hi
i'm a bit confused about normalizing PVectors. I am rotating objects by adding a speed vector, then normalizing it to constrain their movement. but the objects come to a halt after a few seconds of movement. though the variables are still showing movement. everything is happening in the update() function.

Code:

class Circs{
  float x, y, z, angle; 
  float sphereSize;
  int me;
  PVector pos, speed, reverseSpeed;

  Circs(int _me, float _angle) {
    angle = _angle;
    me = _me;
    sphereSize= random(15, 30); 
    float xspeed =random(-5, 5);
    //xspeed = constrain(abs(xspeed),0.5,4);
    float yspeed =random(-4, 4);
    //yspeed = constrain(abs(yspeed),0.5,4);
    float zspeed =random(-4, 4);
    // zspeed = constrain(abs(zspeed),0.5,4);
    speed = new PVector (xspeed, yspeed, zspeed);

    x=cos(angle)*(radius/2+sphereSize/2); 
    y=sin(angle)*(radius/2+sphereSize/2); 
    z= random(-radius/2, radius/2);

    pos = new PVector (x,y,z);
  } 

  void draw(){
    update();
    fill(255); 

    pushMatrix(); 
    translate(pos.x, pos.y, pos.z);
    sphere(sphereSize);
    popMatrix();   

  }

  void update(){
    pos.add(speed);
    pos.normalize();
    pos.mult(radius/2);
    println ("x: " + pos.x + "y: " + pos.y + "z: " + pos.z);
    println ("speedx: " + speed.x + " speedy: " + speed.y + " speedz: " + speed.z );
  }

}


 

ok, sorry, here's the whole shebang:
Code:

//import processing.opengl.*;

float radius = 200; 
int balls = 15; 

Circs[] circs;

void setup(){ 
size(640, 480, P3D); 
circs = new Circs[balls];
float ang = 0; 
for(int i=0; i< balls; i++){ 
circs[i] = new Circs(i, ang); 
ang+=radians(random(180));
} 
noStroke(); 
sphereDetail(8); 
} 
void draw(){ 
background(255); 
lights(); 
translate(width/2,height/2); 
//rotateY(radians(frameCount)); 
//rotateX(radians(frameCount)); 
sphereDetail(80, 80);
sphere(radius/2);
sphereDetail(15,15);
for (int i=0; i<circs.length; i++){
circs[i].draw();  
}

} 

void mousePressed(){
 println("frameRate: "+frameRate); 
}

class Circs{
  float x, y, z, angle; 
  float sphereSize;
  int me;
  PVector pos, speed, reverseSpeed;

  Circs(int _me, float _angle) {
    angle = _angle;
    me = _me;
    sphereSize= random(15, 30); 
    float xspeed =random(-5, 5);
    //xspeed = constrain(abs(xspeed),0.5,4);
    float yspeed =random(-4, 4);
    //yspeed = constrain(abs(yspeed),0.5,4);
    float zspeed =random(-4, 4);
    // zspeed = constrain(abs(zspeed),0.5,4);
    speed = new PVector (xspeed, yspeed, zspeed);

    x=cos(angle)*(radius/2+sphereSize/2); 
    y=sin(angle)*(radius/2+sphereSize/2); 
    z= random(-radius/2, radius/2);

    pos = new PVector (x,y,z);
  } 

  void draw(){
    update();
    fill(255); 

    pushMatrix(); 
    translate(pos.x, pos.y, pos.z);
    sphere(sphereSize);
    popMatrix();   

  }

  void update(){
    pos.add(speed);
    pos.normalize();
    pos.mult(radius/2);
    println ("x: " + pos.x + "y: " + pos.y + "z: " + pos.z);
    println ("speedx: " + speed.x + " speedy: " + speed.y + " speedz: " + speed.z );
  }

}




 

You could also look at it from a vector-calculation point of view.
If you print out the cross-prodcut of your position and velocity vectors by
println("cross:"+pos.cross(speed));
you will see that this vector is getting smaller and smaller. Now, the cross-product of vector a with vector b is a vector perpendicular to the plane given by the both vectors and with a length calculated as
|a| * |b| * sin(phi<ab>)  with | | beeing the length and phi<> beeing the angle between the vectors.
The vector going "smaller and smaller" means that a & b (or pos & velocity) become more and more parallel as you continue which is exactly what koggy was posting...

What you want to achieve is a "speedvector" which is always normal to the direction of the position vector.

Since you are constraining movement to the surface of a sphere then rather than defining the velocity as a change in XYZ position, it is probably better to consider the velocity as the rate of change in longitude an latitude.

There would be no need to record the XYZ position of the sphere rather this would be calculated from the angles for latitude and longitude and these angles can be simply incremented/decremented each update according to the velocity..

Are you wanting to rotate the small spheres to face the direction of movement? That would add a little more work.