int size = 20;       // Width of the shape
float xpos, ypos;    // Starting position of shape    

float xspeed = 0.75;  // Speed of the shape
float yspeed = 0.75;  // Speed of the shape

int xdirection = 1;  // Left or Right
int ydirection = 1;  // Top to Bottom


void setup() 
{
  size(200, 200);
  stroke(0,0,0);
  line(0,height/2, width,height/2);
  
  // Set the starting position of the shape
  xpos = 0;
  ypos = height/2-size/2;
}

void loop() {
  background(120,120,120);
  
  xpos+=(xspeed*xdirection);
  ypos+=(yspeed*ydirection);
  
  if(xpos>width-size || xpos<0) {
  xdirection*=-1;
  }
  if(ypos>height-size || ypos<0) {
  ydirection*=-1;
  }
  noStroke(); 
  rect(xpos,ypos, size,size);
  
  //stroke(0,0,0);
  line(0,height/2, width,height/2);
  line(width/2,0, width/2,height);
}