float radijus = 30;
float xc, yc;    

float xbrzina = 2;
float ybrzina = 2;

int xsmer = 1;
int ysmer = 1;

void setup() 
{
  size(640, 360);
  stroke(255);
  frameRate(30);
  strokeWeight(2);
  ellipseMode(RADIUS);
  noFill();
  xc = width / 3;
  yc = height / 3;
}

void draw() {
  background(0);
  xc = xc + xbrzina * xsmer;
  yc = yc + ybrzina * ysmer;

  if (xc > width-radijus || xc < radijus)
    xsmer = -xsmer;

  if (yc > height-radijus || yc < radijus)
    ysmer = -ysmer;
    
  ellipse(xc, yc, radijus, radijus);
}