

Code :

Code: BlitzBasic

1. ;------------------------------------------
2. ;2D COLLISION DEMO CODE
3. ;------------------------------------------
4. ; By Joseph 'Phish' Humfrey
5. ; This type of collision response isn't
6. ; just useful for pool and billiard style
7. ; games, and in fact, I didn't write it for
8. ; that reason at all. I wrote it because I
9. ; needed to have collision for the space
10. ; ships in my game 'Unity'. When their
11. ; shields are up, they use this relatively
12. ; simple method. It can be used for almost
13. ; anything - player character bouncing off
14. ; enemies in a platform game, to space
15. ; ship collision, to a game which does
16. ; actually involve balls. The basic
17. ; algorithm which I used would work for
18. ; both 2d and 3d, so if you would like to
19. ; see it, email me at phish@aelius.com
20. ;------------------------------------------
21. ; The actual code which works out what
22. ; happens after a collision is in the
23. ; UpdateCirclePhysics() function.
24. ;------------------------------------------
25. ; Enjoy!
26. ;------------------------------------------
27. ;------------------------------------------
28.  
29. Graphics 800, 600, 16, 2
30. SetBuffer BackBuffer()
31. SeedRnd MilliSecs()
32.  
33.  
34.  
35. ;------------------------------------------
36. ; MAIN DATA TYPE
37. ;------------------------------------------
38. ; This exact type isn't supposed to be used
39. ; Instead, you should use some of the fields
40. ; in your own type, or just use this one
41. ; for reference, to see what each field does
42. Type circle
43.         Field x#, y#            ;position
44.         Field dx#, dy#          ;x and y speeds
45.        
46.         Field radius#           ;radius of circle
47.         Field mass#                     ;mass of circle
48. End Type
49. ;------------------------------------------
50. ;------------------------------------------
51.  
52.  
53.  
54.  
55.  
56.  
57. ;------------------------------------------
58. ; SET UP BALLS INTO A POOL STYLE ARRANGEMENT
59. ; FOR DEMO
60. ;------------------------------------------
61. .Setup
62. ballTriangleSize=5
63. For xloop = ballTriangleSize To 1 Step -1
64.         For yloop = 1 To xloop
65.                 c.circle = New circle
66.                 cx = (5-xloop)*27 + 200
67.                 cy = yloop*31-(xloop*31)/2.0 + 300
68.                 cdx=0
69.                 cdy=0
70.                 c
71. adius = 15
72.                 cmass = 50
73.         Next
74. Next
75.  
76. ;Cue ball (smaller so you know which it is :)
77. cue.circle = New circle
78. cuex = 800
79. cuey = 300 +20
80. cuedx = -20
81. cuedy = Rnd(4)-2
82. cue
83. adius = 14
84. cuemass = 50
85. ;------------------------------------------
86. ;------------------------------------------
87.  
88.  
89.  
90.  
91.  
92.  
93.  
94. ;------------------------------------------
95. ;MAIN LOOP
96. ;------------------------------------------
97. ; This is the main While..Wend game loop
98. While Not KeyDown(1)
99.  
100.         Cls
101.        
102.         UpdateCirclePhysics()
103.         RenderCircles()
104.        
105.         ;------------
106.         ; Reset button
107.         Text 10, 10, "Press a mouse button to reset."
108.         Text 10, 25, "Press Esc to exit."
109.         If GetMouse() Then
110.                 For c.circle = Each circle
111.                         Delete c
112.                 Next
113.                 Goto setup
114.         End If
115.         ;------------
116.        
117.         Flip
118.        
119. Wend
120. ;------------------------------------------
121. ;------------------------------------------
122. End
123.  
124.  
125.  
126.  
127.  
128.  
129.  
130.  
131.  
132. ;------------------------------------------
133. Function UpdateCirclePhysics()
134. ;------------------------------------------
135. ; This is the main physics function for the
136. ; circles. It contains the very basic
137. ; movement physics as well as the collision
138. ; response code.
139. ;------------------------------------------
140.  
141.         For c.circle = Each circle
142.        
143.                 ;update positions
144.                 cx=cx+cdx
145.                 cy=cy+cdy
146.                
147.                 ;gradually slow down
148.                 cdx=cdx*0.99
149.                 cdy=cdy*0.99
150.                
151.                 ;------------------------------------------
152.                 ;COLLISION CHECKING
153.                 ;------------------------------------------
154.                 ; Check each circle in the loop against
155.                 ; every other (c against c2)
156.                 For c2.circle = Each circle
