title: How does Qt draw its GUI Components ( Basic Idea )?
body: <p>When I browsed the source code of Qt I didn't find how it actually draws a GUI component, but I know it uses OpenGL.</p>
  
  <p>I want to know how does a GUI library like Qt draw its GUI components (ex : QPushButton ,QWidget)?</p>
  
  <p>Can any one help me with a basic idea ?</p>