int px,py;
void setup(){
  size(250,250);
  noStroke();
  rectMode(CENTER);
  background(0,0,255);
}
void draw(){
  for(int i=0;i<=240;i+=10){
      px=int(mouseX/250.0*i+(250-i)/2);
      py=int(mouseY/250.0*i+(250-i)/2);
      fill(0,i,0);
      rect(px,py,250-i,250-i);
  }
}