Der Denkfehler ist behoben. Jetzt ist es fast perfekt. Naja, wenn der Ball gegen die Laufrichtung abprallt, läuft er auf seiner Kreisbahn Richtung Decke weiter. Die physikalischen Gesetze sind aufgehoben... Außerdem werden ein paar Bälle manchmal aus dem Bereich heraus gekickt.

Code B

Ball[] ball = new Ball[15];
int a,b,c; //Abstände der Ebenen

void setup() {
  size(1280,1024);
  //size(1920,1200);
  smooth();
  noStroke();
  colorMode(HSB,360,100,100,100);
  
  a = 500; //700 Größe der ersten Ebene
  b = 400; //600 Größe der zweiten Ebene
  c = 40; //60 Lücke
  
  for (int i=0; i < 5; i++) {
    ball[i] = new Ball();
    //in erster Ebene
    ball[i].x = random(ball[i].g1,a-ball[i].g1);
    ball[i].y = random(ball[i].g2,height-ball[i].g2);
    //Farbe
    ball[i].h = (int)random(120);
  }
  
  for (int i=5; i < 10; i++) {
    ball[i] = new Ball();
    //in zweiter Ebene
    ball[i].x = random(a+c+ball[i].g1,a+c+b-ball[i].g1);
    ball[i].y = random(ball[i].g2,height-ball[i].g2);
    //Farbe
    ball[i].h = (int)random(120,240);
  }
  
  for (int i=10; i < ball.length; i++) {
    ball[i] = new Ball();
    //in dritter Ebene
    ball[i].x = random(a+c+b+c+ball[i].g1,width-ball[i].g1);
    ball[i].y = random(ball[i].g2,height-ball[i].g2);
    //Farbe
    ball[i].h = (int)random(240,360);
  }
}

void draw() {
  background(0);
  fill(255);
  rect(a,0,c,height);
  rect(a+c+b,0,c,height);
  
  for (int i=0; i < 15; i++) {
    ball[i].position[0] = ball[i].x-ball[i].g1/2;
    ball[i].position[1] = ball[i].x+ball[i].g1/2;
    ball[i].position[2] = ball[i].y-ball[i].g2/2;
    ball[i].position[3] = ball[i].y+ball[i].g2/2;
  }
  
  for (int i=0; i < 5; i++) {
    //Bewegen
    if (ball[i].angle <= 360) {
      ball[i].angle -= 2;
    }
    if (ball[i].angle <= 0) {
      ball[i].angle = 360;
    }
    //im Bogen
    ball[i].vx = cos(radians(ball[i].angle))*ball[i].g1/6;
    ball[i].vy = sin(radians(ball[i].angle))*ball[i].g2/6;
    ball[i].x = ball[i].x + ball[i].vx;
    ball[i].y = ball[i].y + ball[i].vy;
    //Ballgröße zurücksetzen
    if (ball[i].g1 != 80) {
      ball[i].g1 = 80;
    }
    //Rand oben
    if (ball[i].x <= 0+ball[i].g1/2) {
      ball[i].angle = 0;
    }
    //Rand unten
    if (ball[i].x >= a-ball[i].g1/2) {
      ball[i].angle = 180;
      ball[i].g1 = ball[i].g1-20;
    }
    //links raus
    if (ball[i].y <= 0-ball[i].g2) {
      ball[i].y = height+ball[i].g2/2;
    }
    //rechts raus
    if (ball[i].y >= height+ball[i].g2) {
      ball[i].y = 0-ball[i].g2/2;
    }
    //Bälle stoßen aneinander
    for (int j=i+1; j < 5; j++) {
      if ((ball[i].position[1] >= ball[j].position[0]) && (ball[i].position[3] >= ball[j].position[2]) && 
      (ball[i].position[1] <= ball[j].position[1]) && (ball[i].position[3] <= ball[j].position[3])) {
        ball[i].angle = 225;
      }
      if ((ball[i].position[0] <= ball[j].position[1]) && (ball[i].position[3] >= ball[j].position[2]) && 
      (ball[i].position[0] >= ball[j].position[0]) && (ball[i].position[3] <= ball[j].position[3])) {
        ball[i].angle = 315;
      }
      if ((ball[i].position[1] >= ball[j].position[0]) && (ball[i].position[2] <= ball[j].position[3]) && 
      (ball[i].position[1] <= ball[j].position[1]) && (ball[i].position[2] >= ball[j].position[2])) {
        ball[i].angle = 135;
      }
      if ((ball[i].position[0] <= ball[j].position[1]) && (ball[i].position[2] <= ball[j].position[3]) && 
      (ball[i].position[0] >= ball[j].position[0]) && (ball[i].position[2] >= ball[j].position[2])) {
        ball[i].angle = 45;
      }
    }
    ball[i].maleBall();
  }
  
