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Verlet Physics

BlitzMax Forums/BlitzMax Programming/Verlet Physics

SSS(Posted 2006) [#1]
Hey everyone! I posted this in the maxphysics thread. However, since I know a lot of you don't frequent it I'm posting it here aswell. Anyway, I've tried to design several physics systems ad-hoc before and they've failed in one way or another. This one seems to work at the moment and I'll be adding collisions and whatever else one adds to a physics system. Anyway, I used the advanced character physics article to write this. I hope you guys like what i've done so far. I definately plan on improving it. Anyone can use it in whatever you guys want (even though its still new and not so good.) I know that it's not such a large accomplishment but I have been trying to get this working for a while so I'm pretty happy with the results :D.

Here's the vector library on which the code runs:
Strict
Import BRL.Basic
Type TVector
	'VARIABLES
	Field _x:Double, _y:Double
	
	'METHODS
	Method New()
		_x = 0
		_y = 0
	End Method
	
	Method Set(x:Double,y:Double)
		_x=x
		_y=y
	End Method
	
	Method Get(x:Double Var, y:Double Var)
		x = _x
		y = _y
	End Method
	
	Method Multiply:TVector(factor:Double)
		_x:*factor
		_y:*factor
		Return Self
	End Method
	
	Method Add:TVector(v:TVector,bself=False)
		If bself = False
			Return TVector.Create(_x+v._x,_y+v._y)
		Else
			_x:+v._x
			_y:+v._y
			Return Self 
		EndIf
	End Method
	
	Method Subtract:TVector(v:TVector,bself=False)
		If bself = False
			Return TVector.Create(_x-v._x,_y-v._y)
		Else
			_x:-v._x
			_y:-v._y
			Return Self
		EndIf
	End Method
	
	Method Copy:TVector()
		Return Create(_x,_y)
	End Method
	
	Method DotProduct:Double(v:TVector)
		Return _x*v._x+_y*v._y
	End Method
	
	Method AngleBetween:Double(v:TVector)
		Return ACos(DotProduct(v)/(Magnitude()*v.Magnitude()))
	EndMethod
	
	Method SetAngle(angle:Double)
		Local mag:Double = Magnitude()
		_x = Cos(angle)*mag
		_y = -Sin(angle)*mag
	End Method
	
	Method GetAngle:Double()
		Return ATan2(-_y,_x)
	End Method
	
	Method SetMagnitude(mag:Double)
		Normalise()
		_x:*mag
		_y:*mag
	End Method
	
	Method Magnitude:Double()
		Return Sqr(_x^2+_y^2)
	End Method
	
	Method MagSquared:Double()
		Return _x^2+_y^2
	End Method
	
	Method Normalise:TVector(bself=True)
		Local m:Double = Magnitude()
		If bself = False
			Return TVector.Create(_x/m,_y/m)
		Else
			_x:/m
			_y:/m
		EndIf
	End Method

	Method PrintVector()
		Print "(" + _x + "," + _y + ")"
	End Method
	
	Method Draw(xoff,yoff)
		DrawLine xoff,yoff,xoff+_x,yoff+_y
	End Method

	'FUNCTIONS
	Function Create:TVector(x:Double,y:Double)
		Local o:TVector = New TVector
		o._x=x
		o._y=y
		Return o
	End Function
	
	Function FromTo:TVector(v1:TVector,v2:TVector)
		Return Create(v2._x-v1._x,v2._y-v2._x)
	End Function
End Type


Here's the actual system:
Strict
Framework BRL.Max2D
Import BRL.D3D7Max2D
Import BRL.Basic
Import BRL.System
Import BRL.Timer
Import BRL.LinkedList
Import "vector.bmx"

Type TParticle
	Global list:TList
	Global bHidden=False
	
	Field oldposition:TVector
	Field position:TVector
	Field acceleration:TVector
	Field mass#
	Field bLocked
	
	Method New()
		If Not list Then list = CreateList()
		ListAddLast(list,Self)
		position = TVector.Create(0,0)
		oldposition = TVector.Create(0,0)
		acceleration = TVector.Create(0,0)
		mass# = 1
		bLocked = False
	End Method
	
	Method Push(Fx#,Fy#)
		acceleration.Add(TVector.Create(Fx/mass,Fy/mass),True)
	End Method
	
