title: How can I make the background of a surface transparent in pygame
body: <p>In pygame I draw a figure on a Surface. I want the Surface itself to be transparent but the image visible. I've tried to use <code>set_alpha</code> on the Surface, but the figure itself stays transparent.</p>
  
  <pre><code>    self.original_image = Surface((self.rect.w, self.rect.h))
      self.original_image.set_alpha(0)
  
      pg.draw.polygon(self.original_image,
                      self.color,
                      [Vector2(gui.PATCH_SIZE, gui.PATCH_SIZE),
                       Vector2(gui.HALF_PATCH_SIZE( ), 0),
                       Vector2(0, gui.PATCH_SIZE),
                       Vector2(gui.HALF_PATCH_SIZE( ), gui.PATCH_SIZE*3/4),
                       ])
  
      self.image = self.original_image
  </code></pre>
  
  <p>When I draw it I see nothing. When I leave out <code>self.original_image.set_alpha(0)</code> I get the figure I want, but on a black background rectangle, which obscures whatever it is over.  I get the same result if I try <code>self.original_image.fill((0, 0, 0, 0))</code> instead of <code>self.original_image.set_alpha(0)</code>. Any suggestions would be appreciated. Thanks.</p>
  
  <p>P.S. I'm trying to draws a NetLogo standard "Turtle."</p>