// Dartmouth CS 2, Winter 2009, Chris Bailey-Kellogg
// Notes 5 | Sketch 5

// The current coordinates of the ball, starting at the center (in setup)
float x, y;
// The current color of the ball, and whether it's increasing or decreasing
int gray=255, dgray=-1;
// The current size of the ball, and how much it's increasing or decreasing
float sz=5, dsz=0.1;

void setup()
{
  smooth();
  noStroke();
  background(0);
  x=width/2; y=height/2;
}

void draw()
{
  fill(0,3);
  rect(0,0,width,height);
  fill(gray);
  ellipse(x,y,sz,sz);
  // Move the position by random steps in x and y
  x += random(-2,2);
  y += random(-2,2);
  // Increase/decrease the color, changing direction at the extremes
  gray += dgray;
  if (gray == 128) {
    // Decreased too far; start increasing
    dgray = 1;
  }
  else if (gray == 255) {
    // Increased too far; start decreasing
    dgray = -1;
  }
  // Increase/decrease the size, changing direction at the extremes
  sz += dsz;
  if (sz <= 2 || sz >= 15) dsz = -dsz;
}