	Method Update()
		If bLocked = False Then VerletIntegrator()
		acceleration.set(0,0)
	End Method
	
	Method VerletIntegrator:TVector(update = True)
		If update = True
			Local p:TVector = position.Copy()
			Local op:TVector = oldposition.Copy()
			Local accel:TVector = acceleration.Copy()
			p.Multiply(2-TPhysicsEngine.Resistance)
			op.Multiply(1-TPhysicsEngine.Resistance)
			p.Subtract(op,True)
			accel.Multiply(TPhysicsEngine.DeltaTime()^2)
			p.Add(accel,True)
			oldposition = position.Copy()
			position = p.Copy()
			Return Null
		Else
			Local p:TVector = position.Copy()
			Local op:TVector = oldposition.Copy()
			Local accel:TVector = acceleration.Copy()
			p.Multiply(2-TPhysicsEngine.Resistance)
			p.Subtract(op.Multiply(1-TPhysicsEngine.Resistance),True)
			p.Add(accel.Multiply(TPhysicsEngine.DeltaTime()^2))
			Return p
		EndIf
	End Method
	
	Method Lock()
		bLocked = True
	End Method
		
	Method Unlock()
		blocked = False
	End Method
	
	Method GetVelocity:TVector()
		Local v:TVector = VerletIntegrator(False).Subtract(oldposition)
		v.Multiply(1/Float(2*TPhysicsEngine.DeltaTime()))
		Return v
	End Method
	
	Method SetPosition(x#,y#)
		position._x=x
		position._y=y
		oldposition = position.Copy()
	End Method
	
	Method GetMomentumX:Float()
		Return GetVelocity()._x*mass
	End Method
	
	Method GetMomentumY:Float()
		Return GetVelocity()._y*mass
	End Method
	
	Method Render()
		DrawRect position._x-5,position._y-5,10,10
	End Method
	
	Function Create:TParticle(x#,y#,mass#=1,vx#=0,vy#=0)
		Local o:TParticle = New TParticle
		o.SetPosition(x,y)
		o.mass = mass
		o.acceleration.set(vx,vy)
		Return o
	End Function
	
	Function DoFrame()
		For Local o:TParticle = EachIn(list)
			o.Update()
			If bHidden = False Then o.Render()
		Next
	End Function
End Type

Type TConstraint
	Global list:TList
	Global bHidden=False

	Field _distance:Float
	Field _p1:TParticle = Null
	Field _p2:TParticle = Null
	
	Method New()
		If Not list Then list = CreateList()
		list.addlast(Self)
	EndMethod
	
	Method Update()
		Local vdistance:TVector
		Local ndistance#
		Local difference#
		Local invmass1# = 1.0/_p1.mass
		Local invmass2# = 1.0/_p2.mass
		
		vdistance = _p1.position.Subtract(_p2.position)
		ndistance = vdistance.Magnitude()
		difference = (ndistance-_distance)/(ndistance*(invmass1+invmass2))
		If _p1.bLocked = False
			_p1.position._x=_p1.position._x-vdistance._x*invmass1*difference
			_p1.position._y=_p1.position._y-vdistance._y*invmass1*difference
		EndIf
		If _p2.bLocked = False
			_p2.position._x=_p2.position._x+vdistance._x*invmass2*difference
			_p2.position._y=_p2.position._y+vdistance._y*invmass2*difference
		EndIf
	End Method
	
	Method Render()
		DrawLine _p1.position._x,_p1.position._y,_p2.position._x,_p2.position._y
	End Method	
	
	Function Create:TConstraint(p1:TParticle,p2:TParticle,dist:Float = 0)
		Local c:TConstraint = New TConstraint
		c._p1 = p1
		c._p2 = p2
		If dist = 0
			c._distance = Sqr((p1.position._x-p2.position._x)^2+(p1.position._y-p2.position._y)^2)
		Else
			c._distance = dist
		EndIf
		Return c
	End Function
	
	Function DoFrame()
		For Local o:TConstraint = EachIn(list)
			o.Update()
			If bHidden = False Then o.Render()
		Next
	End Function
End Type

	
	

