int N = 1000;   // arbitrary limit
int x[] = new int[N];
int y[] = new int[N];
int count = 0;
boolean active = true;
float r = 3;  // radius for control points

void setup() {
  size(600,400);
  strokeWeight(2);
}

void draw() {
  background(204);
  if (active) {
    fill(0);
    beginShape();
    for (int j=0; j < count; j++) {
      vertex(x[j], y[j]);
      ellipse(x[j], y[j], 2*r, 2*r);
    }
    noFill();
    endShape();
  } else {
    fill(255, 127, 0);
    beginShape();
    for (int j=0; j < count; j++) {
      vertex(x[j], y[j]);
    }
    endShape(CLOSE);
  }
}

void mousePressed() {
   if (active) {
     if (count == 0 || dist(x[0],y[0],mouseX,mouseY) > r) {
       // add a new point
       x[count] = mouseX;
       y[count] = mouseY;
       count++;
     } else {
       // close the polygon
       active = false;
     }
   }
}

void keyPressed() {
   if (key == 'r') {
       count = 0;
       active = true;
   }
}