/**
 FirstAnimation: A program to illustrate motion in Processing
 By: F. Green, 9/2006.
 Last modified: 10/24/2007.
 */
 
//xspeed is the horizontal speed of the ball
float xspeed;
//x its horizontal position
float x;

//Initialize the position and speed of the ball etc.
void setup ()
{
  size(400, 400);
  background(0);
  xspeed = 2.5;
  x = 10;
  frameRate(100);
  ellipseMode(CENTER);
}

void draw ()
{
  background(0);
  smooth();
  //Draw the ball:
  ellipse(x, 200, 20, 20);
  //Update its position:
  x = x + xspeed;
  //Detect collision with the walls:
  if (x > width - 10 || x < 10)
    xspeed = -xspeed;
}