Type TPhysicsEngine
	Global Gravity:Float = 0.0
	Global Resistance:Float = 0.0
	Global LastTime = 0
	Global ThisTime = 0
	
	Function DoFrame()
		If ThisTime = 0 And LastTime = 0
			ThisTime = MilliSecs()
			LastTime = MilliSecs()
		EndIf
		
		ThisTime = MilliSecs()
		If TParticle.list
			For Local p:TParticle = EachIn(TParticle.list)
				p.acceleration._y:+Gravity
				p.Update()
				p.Render()
			Next
		EndIf
		
		If TConstraint.list
			For Local c:TConstraint = EachIn(TConstraint.list)
				c.Render()
				c.Update()
			Next
		EndIf
		LastTime = ThisTime
	End Function
	
	Function DeltaTime#()
		Return Float(ThisTime-LastTime)/1000.0
	End Function
End Type


And here's a small example:
Graphics 640,480,0

Local p:TParticle = TParticle.Create(640/2,480/2)
Local p2:TParticle = TParticle.Create(640/2,480/2+50)
Local p3:TParticle = TParticle.Create(640/2,480/2+100)
Local p4:TParticle = TParticle.Create(640/2,480/2+150)
Local p5:TParticle = TParticle.Create(640/2,480/2+200)
Local p6:TParticle = TParticle.Create(640/2-25,480/2+225)
Local p7:TParticle = TParticle.Create(640/2+25,480/2+225)
TConstraint.Create(p,p2)
TConstraint.Create(p2,p3)
TConstraint.Create(p3,p4)
TConstraint.Create(p4,p5)
TConstraint.Create(p5,p6)
TConstraint.Create(p6,p7)
TConstraint.Create(p5,p7)
p.Lock()
Local x# =640/2
Local y# =480/2
TPhysicsEngine.Gravity = 70
TPhysicsEngine.Resistance = 0.005
	
While Not KeyDown(KEY_ESCAPE)
	Cls
	If MouseDown(1) Then p5.SetPosition(MouseX(),MouseY())
	TPhysicsEngine.DoFrame()
	Flip
Wend



ImaginaryHuman(Posted 2006) [#2]
Can you explain in simple terms what this is all about?


SSS(Posted 2006) [#3]
Hey, I'll do my best to explain.
It's the begining of a physics engine in two dimensions. It will eventually allow objects that react as they would in the real world. For the moment, it does a lot to that extent but is missing some features.

For example, you could make some boxes and shoot them and they'd bounce around realistically. (well no collision yet but you WOULD be able to :P). You could make chains or cloth that blows in the wind. You could make space ships that interact realistically.

The code is pretty simple at the moment (to use). It works using a combination of particles and constraints. Particles are points that have masses, velocities, accelerations and positions. Constraints keep particles a specific distance apart. So, if you wanted to create a very simple ship (triangular in shape) you could use the following code:

Graphics 640,480
'creates particles at the positions (x,y)
Local top:TParticle = TParticle.Create(640/2,480/2-10)
Local Left:TParticle = TParticle.Create(640/2-10,480/2+10)
Local Right:TParticle = TParticle.Create(640/2+10,480/2+10)

'create constraints between the particles to maintain the shape
TConstraint.Create(top,Left)
TConstraint.Create(top,Right)
TConstraint.Create(Left,Right)

While Not KeyDown(KEY_ESCAPE)
	Cls
	'get a vector from the top vertex to the mouse
	Local x# = MouseX()-top.position._x
	Local y# = MouseY()-top.position._y
	'normalise the vector
	x = x/Sqr(x^2+y^2)
	y = y/Sqr(x^2+y^2)
	
	'push the ship towards the mouse
	top.Push(20*x,20*y)
	
	'process the frame
	TPhysicsEngine.DoFrame()
	'flip
	Flip
Wend

(btw this is a terrible model... i just made it in two minutes as an example. It really doesnt show what this is capable of, only how it works.)


BlitzSupport(Posted 2006) [#4]
Looking good so far!


REDi(Posted 2006) [#5]
Most impressive!


poopla(Posted 2006) [#6]
I added you to my hotmail SSS, perhaps integrating my collision routines into a unified physics system will be a good way to go, we can talk if/when you come online sometime ^_^.


SSS(Posted 2006) [#7]
Thanks for all the positive feedback!