157.                
158.                         collisionDistance# = c
159. adius+c2
160. adius
161.                         actualDistance# = Sqr((c2x-cx)^2+(c2y-cy)^2)
162.                        
163.                         ;Collided or not?
164.                         If actualDistance<collisionDistance Then
165.                                
166.                                 collNormalAngle#=ATan2(c2y-cy, c2x-cx)
167.  
168.                                 ;Position exactly touching, no intersection
169.                                 moveDist1#=(collisionDistance-actualDistance)*(c2mass/Float((cmass+c2mass)))
170.                                 moveDist2#=(collisionDistance-actualDistance)*(cmass/Float((cmass+c2mass)))
171.                                 cx=cx + moveDist1*Cos(collNormalAngle+180)
172.                                 cy=cy + moveDist1*Sin(collNormalAngle+180)
173.                                 c2x=c2x + moveDist2*Cos(collNormalAngle)
174.                                 c2y=c2y + moveDist2*Sin(collNormalAngle)
175.                                
176.                                
177.                                 ;------------------------------------------
178.                                 ;COLLISION RESPONSE
179.                                 ;------------------------------------------
180.                                 ;n = vector connecting the centers of the circles.
181.                                 ;we are finding the components of the normalised vector n
182.                                 nX#=Cos(collNormalAngle)
183.                                 nY#=Sin(collNormalAngle)
184.                                
185.                                 ;now find the length of the components of each movement vectors
186.                                 ;along n, by using dot product.
187.                                 a1# = cdx*nX  +  cdy*nY
188.                                 a2# = c2dx*nX +  c2dy*nY
189.                                
190.                                 ;optimisedP = 2(a1 - a2)
191.                                 ;             ----------
192.                                 ;              m1 + m2
193.                                 optimisedP# = (2.0 * (a1-a2)) / (cmass + c2mass)
194.                                
195.                                 ;now find out the resultant vectors
196.                                 ;r1 = c1v - optimisedP * mass2 * n
197.                                 cdx = cdx - (optimisedP*c2mass*nX)
198.                                 cdy = cdy - (optimisedP*c2mass*nY)
199.                                
200.                                 ;r2 = c2v - optimisedP * mass1 * n
201.                                 c2dx = c2dx + (optimisedP*cmass*nX)
202.                                 c2dy = c2dy + (optimisedP*cmass*nY)
203.  
204.                         End If
205.  
206.                 Next
207.                 ;------------------------------------------
208.                 ;------------------------------------------
209.                
210.                
211.                 ;Simple Bouncing off walls.
212.                 If cx<c
213. adius Then
214.                         cx=c
215. adius
216.                         cdx=cdx*-0.9
217.                 End If
218.                 If cx>GraphicsWidth()-c
219. adius Then
220.                         cx=GraphicsWidth()-c
221. adius
222.                         cdx=cdx*-0.9
223.                 End If
224.                 If cy<c
225. adius Then
226.                         cy=c
227. adius
228.                         cdy=cdy*-0.9
229.                 End If
230.                 If cy>GraphicsHeight()-c
231. adius Then
232.                         cy=GraphicsHeight()-c
233. adius
234.                         cdy=cdy*-0.9
235.                 End If
236.                
237.         Next
238.  
239. End Function
240.  
241.  
242.  
243.  
244.  
245. ;------------------------------------------
246. Function RenderCircles()
247. ;------------------------------------------
248. ; Simple function draws all the circles
249. ; on the screen.
250. ;------------------------------------------
251.  
252.         For c.circle = Each circle
253.                 If c
254. adius=15 Then Color 200, 50, 50 Else Color 255, 255, 255
255.                 Oval cx-c
256. adius, cy-c
257. adius, c
258. adius*2, c
259. adius*2
260.         Next
261.        
262. End Function
263. ;------------------------------------------
264. ;------------------------------------------

Comments :


Ching(Posted 1+ years ago)

 I could have scored much better in my advanced graphics module if I'd found this code a couple of years ago.This code is really good.


