controlP5 slider doesn't show up:

1. import controlP5.*;
   ControlP5 cp5;
   
   public float ptSize = 1.0; // point size
   
   //_______________________________________________________________________________________________
   void setup(){
     size(500,500, P3D);
     smooth();
   
     // initiate controller
     cp5 = new ControlP5(this);
    
     cp5.addSlider("point_size")
        .setRange(0, 10)
        .setValue(ptSize)
        .setPosition(width-200, height-100)
        .setSize(100, 15)
        .showTickMarks(true)
        .setNumberOfTickMarks(11)
        .snapToTickMarks(true)
        ;
       
     cp5.setAutoDraw(false);
   }
   
   //_______________________________________________________________________________________________
   void draw(){
     background(100);
     println("pt size : " + ptSize);
   }
   
   //_______________________________________________________________________________________________
   // events from controller point_size are received here
   public void point_size (int theValue) {
     ptSize = theValue; 
   }
   

it seems only when setAutoDraw is set as true, will i see the slider...

Can anybody kindly help to explain why?

Thanks!