Hi,guy:
I wrote the project for the test of the ball following,
I don't know how can to make the ball effect like falling star
thanks.~:)
 


boolean flagball=true; // playing ball game?
   
float ballpx=320; // position x
float ballpy=240; // position y
float ballvx=0.0; // velocity x
float ballvy=0.0; // velocity y
float ballgy=0.05; //  gravitation
float ballsz=30.0; // size
float ballfv=1.0; //  rebound factor

// x and y positions
// Declare two arrays with 50 elements.
int[] xpos = new int[30];
int[] ypos = new int[30];
 
void setup(){
     size(640, 480, P2D);
     smooth();
     for (int i = 0; i < xpos.length; i ++ ) {
     xpos[i] = 0;
     ypos[i] = 0;
  }
}
 
void draw(){
 
     background(0);
    // ball movement : not essential for optical flow
    // Shift array values
    for (int i = 0; i < xpos.length-1; i ++ ) {
    // Shift all elements down one spot.
    // xpos[0] = xpos[1], xpos[1] = xpos = [2], and so on. Stop at the second to last element.
    xpos[i] = xpos[i+1];
    ypos[i] = ypos[i+1];
  }
 
   ballpx[xpos.length-1] ++;
   ballpy[ypos.length-1] ++;

     //position and velocity
      ballpx += ballvx;
      ballpy += ballvy;
      ballvy += ballgy;
     
     
     
 
      // reflecton
        if(ballpy>480-ballsz) {
           ballpy=480-ballsz;
           ballvy=-ballvy*ballfv;
    }
     
      // draw the ball
     
      fill(0,0,255,200);
     
      ellipse(ballpx,ballpy,ballsz,ballsz);
}