Matty(Posted 1+ years ago)

 The magic numbers in there for friction and elasticity of collision would be better replaced with parameters/fields that can be set as required.


BlitzSupport(Posted 1+ years ago)

 This is great stuff! Here are some quick-and-dirty BMX/Monkey ports...BlitzMax:

Code: [Select]


' By Joseph 'Phish' Humfrey

Graphics 800, 600

SeedRnd MilliSecs()



'------------------------------------------
' Main DATA Type
'------------------------------------------
' This exact Type isn't supposed to be used
' Instead, you should use some of the fields
' in your own Type, Or just use this one
' For reference, To see what each Field does
Type circle
Field x#, y# 'position
Field dx#, dy# 'x And y speeds

Field radius# 'radius of circle
Field mass# 'mass of circle
End Type

Global Circles:TList = CreateList ()

'------------------------------------------
'------------------------------------------






'------------------------------------------
' SET UP BALLS INTO A POOL STYLE ARRANGEMENT
' For DEMO
'------------------------------------------
Function Setup ()

ballTriangleSize=5

For xloop = ballTriangleSize To 1 Step -1
For yloop = 1 To xloop
c:circle = New circle
c.x = (5-xloop)*27 + 200
c.y = yloop*31-(xloop*31)/2.0 + 300
c.dx=0
c.dy=0
c.radius = 15
c.mass = 50
ListAddLast Circles, c
Next
Next

'Cue ball (smaller so you know which it is :)
cue:circle = New circle
cue.x = 800
cue.y = 300 +20
cue.dx = -20
cue.dy = Rnd(4)-2
cue.radius = 14
cue.mass = 50
ListAddLast Circles, cue

End Function

'------------------------------------------
'------------------------------------------



Setup



'------------------------------------------
'Main LOOP
'------------------------------------------
' This is the Main While..Wend game loop
While Not KeyDown(KEY_ESCAPE)

Cls

UpdateCirclePhysics()
RenderCircles()

'------------
' Reset button
DrawText "Press a mouse button to reset.", 10, 10
DrawText "Press Esc to exit.", 10, 25
If MouseHit(1) Then
For c:circle = EachIn Circles
ClearList Circles
Next
Setup
End If
'------------

Flip

Wend
'------------------------------------------
'------------------------------------------
End









'------------------------------------------
Function UpdateCirclePhysics()
'------------------------------------------
' This is the Main physics Function For the
' circles. It contains the very basic
' movement physics as well as the collision
' response code.
'------------------------------------------

For c:circle = EachIn Circles

'update positions
c.x=c.x+c.dx
c.y=c.y+c.dy

'gradually slow down
c.dx=c.dx*0.99
c.dy=c.dy*0.99

'------------------------------------------
'COLLISION CHECKING
'------------------------------------------
' Check each circle in the loop against
' every other (c against c2)
For c2:circle = EachIn Circles

collisionDistance# = c.radius+c2.radius
actualDistance# = Sqr((c2.x-c.x)^2+(c2.y-c.y)^2)

'Collided Or Not?
If actualDistance<collisionDistance Then

collNormalAngle#=ATan2(c2.y-c.y, c2.x-c.x)

'Position exactly touching, no intersection
moveDist1#=(collisionDistance-actualDistance)*(c2.mass/Float((c.mass+c2.mass)))
moveDist2#=(collisionDistance-actualDistance)*(c.mass/Float((c.mass+c2.mass)))
c.x=c.x + moveDist1*Cos(collNormalAngle+180)
c.y=c.y + moveDist1*Sin(collNormalAngle+180)
c2.x=c2.x + moveDist2*Cos(collNormalAngle)
c2.y=c2.y + moveDist2*Sin(collNormalAngle)


'------------------------------------------
'COLLISION RESPONSE
'------------------------------------------
'n = vector connecting the centers of the circles.
'we are finding the components of the normalised vector n
nX#=Cos(collNormalAngle)
nY#=Sin(collNormalAngle)