Mike, I think that's a great idea as collision routines are something I dread. My hotmail address (for msn) is littleboy55 AT hotmail D0T com (i know, i know but i made it when i was ten.) If you'd like my e-mail it's: Sam D0T Schoenholz AT gmail DOT com.

Hope to hear from you soon.


smilertoo(Posted 2006) [#8]
you appear to have forgotten includes or something, the program does nothing for me.

ah, the problem is youve presented it as 3 files when the 3rd part should be included in the 2nd part.


SSS(Posted 2006) [#9]
Hey everyone,

I got angular constraints working (i think.. dunno exactly how their supposed to act.) The creation is through the function TAngularConstraint.Create(p1,p2,p3,angle) where p1 is the pivot and p2, and p3 are the two particles that you want to separate by a certain angle. The angle is only needed if you dont want them at their rest angle but some other artificial angle. Anyway, without further adue here is the code:

Type TAngularConstraint
	Global list:TList

	Field _angle:Float
	Field _p1:TParticle = Null
	Field _p2:TParticle = Null
	Field _p3:TParticle = Null
	
	Method New()
		If Not list Then list = CreateList()
		list.addlast(Self)
	EndMethod
	
	Method Update()
		Local t1:TVector = _p3.Position.Subtract(_p1.Position)
		Local t2:TVector = _p2.Position.Subtract(_p1.Position)

		Local Angle:Double = t1.AngleBetween(t2)
		
		If Angle<_angle
			Local da:Double = (_angle-Angle)/(2*(1.0/_p2.mass+1.0/_p3.mass))
			If t1.GetAngle()>t2.GetAngle()
				t1.SetAngle(t1.GetAngle() + da/_p3.mass)
				t2.SetAngle(t2.GetAngle() - da/_p2.mass)
			Else
				t1.SetAngle(t1.GetAngle() - da/_p3.mass)
				t2.SetAngle(t2.GetAngle() + da/_p2.mass)
			EndIf
			t1.Add(_p1.Position,True)
			t2.Add(_p1.Position,True)
			
			If _p3.bLocked = False
				_p3.Position = t1
			EndIf
			If _p2.bLocked = False
				_p2.Position = t2
			EndIf
		EndIf
	End Method
	
	Method CalculateAngle:Double()
		Local t1:TVector = _p3.Position.Subtract(_p1.Position)
		Local t2:TVector = _p2.Position.Subtract(_p1.Position)
		
		Return t1.AngleBetween(t2)
	End Method 		
	
	Function Create:TAngularConstraint (p1:TParticle,p2:TParticle,p3:TParticle,angle:Float = 0)
		Local c:TAngularConstraint = New TAngularConstraint
		c._p1 = p1
		c._p2 = p2
		c._p3 = p3
		If angle = 0
			c._angle = c.CalculateAngle()
		Else
			c._angle = angle
		EndIf
		Return c
	End Function
	
	Function DoFrame()
		For Local o:TAngularConstraint = EachIn(list)
			o.Update()
		Next
	End Function
End Type


and an example
Graphics 640,480,0
Local p:TParticle = TParticle.Create(640/2,480/2)
Local p2:TParticle = TParticle.Create(640/2+25,480/2+50)
Local p3:TParticle = TParticle.Create(640/2-25,480/2+50)
TConstraint.Create(p,p2)
TConstraint.Create(p,p3)
p2.mass = 2
Local l:TAngularConstraint = TAngularConstraint.Create(p,p2,p3)
p.Lock()
Local x# =640/2
Local y# =480/2
TPhysicsEngine.Gravity = 70
TPhysicsEngine.Resistance = 0.005
	
While Not KeyDown(KEY_ESCAPE)
	Cls
	DrawText l.CalculateAngle(),10,10
	If MouseDown(1) Then p2.SetPosition(MouseX(),MouseY())
	TPhysicsEngine.DoFrame()
	Flip
Wend



SSS(Posted 2006) [#10]
UPDATE!!
I managed to add breakable constraints when there is to much tension. Once again, I didn't follow an article, just used my own knowledge so it might not be strictly 'correct'. Anyway, here's the code
Type TConstraint
	Global list:TList
	Global bHidden=False

