title: What is wrong with my Processing sketch?
body: <p>I have a very basic Processing sketch that will allow me to draw a line like in MS-Paint. Here it is:  </p>
  
  <pre><code>void setup(){
   size(640,480); 
   background(255);
  }
  
  void mouseDragged(){
   line(pmouseX,pmouseY,mouseX,mouseY); 
  }  
  </code></pre>
  
  <p>However, when I click and drag the mouse nothing happens.<br>
  <strong>What is going wrong ?</strong></p>