'now find the length of the components of each movement vectors
'along n, by using dot product.
a1# = c.dx*nX  +  c.dy*nY
a2# = c2.dx*nX +  c2.dy*nY

'optimisedP = 2(a1 - a2)
'             ----------
'              m1 + m2
optimisedP# = (2.0 * (a1-a2)) / (c.mass + c2.mass)

'now find out the resultant vectors
'r1 = c1.v - optimisedP * mass2 * n
c.dx = c.dx - (optimisedP*c2.mass*nX)
c.dy = c.dy - (optimisedP*c2.mass*nY)

'r2 = c2.v - optimisedP * mass1 * n
c2.dx = c2.dx + (optimisedP*c.mass*nX)
c2.dy = c2.dy + (optimisedP*c.mass*nY)

End If

Next
'------------------------------------------
'------------------------------------------


'Simple Bouncing off walls.
If c.x<c.radius Then
c.x=c.radius
c.dx=c.dx*-0.9
End If
If c.x>GraphicsWidth()-c.radius Then
c.x=GraphicsWidth()-c.radius
c.dx=c.dx*-0.9
End If
If c.y<c.radius Then
c.y=c.radius
c.dy=c.dy*-0.9
End If
If c.y>GraphicsHeight()-c.radius Then
c.y=GraphicsHeight()-c.radius
c.dy=c.dy*-0.9
End If

Next

End Function





'------------------------------------------
Function RenderCircles()
'------------------------------------------
' Simple Function draws all the circles
' on the screen.
'------------------------------------------

For c:circle = EachIn Circles
If c.radius=15 Then SetColor 200, 50, 50 Else SetColor 255, 255, 255
DrawOval c.x-c.radius, c.y-c.radius, c.radius*2, c.radius*2
Next

End Function
Monkey:

Code: [Select]


' By Joseph 'Phish' Humfrey

Import mojo

Global Circles:List <circle>

Class Pool Extends App

Method OnCreate ()
SetUpdateRate 60
Circles = New List <circle>
Seed = Millisecs ()
Setup
End

Method OnUpdate ()

UpdateCirclePhysics

If MouseHit ()

For Local c:circle = Eachin Circles
Circles.Clear
Next

Setup

Endif

End

Method OnRender ()

Cls

RenderCircles()

DrawText "Press a mouse button to reset.", 10, 10
DrawText "Press Esc to exit.", 10, 25

End

End




'------------------------------------------
' Main DATA Class
'------------------------------------------
' This exact Class isn't supposed to be used
' Instead, you should use some of the fields
' in your own Class, Or just use this one
' For reference, To see what each Field does
Class circle
Field x#, y# 'position
Field dx#, dy# 'x And y speeds

Field radius# 'radius of circle
Field mass# 'mass of circle
End


'------------------------------------------
' SET UP BALLS INTO A POOL STYLE ARRANGEMENT
' For DEMO
'------------------------------------------
Function Setup ()

Local ballTriangleSize=5

For Local xloop = ballTriangleSize To 1 Step -1
For Local yloop = 1 To xloop
Local c:circle = New circle
c.x = (5-xloop)*27 + 200
c.y = yloop*31-(xloop*31)/2.0 + 300
c.dx=0
c.dy=0
c.radius = 15
c.mass = 50
Circles.AddLast c
Next
Next

'Cue ball (smaller so you know which it is :)
Local cue:circle = New circle
cue.x = 800
cue.y = 300 +20
cue.dx = -20
cue.dy = Rnd(4)-2
cue.radius = 14
cue.mass = 50
Circles.AddLast cue

End

Function Main ()
New Pool
End









'------------------------------------------
Function UpdateCirclePhysics()
'------------------------------------------
' This is the Main physics Function For the
' circles. It contains the very basic
' movement physics as well as the collision
' response code.
'------------------------------------------

For Local c:circle = Eachin Circles

'update positions
c.x=c.x+c.dx
c.y=c.y+c.dy

'gradually slow down
c.dx=c.dx*0.99
c.dy=c.dy*0.99

'------------------------------------------
'COLLISION CHECKING
'------------------------------------------
' Check each circle in the loop against
' every other (c against c2)
For Local c2:circle = Eachin Circles