  for (int i=5; i < 10; i++) {
    //Bewegen
    if (ball[i].angle <= 360) {
      ball[i].angle -= 3;
    }
    if (ball[i].angle <= 0) {
      ball[i].angle = 360;
    }
    //im Bogen
    ball[i].vx = cos(radians(ball[i].angle))*ball[i].g1/6;
    ball[i].vy = sin(radians(ball[i].angle))*ball[i].g2/6;
    ball[i].x = ball[i].x + ball[i].vx;
    ball[i].y = ball[i].y + ball[i].vy;
    //Ballgröße zurücksetzen
    if (ball[i].g1 != 80) {
      ball[i].g1 = 80;
    }
    //Rand oben
    if (ball[i].x <= a+c+ball[i].g1/2) {
      ball[i].angle = 0;
    }
    //Rand unten
    if (ball[i].x >= a+c+b-ball[i].g1/2) {
      ball[i].angle = 180;
      ball[i].g1 = ball[i].g1-20;
    }
    //links raus
    if (ball[i].y <= 0-ball[i].g2) {
      ball[i].y = height+ball[i].g2/2;
    }
    //rechts raus
    if (ball[i].y >= height+ball[i].g2) {
      ball[i].y = 0-ball[i].g2/2;
    }
    //Bälle stoßen aneinander
    for (int j=i+1; j < 10; j++) {
      if ((ball[i].position[1] >= ball[j].position[0]) && (ball[i].position[3] >= ball[j].position[2]) && 
      (ball[i].position[1] <= ball[j].position[1]) && (ball[i].position[3] <= ball[j].position[3])) {
        ball[i].angle = 225;
      }
      if ((ball[i].position[0] <= ball[j].position[1]) && (ball[i].position[3] >= ball[j].position[2]) && 
      (ball[i].position[0] >= ball[j].position[0]) && (ball[i].position[3] <= ball[j].position[3])) {
        ball[i].angle = 315;
      }
      if ((ball[i].position[1] >= ball[j].position[0]) && (ball[i].position[2] <= ball[j].position[3]) && 
      (ball[i].position[1] <= ball[j].position[1]) && (ball[i].position[2] >= ball[j].position[2])) {
        ball[i].angle = 135;
      }
      if ((ball[i].position[0] <= ball[j].position[1]) && (ball[i].position[2] <= ball[j].position[3]) && 
      (ball[i].position[0] >= ball[j].position[0]) && (ball[i].position[2] >= ball[j].position[2])) {
        ball[i].angle = 45;
      }
    }
    ball[i].maleBall();
  }
  
  for (int i=10; i < 15; i++) {
    //Bewegen
    if (ball[i].angle <= 360) {
      ball[i].angle -= 4;
    }
    if (ball[i].angle <= 0) {
      ball[i].angle = 360;
    }
    //im Bogen
    ball[i].vx = cos(radians(ball[i].angle))*ball[i].g1/6;
    ball[i].vy = sin(radians(ball[i].angle))*ball[i].g2/6;
    ball[i].x = ball[i].x + ball[i].vx;
    ball[i].y = ball[i].y + ball[i].vy;
    //Ballgröße zurücksetzen
    if (ball[i].g1 != 80) {
      ball[i].g1 = 80;
    }
    //Rand oben
    if (ball[i].x <= a+c+b+c+ball[i].g1/2) {
      ball[i].angle = 0;
    }
    //Rand unten
    if (ball[i].x >= width-ball[i].g1/2) {
      ball[i].angle = 180;
      ball[i].g1 = ball[i].g1-20;
    }
    //links raus
    if (ball[i].y <= 0-ball[i].g2) {
      ball[i].y = height+ball[i].g2/2;
    }
    //rechts raus
    if (ball[i].y >= height+ball[i].g2) {
      ball[i].y = 0-ball[i].g2/2;
    }
    //Bälle stoßen aneinander
    for (int j=i+1; j < 15; j++) {
      if ((ball[i].position[1] >= ball[j].position[0]) && (ball[i].position[3] >= ball[j].position[2]) && 
      (ball[i].position[1] <= ball[j].position[1]) && (ball[i].position[3] <= ball[j].position[3])) {
        ball[i].angle = 225;
      }
      if ((ball[i].position[0] <= ball[j].position[1]) && (ball[i].position[3] >= ball[j].position[2]) && 
      (ball[i].position[0] >= ball[j].position[0]) && (ball[i].position[3] <= ball[j].position[3])) {
        ball[i].angle = 315;
      }
      if ((ball[i].position[1] >= ball[j].position[0]) && (ball[i].position[2] <= ball[j].position[3]) && 
      (ball[i].position[1] <= ball[j].position[1]) && (ball[i].position[2] >= ball[j].position[2])) {
        ball[i].angle = 135;
      }
      if ((ball[i].position[0] <= ball[j].position[1]) && (ball[i].position[2] <= ball[j].position[3]) && 
      (ball[i].position[0] >= ball[j].position[0]) && (ball[i].position[2] >= ball[j].position[2])) {
        ball[i].angle = 45;
      }
    }
    ball[i].maleBall();
  }
}

Objekt Ball

class Ball {
  
  int g1,g2; //Größe
  float x,y; //Position
  
  int h,s,b,a; //Farbe
  
  float vx,vy; //Geschwindigkeit
  float angle; //Gradzahl
  
  float[] position = new float[4]; // die 4 Ecken des Balls;
  
  Ball() {
    g1 = 80;
    g2 = g1;
    
    s = 100;
    b = 100-g1; 
    a = 100;
    
    angle = 90;
  }
  
  void maleBall() {
    for (int i=0; i<g1; i+=5) { //Abrundungen bzw. Schatten
       fill(h,s,b+i,a);
       ellipse(x,y,g1-i,g2-i);
    }
  }
}