I was making a simple button interface and found myself handcoding the relative points, so I made a primitive tool to address this.

The program will print a short code to draw the shape using vertex function.

/////////////////////////////////////////
import java.util.Iterator;

ArrayList points;

void setup(){
 size(640,480);
 frame.setResizable(true);
 cursor(CROSS);
 points = new ArrayList();
}

void draw(){
 background(200);
 Iterator ite =  points.iterator();

 beginShape();
 
 while(ite.hasNext()){
   Vector v = (Vector)ite.next();
   float x = width*v.x;
   float y = width*v.y;

   vertex(x, y);
 }
 
 endShape();
}

class Vector{
 public float x;
 public float y;
}

void mousePressed(){
 if (mouseButton == RIGHT){
   // erase last point
   int i = points.size();
   if (i > 0){
     points.remove(i-1);
   }
   println("****point deleted. the new code is:");
   Iterator ite =  points.iterator();
   while(ite.hasNext()){
     Vector v = (Vector)ite.next();
     println("vertex("+v.x+"*f,"+v.y+"*f);");
   }
 }else{
   // add a point
   Vector v = new Vector();
   v.x = (float)mouseX/(float)width;
   v.y = (float)mouseY/(float)width;
   points.add(v);
   println("vertex("+v.x+"*f,"+v.y+"*f);");
 }
}
/////////////////////////////////////////

Maybe this has been done before in the forum, but I couldn't find it immediately, so I posted this cookbook method.