int opacF = 255;
int opacC = 0;
int opac;


boolean opacState;

void setup(){
  size (400,400);
  background (255);
  opac = opacF;
  opacState = true;
}

void draw(){
  fill (0,opac);
  rect(0,0,400,400);
  //println (opac);
}

void mouseClicked(){
  if (opacState==true){
    opacState = false;
    opac = opacC;
    println  ("false"+" "+opac);
  }
  else{
    opacState = true;
    opac = opacF;
    println ("true"+" "+opac);
  }

}