	Field _distance:Float
	Field _stress:Float = 0
	Field _p1:TParticle = Null
	Field _p2:TParticle = Null
	
	Method New()
		If Not list Then list = CreateList()
		list.addlast(Self)
	EndMethod
	
	Method Update()
		Local vdistance:TVector
		Local ndistance#
		Local difference#
		Local invmass1# = 1.0/_p1.mass
		Local invmass2# = 1.0/_p2.mass
		
		vdistance = _p1.position.Subtract(_p2.position)
		ndistance = vdistance.Magnitude()
		difference = (ndistance-_distance)/(ndistance*(invmass1+invmass2))
		
		If _stress>0
			If Stress()>_stress Then list.remove(Self)
		EndIf		
			
		If _p1.bLocked = False
			_p1.position._x=_p1.position._x-vdistance._x*invmass1*difference
			_p1.position._y=_p1.position._y-vdistance._y*invmass1*difference
		EndIf
		If _p2.bLocked = False
			_p2.position._x=_p2.position._x+vdistance._x*invmass2*difference
			_p2.position._y=_p2.position._y+vdistance._y*invmass2*difference
		EndIf
	End Method
	
	Method Stress:Float()
		Local tofrom:TVector = TVector.FromTo(_p1.position,_p2.position)
		Local acceleration1:TVector = TVector.FromTo(_p1.oldposition,_p1.position)
		Local acceleration2:TVector = TVector.FromTo(_p2.oldposition,_p2.position)
		
		acceleration1.Multiply(_p1.mass)
		acceleration2.Multiply(_p2.mass)
		
		Local angle:Float = 180.0-acceleration1.AngleBetween(tofrom)
		Local Force1:Float = acceleration1.Magnitude()*Cos(angle)
		angle:Float = 180.0-acceleration2.AngleBetween(tofrom)
		Local Force2:Float = acceleration2.Magnitude()*Cos(angle)
		Force1:+Force2
		Return Force1
	End Method
	
	Method Breakable(stress:Float)
		_stress = stress
	End Method
	
	Method Unbreakable()
		_stress = 0
	End Method
	
	Method Render()
		DrawLine _p1.position._x,_p1.position._y,_p2.position._x,_p2.position._y
	End Method	
	
	Function Create:TConstraint(p1:TParticle,p2:TParticle,dist:Float = 0)
		Local c:TConstraint = New TConstraint
		c._p1 = p1
		c._p2 = p2
		If dist = 0
			c._distance = Sqr((p1.position._x-p2.position._x)^2+(p1.position._y-p2.position._y)^2)
		Else
			c._distance = dist
		EndIf
		Return c
	End Function
	
	Function DoFrame()
		For Local o:TConstraint = EachIn(list)
			o.Update()
			If bHidden = False Then o.Render()
		Next
	End Function
End Type


Here's an example,
Graphics 640,480,0
Local p:TParticle = TParticle.Create(640/2,480/2)
Local p2:TParticle = TParticle.Create(640/2,480/2+50)
Local p3:TParticle = TParticle.Create(640/2,480/2+100)
Local p4:TParticle = TParticle.Create(640/2,480/2+150)
Local p5:TParticle = TParticle.Create(640/2,480/2+200)
Local p6:TParticle = TParticle.Create(640/2-25,480/2+225)
Local p7:TParticle = TParticle.Create(640/2+25,480/2+225)
TConstraint.Create(p,p2)
Local c:TConstraint = TConstraint.Create(p2,p3)
c.Breakable(7)
TConstraint.Create(p3,p4)
TConstraint.Create(p4,p5)
TConstraint.Create(p5,p6)
TConstraint.Create(p6,p7)
TConstraint.Create(p5,p7)
p.Lock()
Local x# =640/2
Local y# =480/2
TPhysicsEngine.Gravity = 70
TPhysicsEngine.Resistance = 0.005
Local pu# = 0
While Not KeyDown(KEY_ESCAPE)
	Cls
	If p5.position._y>=300 Then p5.Push(pu,0)
	If KeyDown(KEY_UP) Then pu#:-50
	TPhysicsEngine.DoFrame()
	Flip
Wend

hold the upkey to make the thing go around faster until it breaks. I think it's quite cool.

[edit] just replace the previous constraint code [/edit]