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POFlash Actionscript 3.0 : Number Type and Decimal Precision (Vector Fields and Particles)
 

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  1. POFlash Actionscript 3.0 : Number Type and Decimal Precision (Vector Fields and Particles)
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    <p>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 <em>F(x,y)=yi-xj</em></p> <p>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.</p> <p>This code spawns the particles on the screen (which is 800x450). This code probably isn't in trouble, but for completeness I included it.</p> <pre><code>//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&lt;=800/spread; i++) { for (j=0; j&lt;=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); } } </code></pre> <p>This is the "dot" class which contains everything important about the particles being spawned.</p> <pre><code>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; } } } </code></pre> <p>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!</p>
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    1. COI think your assumption is correct. Seems floating point inaccuracies are the cause. As to how you can fix this.....
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    2. COI am still suspicious though, my calculations shouldn't produce numbers which are too large for the Number data type. Unless the inaccuracy can be seen with fractional values such as 1/3. Then it would be impossible to model vector fields in flash using the native data types.
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