I've been messing with this script for a few days and I'm trying to move from 2D to 3D. My first step was to at least get PeasyCam so I could revolve around the objects drawn in a 3D environment. The problem I'm running in to is if I add "background(255);" to the void draw ( ) expression then I lose the series of boxes drawn as it moves across the sketch (which I'd like to keep), but the red cross-hairs (which denote attractors) remain and I'm able to orbit/pan/zoom. If I remove "background(255);" from void draw ( ) then i get the series of boxes but my cross-hairs leave a trail when i obit/pan/zoom. This is due to the way i've constructed the script. So my question is : What is the best way to rewrite this so that I can keep both the series of boxes and cross-hairs without either of them leaving a trail as I orbit/pan/zoom. I'm guessing that somehow I need to figure out how to draw the boxes like the cross-hairs so the geometry continues to update - but without computing all the acceleration stuff. I hope that is clear. The script below allows the camera to orbit/pan/zoom but leaves cross hair trails. 

PS - i know i'm using a couple libraries (PeasyCam and Old3D) but my question is about the structure of my code - not library specific. That's why I posted here. 

2. import ixagon.renderer.*;
3. import peasy.*;
4. PeasyCam cam;
5. 
   
6. ForcePoly myPoly;
7. 
   
8. Attractor[] attractor = new Attractor[100];
9. 
   
10. int G = 10000; 
11. 
    
12. /////////////////////////////////////////////////////
13. void setup() {
14.   cam = new PeasyCam(this,width/2, height/2, 0, 300);
15.   size(600, 240, OldP3D.NOSTALGIA);
16.   smooth(); 
17.   background(255);
18.   for (int i=0; i < attractor.length; i++) {
19.     attractor[i] = new Attractor( random(width), random(height)  );
20.   }
21.   myPoly = new ForcePoly (0, 25, 2, 0);
22.   
23. }
24. 
    
25. void draw() {
26.   //background(255);
27.   myPoly.update();
28.   for (int i=0; i < attractor.length; i++) {
29.     attractor[i].update();
30.   }
31. }
32. 
    
33. void keyPressed() {
34.   setup();
35. }
36. 
    
37. /////////////////////////////////////////////////////
38. class ForcePoly {
39.   float x,y, xspeed, yspeed, angA; 
40.   float angV = 0;
41.   boolean dead = false; 
42.   PVector a; 
43.   
44.   
45.   ForcePoly(float _x, float _y, float _xspeed, float _yspeed) {
46.     x = _x;
47.     y = _y;
48.     xspeed = _xspeed; 
49.     yspeed = _yspeed; 
50.   }
51.   void update() {
52.     if (!dead) {
53.       x += xspeed; 
54.       y += yspeed;
55.       a = new PVector(); 
56.       for (int i = 0; i < attractor.length; i++) {
57.         a.add(attractor[i].getAccel(x,y));
58.       }
59.       angA = a.heading2D();
60.       angV +=  angA/5;
61.       render();
62.       if (x > 300) {
63.         dead = true;  
64.       }
65.       angV  *= .9;
66.       
67.     } 
68.   }
69.   
70.   void render() {
71.     rectMode(CENTER);    
72.     stroke(0);
73.     fill(255); 
74.     pushMatrix(); 
75.       translate(x*2,y); 
76.       rotate(angV);
77.       box(60, 30, 1);
78.       popMatrix(); 
79.       
80.   }
81. }
82. 
    
83. ///////////////////////////////////////////////////
84. class Attractor {
85.   float x,y;
86.   
87.   Attractor(float _x, float _y ) {
88.     x = _x;
89.     y = _y;
90.   }
91.   
92.   void update() {
93.     render(); 
94.   }
95.   void render() {
96.       stroke(255,0,0);
97.       line(x-4, y, x+4, y);
98.       line(x, y-4, x, y+4);
99.   }
100.   PVector getAccel(float px, float py) {
101.     PVector a = new PVector(0,0,0); 
102.       float d2 = sq(dist(px,py,x,y));
103.       if (d2 > 10) {
104.         a.x = G * (x-px) / d2;
105.         a.y = G * (y-py) / d2;
106.       }
107.       return a; 
108.    }
109. }