Bouncing collision response

BlitzMax Forums/BlitzMax Beginners Area/Bouncing collision response

Leo Santos (Posted 2009) [#1]

Hi everybody. I'm having trouble achieving proper collision response based on a collision normal. I'm a complete noob when it comes to math, so even though I've looked through all examples I could find (Snooker balls physics, the Billiards example by Joseph 'Phish' Humfrey, etc.) I end up in trouble trying to adapt their solutions to mine, simply because I'm doing it blindly - I don't really understand what's going on, so when something doesn't work, it's very hard to know what to do... That said, I got much farther than I had before, this time using polygonal/circle collisions, mostly thanks to Dave 'Oddball' Williamson poly collisions code. The ONLY thing I'm still having trouble is achieving a bounce against the collision object. Here's some example code I took from the main program and simplified so it only has circle to circle collisions. Please note that I'm not trying to move both objects in the collision, only the "MainBall" object. '******************* Basic setup ******************************** SuperStrict Const Xres:Int = 800 Const Yres:Int = 600 Graphics Xres,Yres,0,60 '******************* Create Scene ******************************** Local BallList:TList = CreateList() For Local N:Int = 1 To 100 Local NewBall:Ball = New Ball NewBall.X = Rand(0,Xres) NewBall.Y = Rand(0,Yres) NewBall.R = Rand(5,20) BallList.Addlast(NewBall) Next Local MainBall:Ball = New Ball MainBall.X = Xres/2 MainBall.Y = Yres/2 MainBall.R = 10 MainBall.SpeedX = 1 MainBall.SpeedY = 1 '******************* Main Loop ************************************ While Not AppTerminate() And Not KeyDown(Key_ESCAPE) Cls MainBall.Move() For Local Obj:Ball = EachIn BallList Obj.Draw() MainBall.Collide(Obj) Next MainBall.Draw() MainBall.DisplayInfo() Flip Wend '******************* Types **************************************** Type Ball Field X:Float Field Y:Float Field R:Float Field SpeedX:Float Field SpeedY:Float Field SpeedTotal:Float Field CollisionAngle:Float Method Move() 'Applies speed and keeps ball inside screen X :+ SpeedX Y :+ SpeedY SpeedTotal = Sqr((SpeedX*SpeedX)+(SpeedY*SPeedY)) If X > Xres X= Xres SpeedX :* -1 EndIf If X < 0 X=0 SpeedX :*-1 EndIf If Y > Yres Y = Yres SpeedY :*-1 EndIf If Y < 0 Y=0 SpeedY :*-1 EndIf End Method Method Draw() 'Draws a hollow circle SetColor 0,150,255 Plot X,Y Local PX:Int = X Local Py:Int = R + Y For Local Angle:Int = 0 To 360 Step 5 Local Nx:Int = Sin(Angle) * R + X Local Ny:Int = Cos(Angle) * R + Y DrawLine (Px, Py, Nx, Ny, False) Px = Nx Py = Ny Next EndMethod Method Collide(Obj:Ball) Local Distance:Float = Sqr((Obj.X-X)*(Obj.X-X)+(Obj.Y-Y)*(Obj.Y-Y)) Local TouchDistance:Float = R + Obj.R If Distance < TouchDistance 'Collision Ocurred, 'Detects angle of collision CollisionAngle = ATan2(Obj.Y-Y, Obj.X-X)+180 'Move out of collision X :+ (TouchDistance - Distance) * Cos(CollisionAngle) Y :+ (TouchDistance - Distance) * Sin(CollisionAngle) 'Sets new Speed SpeedX = SpeedTotal * Cos(CollisionAngle) SpeedY = SpeedTotal * Sin(CollisionAngle) EndIf EndMethod Method DisplayInfo() SetColor 240,240,0 DrawText "SpeedX: "+SpeedX,10,Yres-20 DrawText "SpeedX: "+SpeedY,200,Yres-20 DrawText "SpeedTotal: "+SpeedTotal,400,Yres-20 SetColor 0,250,50 DrawText "CollisionAngle: "+Int(CollisionAngle),10,Yres-40 EndMethod End Type It seems to work at first, but then you realize that the ball is bouncing exactly at the collision normal angle, instead of "mirroring" its original movement at the collision point. I know where the problem is (at the "Set new speed" section), I just don't know how to fix it... Any help would be greatly appreciated. Thanks!

