void setup(){
  size(300,300);
}
void loop(){
if(mousePressed){
  //background(mouseX,mouseY,0);
  ellipse(mouseX-5,mouseY,10,10);
  stroke(255,255,255);
  line(0,0,mouseX,mouseY);
  line(300,0,mouseX,mouseY);
  smooth();
  }else{
  rect(mouseX-5,mouseY,10,10);
  rect(mouseY-5,mouseX,10,10);
  stroke(255,0,0);
  line(mouseX,mouseY,mouseY,mouseX);
  }
}