Local collisionDistance# = c.radius+c2.radius
Local actualDistance# = Sqrt(Pow(c2.x-c.x, 2)+Pow(c2.y-c.y, 2))

'Collided Or Not?
If actualDistance<collisionDistance Then

Local collNormalAngle#=ATan2(c2.y-c.y, c2.x-c.x)

'Position exactly touching, no intersection
Local moveDist1#=(collisionDistance-actualDistance)*(c2.mass/Float((c.mass+c2.mass)))
Local moveDist2#=(collisionDistance-actualDistance)*(c.mass/Float((c.mass+c2.mass)))
c.x=c.x + moveDist1*Cos(collNormalAngle+180)
c.y=c.y + moveDist1*Sin(collNormalAngle+180)
c2.x=c2.x + moveDist2*Cos(collNormalAngle)
c2.y=c2.y + moveDist2*Sin(collNormalAngle)


'------------------------------------------
'COLLISION RESPONSE
'------------------------------------------
'n = vector connecting the centers of the circles.
'we are finding the components of the normalised vector n
Local nX#=Cos(collNormalAngle)
Local nY#=Sin(collNormalAngle)

'now find the length of the components of each movement vectors
'along n, by using dot product.
Local a1# = c.dx*nX  +  c.dy*nY
Local a2# = c2.dx*nX +  c2.dy*nY

'optimisedP = 2(a1 - a2)
'             ----------
'              m1 + m2
Local optimisedP# = (2.0 * (a1-a2)) / (c.mass + c2.mass)

'now find out the resultant vectors
'r1 = c1.v - optimisedP * mass2 * n
c.dx = c.dx - (optimisedP*c2.mass*nX)
c.dy = c.dy - (optimisedP*c2.mass*nY)

'r2 = c2.v - optimisedP * mass1 * n
c2.dx = c2.dx + (optimisedP*c.mass*nX)
c2.dy = c2.dy + (optimisedP*c.mass*nY)

End If

Next
'------------------------------------------
'------------------------------------------


'Simple Bouncing off walls.
If c.x<c.radius Then
c.x=c.radius
c.dx=c.dx*-0.9
End If
If c.x>DeviceWidth()-c.radius Then
c.x=DeviceWidth()-c.radius
c.dx=c.dx*-0.9
End If
If c.y<c.radius Then
c.y=c.radius
c.dy=c.dy*-0.9
End If
If c.y>DeviceHeight()-c.radius Then
c.y=DeviceHeight()-c.radius
c.dy=c.dy*-0.9
End If

Next

End





'------------------------------------------
Function RenderCircles()
'------------------------------------------
' Simple Function draws all the circles
' on the screen.
'------------------------------------------

For Local c:circle = Eachin Circles
If c.radius=15 Then SetColor 200, 50, 50 Else SetColor 255, 255, 255
DrawOval c.x-c.radius, c.y-c.radius, c.radius*2, c.radius*2
Next

End



Matty(Posted 1+ years ago)

 In the monkey example my browser freezes when compiled either for html5 or flash as soon as the user clicks the mouse after the balls have moved.  I'm using Chrome, and Monkey 45c.  Otherwise looks good.


BlitzSupport(Posted 1+ years ago)

 Bit odd -- I'm on Chrome and it's fine here. I also ran it without problems on GLFW and my Android phone.


Jesse(Posted 1+ years ago)

 works fine here in Safari and Chrome using monkey 53.I have been using this code for a few years although not for pool. Actually this code was what motivated me to do more research on vector physics and start learning all about vectors.it's good for fairly simple simulation. I wasn't able to use it as it moves balls apart when they over lap(causes a collision inaccuracy) instead of finding the exact point of collision and it fails completely if the speed of a ball exceed its diameter. I needed something that works dynamically.I find it useful for other things but was not able to use it for my pool game where I needed to find the exact point of collision.I am not nocking it down as it has it's uses and the collision response is really good but not for acceptable pool physics, at least for me it wasn't.


Hardcoal(Posted 1+ years ago)

 great submission


Captain Wicker (crazy hillbilly)(Posted 1+ years ago)

 lovely! [/i]