Simulate a flock of objects


Have you ever wished that you could get a group of objects to move as though they had a collective mind, like a school of fish or a herd of buffalo? You've probably realized that it's a flocking problem! I'll show you the flocking solution to the whole flocking thing!

If you wish to follow along you might want to download the sample source code that goes along with this tutorial. 

There are a number of different ways to go about this, but in general, these are the rules that a flock must follow to exhibit convincing behaviour:

  • Objects must attempt to move towards the center of the group
  • Objects must maintain some minimum distance between themselves and others
  • Objects must move at a speed relative to the speed of the flock

This makes sense intuitively; think about a herd of zebra in Africa. The safest zebra are those in the middle of the herd, but they must maintain a practical distance between each other, and also match speeds (or be left behind). Zebra on the outside want to get inside (so they're as far away from lions as possible!), but won't run over other zebra to do it.

The way you implement these rules is really not important, but I'll show you how I've done it anyway, as an example:


Visual Basic Tutorial Screen1



We'll call our objects "sheep" (because I think sheep are funny). Now, lets randomize the locations of the starting positions of the sheep, as we can see has been done in the image above.

Private Type SHEEPTYPE
   sngX As Single
   sngY As Single
   sngXSpeed As Single
   sngYSpeed As Single
End Type
Dim mudtSheep() As SHEEPTYPE
Const NUM_SHEEP = 19
Dim i As Integer
Dim j As Integer
Dim blnSeperation As Boolean
   ReDim mudtSheep(NUM_SHEEP - 1)
   For i = 0 To UBound(mudtSheep)
      blnSeperation = False
      Do While Not (blnSeperation)
         mudtSheep(i).sngX = Rnd() * frmFlock.ScaleWidth
         mudtSheep(i).sngY = Rnd() * frmFlock.ScaleHeight
         blnSeperation = True
         For j = 0 To i - 1
            If CalcDist(i, j) <= MIN_SEPERATION Then
               blnSeperation = False
               Exit For
            End If
         Next j
   Next i 


A lot of code just to disperse some sheep! First, we set up the UDT (User Defined Type) for our sheep data, and then create a dynamic array and size it according to the value of the NUM_SHEEP constant. Easy, right? Then we step through each sheep with a For loop and assign it a random location. It is imperative that the sheep don't overlap however, so we need ANOTHER For loop to check this! The process continues within a While loop until success is achieved.


Visual Basic Tutorial Screen2


Ooooh, now they're all in a clump! How did that happen?

Private Function CalcDist(intIndex1 As Integer, intIndex2 As Integer) As Single
   CalcDist = Sqr((mudtSheep(intIndex1).sngX - mudtSheep(intIndex2).sngX) ^ 2 + _
                  (mudtSheep(intIndex1).sngY - mudtSheep(intIndex2).sngY) ^ 2)
End Function 


This handy little function returns the distance between any two sheep (identified by their index value within the array). With it, we can determine our closest neighbour (using loops, similar to those used in the randomization step) and check his speed. Remember, we need to keep every sheep's speed similar to that of the flock!

Const MAX_NOISE = 250
Dim sngXSum As Single
Dim sngYSum As Single
Dim sngXAvg As Single
Dim sngYAvg As Single
   For j = 0 To UBound(mudtSheep)
      sngXSum = sngXSum + mudtSheep(j).sngX
      sngYSum = sngYSum + mudtSheep(j).sngY
   Next j
   sngXAvg = (sngXSum / NUM_SHEEP) + (Rnd() * MAX_NOISE) - (MAX_NOISE / 2)
   sngYAvg = (sngYSum / NUM_SHEEP) + (Rnd() * MAX_NOISE) - (MAX_NOISE / 2) 

 This code will find the center of the flock by averaging the X and Y values of all of the sheep. It also adds some noise, so that the sheep appear to jostle around. The reason for the subtraction of (MAX_NOISE / 2) is so that the noise can be positive or negative. Essentially, we're adding a random value between -125 and 125.

All that remains is to move each sheep towards this "center" according to their X and Y speed values. Also, if a movement will cause a sheep to infringe on another's MIN_SEPERATION, then you should abort the movement. Have a look at this sample source code if you'd like to be a shepherd for a day. If you have any questions or comments about this Visual Basic 6 Tutorial please post them below.

This tutorail is released under the GNU Free Documentation License 1.2. The original can be found here.

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no errors here, kid, and this guy is old enough to know what he should and should not do..


'nuf said


xD xD xD xD xD xD xD



natural flock

i am not sure about the statement "Objects must attempt to move towards the center of the group". i think you can even reduce it and say get your closest neighbours. maybe according to distance and try to stay in a safe distance to them and follow their direction.


You should check your tutorials. This one has a runtime error and the source download uses a completly different code. Also you really need to add a tutorial on making the objects move with mouse buttons ><.