Question

I’m modeling a vector field in flash and spawning a mess of particles to visualize the flow of the field. Using the vector field F(x,y)=yi-xj

This vector field has a curl, the particles are to move in circles which they do. My problem is that the particles diverge from the origin, though this particular vector field has no divergence. I suspect that my data types may be losing decimal precision during the very basic caluclations for this field or perhaps I’m making some logic mistake I am unsure.

This code spawns the particles on the screen (which is 800×450). This code probably isn’t in trouble, but for completeness I included it.

//spawn particles
var i:int;
var j:int;
//spread is the spacing between particles
var spread:Number;
spread = 10.0;
//spawn the particles
for (i=0; i<=800/spread; i++)
{
 for (j=0; j<=450/spread; j++)
 {
  //computes the particles position and then constructs the particle.
  var iPos:Number = spread * Number(i) - 400.0;
  var jPos:Number = 225.0 - spread * Number(j);
  var particle:dot = new dot(iPos,jPos,10.0);
  addChild(particle);
 }
}

This is the “dot” class which contains everything important about the particles being spawned.

package 
{
 //import stuff
 import flash.display.MovieClip;
 import flash.events.Event;
 public class dot extends MovieClip
 {
  //variables
  private var xPos:Number;
  private var yPos:Number;
  private var xVel:Number;
  private var yVel:Number;
  private var mass:Number;
  //constructor
  public function dot(xPos:Number, yPos:Number, mass:Number)
  {
   //Defines the function to be called when the stage advances a frame.
   this.addEventListener(Event.ENTER_FRAME, moveDot);
   //Sets variables from the constructor's arguments.
   this.xPos = xPos;
   this.yPos = yPos;
   this.mass = mass;
   //Set these equal to 0.0 so the Number type knows I want a decimal (hopefully).
   xVel = 0.0;
   yVel = 0.0;
  }
  //Controlls the particle's behavior when the stage advances a frame.
  private function moveDot(event:Event)
  {
   //The vector field is a force field. F=ma, so we add F/m to the velocity. The mass acts as a time dampener.
   xVel += yPos / mass;
   yVel +=  -  xPos / mass;
   //Add the velocity to the cartesian coordinates.
   xPos +=  xVel;
   yPos +=  yVel;
   //Convert the cartesian coordinates to the stage's native coordinates.
   this.x = xPos + 400.0;
   this.y = 225.0 + yPos;
  }
 }
}

Ideally the particles would all move in circles about the origin forever. But the code creates a situation where the particles rotate around the origin and spiral outwards, eventually leaving the stage. I’d really appreciate a helping hand.
Thanks!

Answer 1

You can normalize each particle’s distance from origin (maybe not on each step to save calculations). Also it seems your code is not optimized – you are creating ENTER_FRAME listener for every particle, and there is 3600 of them. One listener should be enough. And I would change all these divisions to multiplication by inverse value.