Simulating a hanging chain

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Stevie G

The diff is a distance measurement which tells you how far from it's restlength a link is. It uses this to adjust both points proportionate to their inversemass so that a heavier particle isn't moved as far as a lighter one but they still maintain their distance appart.

Really the 'Mass' field should be 'InverseMass'. Note if you set this to 0 the particle is immovable automatically.

I've changed a couple of things. Your initial particle positions were causing major energy in the system so I fixed this. I also made the inversemasses lower so that gravity has a bigger effect. It seems to work ..

Open in online IDE

Graphics 640, 480, 16, 0
SetBuffer BackBuffer()
SeedRnd MilliSecs()
TFormFilter False
frameTimer = CreateTimer(60)

timeStep# = 0.05 ;0.016666
g# = 9.81
drag# = 0.995
numLinks = 100
massOfEachLink = 1
restLength# = 4
Iterations = 1



Type playerType
Field x#, y#, vX#, vY#, aX#, aY#
Field mass#
End Type

player.playerType = New playerType
player\x = 100
player\y = 250
player\mass = 1


Type clawType
Field x#, y#, aX#, aY#
End Type


Type chainLinkType
Field x#, y#, oldX#, oldY#, aX#, aY#, mass#
End Type

Dim chainArray.chainLinkType(numLinks - 1)
For i = 0 To numLinks - 1

  chainArray(i) = New chainLinkType
  chainArray(i)\x = 120 + i * 4 ;i * 540 / (numLinks - 1)
  chainArray(i)\y = 100
  chainArray(i)\oldX = chainArray(i)\x
  chainArray(i)\oldY = chainArray(i)\y
  chainArray(i)\mass = massOfEachLink

  ;set end points to have infinite mass
  If i = 0 Or i = ( numlinks-1 )
 	  chainArray(i)\mass = 0 
  EndIf	

Next




While Not KeyHit(1)
WaitTimer frameTimer
Cls


  ; Controls



  For i = 0 To numLinks - 1

    ; Apply gravity to particles which don't have an infinite mass
	If chainArray(i)\Mass > 0   chainArray(i)\aY = chainArray(i)\aY + g / chainArray(i)\mass

    ; Verlet Integration
    tempX# = chainArray(i)\x
    tempY# = chainArray(i)\y

    chainArray(i)\x = chainArray(i)\x * (1.0 + drag) - drag * chainArray(i)\oldX + chainArray(i)\aX * timeStep * timeStep
    chainArray(i)\y = chainArray(i)\y * (1.0 + drag) - drag * chainArray(i)\oldY + chainArray(i)\aY * timeStep * timeStep
    chainArray(i)\oldX = tempX
    chainArray(i)\oldY = tempY

    ; Reset acceleration after moving particle
    chainArray(i)\aX = 0 : chainArray(i)\aY = 0
  Next



  ; Make each link experience a force from the surrounding ones
	For j = 1 To Iterations
	  For i = 0 To numLinks - 2
	
	    temp_dx# = chainArray(i + 1)\x - chainArray(i)\x
	    temp_dy# = chainArray(i + 1)\y - chainArray(i)\y
	    temp_Separation# = Sqr(temp_dx * temp_dx + temp_dy * temp_dy)
	    temp_Extension# = temp_Separation - restLength
	
	    ; Start 
	    temp_SpringForce# = temp_Extension / ( temp_Separation * (chainArray(i)\mass + chainArray(i + 1)\mass) )
	    chainArray(i)\x = chainArray(i)\x + chainArray(i)\mass * temp_dx * temp_SpringForce
	    chainArray(i)\y = chainArray(i)\y + chainArray(i)\mass * temp_dy * temp_SpringForce
	    chainArray(i + 1)\x = chainArray(i + 1)\x - chainArray(i + 1)\mass * temp_dx * temp_SpringForce
	    chainArray(i + 1)\y = chainArray(i + 1)\y - chainArray(i + 1)\mass * temp_dy * temp_SpringForce
	
	  Next
	
	 Next

  ; Draw Links
  For i = 0 To numLinks - 1
    Rect chainArray(i)\x, chainArray(i)\y, 1, 1, 1
  Next



Flip
Wend
End

p.s. Use the TAB button to indent your code .. not spaces!!

Stevie