Warpy (Posted 2009) [#2]

making things bounce

Leo Santos (Posted 2009) [#3]

Fantastic! That was quick! :-D I'm adding a bookmark to your page, Christian, it is really helpful... Here's the fixed code, it may help someone else out there: '******************* Basic setup ******************************** SuperStrict Const Xres:Int = 800 Const Yres:Int = 600 Graphics Xres,Yres,0,60 '******************* Create Scene ******************************** Local BallList:TList = CreateList() For Local N:Int = 1 To 100 Local NewBall:Ball = New Ball NewBall.X = Rand(0,Xres) NewBall.Y = Rand(0,Yres) NewBall.R = Rand(5,10) BallList.Addlast(NewBall) Next Local MainBall:Ball = New Ball MainBall.X = Xres/2 MainBall.Y = Yres/2 MainBall.R = 10 MainBall.SpeedX = 2 MainBall.SpeedY = 2 '******************* Main Loop ************************************ While Not AppTerminate() And Not KeyDown(Key_ESCAPE) Cls MainBall.Move() For Local Obj:Ball = EachIn BallList Obj.Draw() MainBall.Collide(Obj) Next MainBall.Draw() MainBall.DisplayInfo() Flip Wend '******************* Types **************************************** Type Ball Field X:Float Field Y:Float Field R:Float Field SpeedX:Float Field SpeedY:Float Field SpeedTotal:Float Field CollisionAngle:Float Method Move() 'Applies speed and keeps ball inside screen X :+ SpeedX Y :+ SpeedY SpeedTotal = Sqr((SpeedX*SpeedX)+(SpeedY*SPeedY)) If X > Xres X= Xres SpeedX :* -1 EndIf If X < 0 X=0 SpeedX :*-1 EndIf If Y > Yres Y = Yres SpeedY :*-1 EndIf If Y < 0 Y=0 SpeedY :*-1 EndIf End Method Method Draw() 'Draws a hollow circle SetColor 0,150,255 Plot X,Y Local PX:Int = X Local Py:Int = R + Y For Local Angle:Int = 0 To 360 Step 8 Local Nx:Int = Sin(Angle) * R + X Local Ny:Int = Cos(Angle) * R + Y DrawLine (Px, Py, Nx, Ny, False) Px = Nx Py = Ny Next EndMethod Method Collide(Obj:Ball) Local Distance:Float = Sqr((Obj.X-X)*(Obj.X-X)+(Obj.Y-Y)*(Obj.Y-Y)) Local TouchDistance:Float = R + Obj.R If Distance < TouchDistance 'Collision Ocurred, 'Detect angle of collision CollisionAngle = ATan2(Obj.Y-Y, Obj.X-X)+180 'Move out of collision X :+ (TouchDistance - Distance) * Cos(CollisionAngle) Y :+ (TouchDistance - Distance) * Sin(CollisionAngle) 'Sets new Speed Local Nx:Float = Cos(CollisionAngle) Local Ny:Float = Sin(CollisionAngle) 'This section adjusted with code by Christian "Warpy" Perfect 'p is the projection of V onto the normal Local Px:Float, Py:Float Local Dotproduct:Float = SpeedX*Nx+SpeedY*Ny Px = Dotproduct*Nx Py = Dotproduct*Ny 'the velocity after hitting the wall is V - 2p, so just subtract 2*p from V SpeedX = SpeedX - 2*Px SpeedY = SpeedY - 2*Py EndIf EndMethod Method DisplayInfo() SetColor 240,240,0 DrawText "SpeedX: "+SpeedX,10,Yres-20 DrawText "SpeedX: "+SpeedY,200,Yres-20 DrawText "SpeedTotal: "+SpeedTotal,400,Yres-20 SetColor 0,250,50 DrawText "CollisionAngle: "+Int(CollisionAngle),10,Yres-40 EndMethod End Type